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Full text of "The power of movement in plants"

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TilE 



POWER OF MOVEMENT 



PLANTS. 



THE 

POWER OF MOVEMENT 
IN PLANTS 



BY 

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

ASSISTED BY 

FRANCIS DARWIN 



WITH ILLUSTRATIONS 



NEW YORK 

D. APPLETON AND COMPANY 
1898 



Authorized Edition. 



CONTENTS. 



INTRODUCTION Page 1-9 

CHAPTER I. 

THE ClRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS. 

Brassica oleracea, circumnutation of the radicle, of the arched hypo- 
cotyl whilst still buried beneath the ground, whilst rising above 
the ground and straightening itself, and when erect Circumnu- 
tation of the cotyledons Rate of movement Analogous obser- 
vations on various organs in species of Githago, Gossypium, 
Oxalis, Tropjeolum, Citrus, ^Esculus, of several Leguminous and 
Cucurbitaceous genera, Opuntia, Helianthus, Primula, Cyclamen, 
Stapelia, Cerinthe, Nolana, Solanum, Beta, Eicinus, Quercus, 
Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaiis, Zea, 
Avena, Nephrodium, and Selaginella 10-66 

CHAPTER II. 

GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF 
SEEDLING PLANTS. 

Generality of the circumnutating movement Eadicles, their cir- 
cumnutation of service Manner in which they penetrate the 
ground Manner in which hypocotyls and other organs break 
through the ground by being arched Singular manner of ger- 
mination in Megarrhiza, &c. Abortion of cotyledons Circum- 
nutation of hypocotyls and epicotyls whilst still buried and 
arched Their power of straightening themselves Bursting of 
the seed-coats Inherited effect of the arching process in hypo- 



2033579 



Vi CONTENTS. 

gean hypocotyls Circnmnutation of hypocotyls and epicotyl? 
when erect Circumnutation of cotyledons Pulvini or joints of 
cotyledons, duration of their activity, rudimentary in Oxalia 
comiculata, their development Sensitiveness of cotyledons to 
light and consequent disturbance of their periodic movements- 
Sensitiveness of cotyledons to contact Page 07-128 



CHAPTER III. 

SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND 
TO OTHER IRRITANTS. 

Manner in which radicles bend when they encounter an obstacle in 
the soil Vicia faba, tips of radicles highly sensitive to con- 
tact and* other irritants Ell'ects of too high a temperature 
Power of discriminating between objects attached on opposite 
sides Tips of secondary radicles sensitive Fisum, tips of 
radicles sensitive Effects of such sensitiveness in overcoming 
geotropism Secondary radicles Phaseolus, tips of radicles 
hardly sensitive to contact, but highly sensitive to caustic and 
to the removal of a slice Tropseolum Gossypiuui Cucurbita 
Kaplianus ./Exjulus, tip not sensitive to slight contact, highly 
sensitive to caustic Quercus, tip highly sensitive to contact- 
Power of discrimination Zea, tip highly sensitive, secondary 
radicles Sensitiveness of radicles to moist air Summary of 
chapter 129-200 



CHAPTER IV. 

THE ClRCUMNOTATING MoVKMENTS OF THE SEVERAL PARTS OF 
MATURK 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 3f leaves of Dionsea Leaves of Cannabis sink at night 
Leaves of Gymnosperms Of Monocotyledons Cryptogams 
Concluding remarks on the Circumnutation of leaves : generally 
rise in the evening and sink in the morning .. .. 201-262 



CONTENTS. vu 

CHAPTER V. 

MODIFIED CIRCUMNUTATION : CLIMBING PLANTS; EPINASTIC AND 
HYPONASTIC MOVEMENTS. 

C/ivcumnutation modified through innate causes or through the action 
of external conditions Lunate causes Climbing plants; simi- 
larity of their movements with those of ordinary plants; in- 
creased amplitude ; occasional points of difference Kpinastic 
growth of young leaves Hyponastic growth of the hypocotyls 
and epicotyls of seedlings Hooked tips of climbing and other 
plants due to modified circumnntatiou Ampelopsis trieuspidata 
Smithia Pmndii Straighten ing of the tip due to hypouasty 
Epinastic growth and circumnutation of the flower-peduncles of 
Trifolium repens and Oxalis carnosa Page 2(53-279 

CHAPTER VI. 

MODIFIED CIRCUMNUTATION : SLEEP OR NYCTITKOPIC MOVEMENTS, 
THEIR USE: SLEEP OF COTYLEDONS. 

Preliminary sketch of the skcp or nyctitropic movements of leaves 
Presence of pulvini The lessening of radiation the final cause 
of nictri tropic movements Manner of trying experiments on 
leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, 
and on the cotyledons of Mimosa Concluding remarks on radia- 
tion from leaves Small differences in the conditions make a 
great difference in the result Description of the nyctitropic 
position and movements of the cotyledons of various plants 
List of species Concluding remarks Independence of the 
nyctitropic movements of the leaves and cotyledons of the same 
species Reasons for believing that the movements have been 
acquired for a special purpose 280-316 



CHAPTER VII. 

MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS 
OF LEAVES. 

Conditions necessary for these movements List of Genera nnd 
Families, which include sleeping plants Description of the 
movements in the several Genera Uxalis: leaflets folded tit 



i CONTENTS. 

niuht Averrhoa: rapid movements of the leaflets Porlierift : 
leaflets close when plant kept very dry Tropavilum: leaves do 
not sleep unless well illuminated during day Lupin us : 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 Banlnnia: 
leaves folded at ni^ht 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 Page 317-417 



CHAPTER VII I. 
MODIFIED CIRCUJINUTATIOX: MOVEMENTS EXCITED BY LIUHT. 

Distinction between heliotropism and the effects of light on the 
periodicity of the movements of leaves Heliotropic movements 
of Beta, Solatium, Zea, and Avena Heliotropic movements 
towards an obscure light in Apios, Brassica, Phalaris, Tn>pax>- 
lum, and Cass : a Aphcliotropic movements of tendrils of Big- 
nonia Of flower- peduncles of Cyclamen Burying of the pods 
Heliotropism and apheliotropi.sm modified forms of circumnu- 
tation Steps by which one movement is converted into the 
other Trans versal-heliotropismus or diaheliotropism influenced 
by epinasty, the weight of the part and apogeotropism Apngeo- 
tropism overcome during the middle of the day by diaheliotro- 
pism Effects of the weight of the blades of cotyledons So- 
called diurnal sleep Chlorophyll injured by intense light 
Movements to avoid intense light 418-448 

CHAPTER IX. 

SENSITIVENESS OF PLANTS TO LIGHT : ITS TRANSMITTED EFFOT. 

Uses of he'iotropism Insectivorous and climbing plants not helic- 
tropic Same organ heliotropic at one age and not at another 
Extraordinary sensitiveness of some plants to light The effects 



CONTENTS. IS 

of light do not correspond with its intensity Effects of previous 
illumination Time required for the Hction of light After-effects 
of light Apogeotropism acts as soon as light fails Accuracy 
with which plants bend to the light This dependent on tlw 
illumination of one whole side of the pait Localised sensitive- 
ness to light and its transmitted effects Cotyledons of Pbalaris, 
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 A vena, curvature of basal 
part due to the illumination of upper part Similar results with 
the hypocotyls of Brassica and Beta Radicles of Sinapis aphelio- 
tropic, due to the sensitiveness of their tips Concluding remarks 
and summary of chapter Means by which circunmutatinn hag 
been converted into heliotropism or apheliotropism Page 449-402 



CHAPTER X. 

MODIFIED CIRCUMNUTATION : MOVEMENTS EXCITED BY 
GRAVITATION. 

Means of observation Apogeotropism Cytisus Verbena Beta 
Gradual conversion of the movement of ciicumnutation into 
apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica 
Apogeotropism retarded by heliotropism Effected by the aid 
of joints or pulvini Movements of flower-peduncles of Oxalis 
General remarks on apogeotropism Geotropism Movements of 
radicles Burying of seed-capsules Use of process Tril'olium 
subterraneum Arachis Amphicarpjea Diageotropism 
Conclusion 493-522 



CHAPTER XL 

LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTER 
EFFECTS. 

General considerations Vicia faba, effects of amputating the tips of 
the radicles Regeneration of the tips Effects of a short ex- 
posure of the tips to geotropic action and their subsequent 
amputation Kffects of amputating the tips obliquely Effects 
of cauterising the tips Effects of grease on the tips Pisuni 



CONTENTS. 

HltivTim, tips of radicles cauterised transversely, and on theit 
upper and lower sides Phaseolus, cauterisation and grease on 
the tips Gossypium Cucurbita, tips cauterised transverse iy, 
and on their upper and lower sides Zea, tips cauterised Con- 
cluding remarks and summary of chapter Advantages of the 
sensibility to geotropism being localised in the tips of the 
radicles Page 23-545 



CHAPTEE XII. 

SUMMARY AND CONCLUDING HEMAUKS. 

Nature of the circumnutating movement History of a germinating 
seed The radicle first protrudes and circunmutates 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 'I he circumnutation of all the parts or organs 
Modified circumnutation Epinasty and liyponasty Movements 
of climbing plants Nyctitropic movements .Movements excited 
by light and gravitation Localised sensitiveness Resemblance 
between the movements of plants and animals The tip of the 
radicle acts like a brain 546-573 

IKDES .. 574-593 



THE MOVEMENTS OF PLANTS. 



INTRODUCTION. 

THE 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 stern of a climbing plant, which bends 
successively to all points of the compass, so that the 
tip revolves. This movement has been called by 
Sachs " revolving nutation ;" but we have found it 
much more convenient to use the terms circumnutation 
and circumnutate. As we shall have to say much 
about this movement, it will be useful here briefly to 
describe its nature. If we observe a circumnutating 
stem, which happens at the time to be bent, we will 
say towards the north, it will be found gradually to 
bend more and more easterly, until it faces the east ; 
and so onwards to the south, then to the west, and 
back again to the north. If the movement had been 
quite regular, the apex would have described a circle, 
or rather, as the stem is always growing upwards, a 
circular spiral. But it generally describes irregular 
elliptical or oval figures ; for the apex, after point- 
ing in any one direction, commonly moves back 
to the opposite side, not, however, returning along 
the same line. Afterwards other irregular ellipses 
or ovals are successively described, with their longer 



2 INTRODUCTION. 

axes directed to different points of the compass. 
Whilst describing such figures, the apex often travels 
in a zigzag line, or makes small subordinate loops or 
triangles. In the case of leaves the ellipses are 
generally narrow. 

Until recently the cause of all such bending move- 
ments was believed to be due to the increased growth 
of the side which becomes for a time convex ; that this 
side does temporarily grow more quickly than the 
concave side has been well established ; but De Vries 
has lately shown that such increased growth follows 
a previously increased state of turgescence on the 
convex side.* In the case of parts provided with a 
so-called joint, cushion or pulvinus, which consists of 
an aggregate of small cells that have ceased to 
increase in size from a very early age, we meet with 
similar movements ; and here, as Pfeffer lias shown f 
and as we shall see in the course of this work, 
the increased turgesceuce 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 i that the 
increased extensibility of the cell-walls is the more 
important element. That such extensibility must 
accompany increased turgescence in order that the part 
may bend is manifest, and this has been insisted on by 
several botanists ; but in the case of unicellular plants 
it can hardly fail to be the more important element, 
Oi the whole we may at present conclude that in- 



* Sachs first showed f'Lchr- 19, 187!), p. 830. 

biich,' &c., 4tli edit. p. -152) the f 'De Perindischen Boivcgun- 

intimftte connection between tnr- gen dor lilattorguue,' 187">. 

gescence and growth. For De * ' Ulitersuchungen fiber den 

Vries' interesting essay, ' Wachs- Holiotropismus,' Sitzb <l-r K., 

tliumskriimmuugcn mehrzelliger Aknd.der Wissi'iischaft. (Vicuna). 

Orgiine,' see ' Dot. Zeituug,' Dec. Jan. 1880. 



INTRODUCTION. 3 

creased growth, first ov 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 circimmutating, though often on a small 
scale. Even the stems of seedlings before they have 
broken through the ground, as well as their buried 
radicles, circurn nutate, as far as the pressure of the 
surrounding earth permits. In this universally pre- 
sent movement we have the basis or groundwork for 
the acquirement, according to the requirements of the 
plant, of the most diversified movements. Thus, the 
great sweeps made by the stems of twining plants, 
and by the tendrils of other climbers, result from 
a mere increase in the amplitude of the ordinary 
movement of circumnutation. The position which 
young leaves and other organs ultimately assume 
is acquired by the circumnutaring 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 

* Sec Mr. Vines excellent dis- Naturkunclo in Wiirtemberg, 1 
cussion ('Arbeitcn des Dot. Insti- 1874,p.211) on the curious move- 
tuts in Wiirzburg,' B. II. pp 142, merits of Spirogyra, a plant con- 
1-13, 1878) on this intiicate subject. sisting of a single row of nells,.aro 
Ilofmoister's observations (' Jah- valuable in relation to this subject, 
reschrifte des Vereins fiir Vaterl. 



4: INTRODUCTION. 

forms of circurnnutation ; 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 
circurnnutation, some other subjects will be discussed. 
The two which have interested us most are, firstly, the 
fact that with some seedling plants the uppermost 
part alone is sensitive to light, and transmits an influ- 
ence to the lower part, causing it to bend. If there- 
fore the upper part be wholly protected from light, 
the lower part may be exposed for hours to it, and yet 
does not become in the least bent, although this would 
have occurred quickly if the upper part had been 
excited by light. Secondly, with the radicles of seed- 
lings, the tip is sensitive to various stimuli, espe- 
cially to very slight pressure, and, when thus excited, 
transmits an influence to the upper part, causing it to 
bend from the pressed side. On the other hand, if 
the tip is subjected to the vapour of water proceeding 
from one side, the upper part of the radicle bends 
towards this side. Again it is the tip, as stated by 
Ciesielski, though denied by others, which is sensitive 
to the attraction of gravity, and by transmission causes 
the adjoining parts of the radicle to bend towards the 
centre of the earth. These several cases of the effects 
of contact, other irritants, vapour, light, and the 



INTRODUCTION. 5 

attraction of gravity being transmitted from the ex- 
cited part for some little distance along the organ in 
question, have an important bearing on the theory of 
all such movements. 

Terminology. A. brief explanation of some terras which will 
l)e 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 
liypocotyl: the stem immediately above the cotyledons will be 
called the epicoiyl or plumule. The radicl: can be distinguished 
from the hypocotyl only by the presence of root-hairs and the 
nature of its covering. The meaning of the word circumnu- 
tation has already been explained. Authors speak of positive 
and negative heliotropism,* that is, the bending of an organ 
to or from the light ; but it is much more convenient to confine 
the word hclu&vupiwb to bending towards the light, and to 
designate as apheliotropiam bending from the light. There is 
another reason for tins change, for writers, as we have 
observed, occasionally drop the adjectives poaitiue and negative, 
and thus introduce confusion into their discussions. Dialielio- 
tropisin, 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 bo 
called by us gKotropism ; apogeotropixm will mean bending in 
opposition to gravity or from the centre of the earth ; and dia- 
jeutropism, a position more or less transverse to the radius of 
the earth. Tiie words heliotropism and geotropism properly 
mean the act of moving ill relation to the light or the earth ; 
but in the same manner as gravitation, though denned 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 
IJcliotwp'sm and Giotropieiri tiajjc zur I'll 
tteic first used by Dr. A. B. 1SG8. 



H INTRODUCTION. 

lower surface, and thus causes it to bend downwards. Hyr>o- 
nasty is the reverse, and implies increased growth along the 
lower surface, causing the part to bend upwards.* 

M tho:ls 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 io 
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 tho 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 slick driven into ths 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 tho 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-plale 
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 don quite accurately, when the movement was much 
magnified, such as 80 times and upwards; yet even in this 
ca.se the general course may be trusted. To test the accuracy 
of tho above method of observation, a filament was fixed to an 



* These terms are used in the ' Wiirzburiy Arbeiten,' Heft ii. 
wise given thctu by Do 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 Jength 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 arc much indebted to lie has reduced and engraved our 
Mr. Cooper fur the care with which diagrams. 



$ INTRODUCTION. 

introduced a superfluous number of diagrams ; but they tako 
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 obliqTiely ; 
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 our 



INTRODUCTION. 

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 CIBCUMNUTATION OF SEEDLINGS. CHAP, t 



CHAPTER I. 

THE ClRCCMXTJTATING MOVEMENTS OF SEEDLIira PLA51*. 

Briis.-ica oleracea, circumnntation of the radicle, of the arched hypo- 
cotyl whilst still buried beneath the ground, whilst rising above the 
ground and straightening itself, and when erect Circnmnutation 
of the cotyledons Rate of movement Analogous observations on 
various organs in species of Githago, Gossypium, Oxalis, Tro- 
juenluin, Citrus, JEsculus, of several Leguminous and Cucurbita- 
ceous ginera, Opuntia, Helianthus, Primula, Cyclamen. Stapc-1'a, 
Cerinthe, Nolana, Solanum, Bita, llicinus, Qucrcus, Corylus, Finns, 
Cycas, Canna, Allium, Asparagus, Phalaris, Zea, A vena, Neph ra- 
dium, and Selagiuella. 

THE following chapter is devoted to the circum- 
nutating movements of the radicles, hypocotyls, and 
cotyledons of seedling plants ; and, when the coty- 
ledons do not rise above the ground, to the movements 
of the epLotyl. But in a future chapter we shall have 
to recur to the movements of certain cotyledons which 
sleep at night. 

Brassica oleracea (Cruciferce). 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. 

Hadide. 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 l>ead at the end of the filament was traced (Fig. 1) 
during sixty hours. In this time the radicle increased in 
length from '05 to -11 inch. Had the filament been attached at 
first close to the apex of the radicle, and if it could have re- 
mained there all the time, the movement exhibited would have 



CHAP. I, 



BRASSJCA. 



11 



Fig. 1. 



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 zino 

plate. As far as we could 

rou glil y ascertain by measure- 

ments made with compasses 

on other seeds, the tip alone, 

for a length of only ^ to 

T a^ of an inch, is acted on 

by geotropism. But the trac- 

ing shows that the basal part 

of the radicle continued to 

circumnutate irregularly dur- 

ing the whole time. The 

actual extreme amount of 

movement of the bead at the 

end of the filament was nearly 




05 inch, but to what extent 
the movement of the radicle 
was magnified by the fila- 
ment, which was nearly t inch 
in length, it was impossible 
to estimate. 

Another seed was treated and observed in the same manner, 
but the radicle in this case protruded '1 inch, and was not 



Brassca oleriacea : circumnutation of 
radicle, traced on horizontal glass, 
from 9 A.M. Jan. 31st to 9 P.M. 
Feb. 2nd. Movement of bead at 
end of filament magnified about 
40 times. 



Fig. 2. 



\ 






Brassica oleracea : oircumnutating and geotropic movement of radicle, 
traced on horizontal glass during 46 hours. 

fastened so as to project quite vertically upwards. The filament 
was affixed close to its base. The tracing (Fig. 2, reduced by 
half) shows the movement from 9 A.M. Jan. 31st to 7 A.M. 
Feb. 2nd; but it continued to move during the whole of the 



12 CIRCUMNUTATION OF SEEDLINGS. CHAR I. 

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 tho 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 ft ; the cotyledons being 
etill 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 tho 
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 soilwill permit; but this was difficult to observe, 
because as soon as the arch is freed from lateral pressure the two 
legs begin to separate, eveu at a very early age, before the arch 
would naturally have reached the surface. Seeds were allowed 
to germinate on the surface of damp earth, and after they had 
fixed themselves by their radicles, and after the, as yet, only 



CHAP. I. BRASS1CA. 13 

slightly arched hypocotyl had become nearly vertical, a glasa 
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 circuro- 
nutating, whilst it was straightening itself by growth along its 
inner or concave surface. 
A somewhat different method of observation was next followed : 

Fig. 3. 




Brassica oleracea : circumnutating movement of buried and arched hypo- 
cotyl (dimly illuminated from above), traced on horizontal glass during 
45 hours. Movement of bead of filament magnified about 25 times, 
and here reduced to one-half of original scale. 

as soon as the earth with seeds in a pot began to crack, the 
surface was removed in parts to the depth of '2 inch ; and a 
filament was fixed to the basal leg of a buried and arched hypo- 
cotyl, just above the summit of the radicle. The cotyledons 
were still almost completely enclosed within the much-cracked 
seed-coats ; and these were again covered up with damp adhesive 
eoil 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 stows that the arched hypocotyl tends at this early 



J 4 CIBCUMNUTATION OF SEEDLINGS. CHAP. 1 

ago lo circumnutate irregularly. On the first day the greatei 
movement (from right to left in the figure) was not in the plane 
of tho vertical and arched hypocotyl, but at right angles to it, or in 
the plane of the two cotyledons, which were still in close contact. 
The basal leg of the arch at the time when the filament was 
affixed to it, was already bowed considerably backwards, or 
from the cotyledons ; had the filament been affixed before this 
bowing occurred, the chief movement would have been at right 
angles to that shown in the figure. A filament was attached to 
another buried hypocotyl of the same age, and it moved in a 
similar general manner, but the line traced was not so complex. 
This hypocotyl became almost straight, and the cotyledons were 
dragged from beneath the ground on the evening of the second day, 




Brassica oleracea : circumnutating movement of buried and arched hypo- 
cotyl, with the two legs of the arch tied together, traced on horizontal 
glass during 33 hours. Movement of the bead of filament magnified 
about 26 times, and here reduced to one-half original scale. 

Before the above observations were made, some arched hypo- 
cotyls buried at the depth of a quarter of an inch were un- 
covered ; and in order to prevent the two legs of the arch 
from beginning to separate at once, they were tied together with 
fine silk. This was done partly because we wished to ascertain 
how long the hypocotyl, in its arched condition, would continue 
to move, and whether the movement when not masked and 
disturbed by the straightening process, indicated circumnu- 
tation. Firstly, a filament was fixed to the basal leg of an 
arched hypocotyl close above the summit of the radicle. The 
cotyledons were still partially enclosed within the seed-coats. 
The movement was traced (Fig. 4) from 9.20 A.M. on Dec. 



CHAP. I. BRASSICA. 15 

23rd to 6.45 A.M. on Dec. 25th. No doubt the natural move- 
ment was much disturbed by the two legs having been tied 
together ; but we see that it was distinctly zigzag, first in one 
direction and then in an almost opposite one. After 3 P.M. on 
the 2ith 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 wo 



Fig. 5. 




flrassica oleracea : cireurauutating 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. 

eee that the course was still zigzag, which indicates a tendency 
to circumnutation. The base of the basal leg by this time had 
almost completely ceased to move. 

As soon as the cotyledons have been naturally dragged from 
beneath the ground, and the hypocotyl has straightened itself 
by growth along the inner or concave surface, there is nothing to 
interfere with the free movements of the parts ; and the circum- 
nutation now becomes much more regular and clearly displayed, 
as shown in the following cases: A seedling was placed in 
front and near a north-east window with a line joining the 



16 



CIECUMNUTATION OF SEEDLINGS. CHAP. I. 



two cotyledons parallel to the window. It was thus left tho 
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 tho first dot was 

Fig. 6. 




Brassicn oferacen : conjoint circumnutation of the hypocotyl and cotyledons 
during 10 hours 45 minutes. Figure here reduced to one-half original 
scale. 

made on the glass. The last was made at 8.45 P.M.; seventeen 
dots being altogether made in this interval of 10 h. 45 m. (see 
Fig. 6). It should be noticed that when I looked shortly after 
P.M the bead was pointing off the glass, but it came on again 
at 5.30 P.M., and the course during this interval of 1 h. 30 m. has 
been filled up by imagination, but cannot be far from correct 
The bead moved seven times from side to side, and thus de- 
scribed 3i ellipses in 10J h.; each being completed on an 
overage in 3 h. 4 m. 

On the previous day another seedling had been observed 
under similar conditions, excepting that the plant was so 



CHAP. I. BEASS1CA. 17 

placed that a line joining the two cotyledons pointed towards 
tJie 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. The 
cotyledons bowed themselves greatly towards the light from 8 to 
10.00 A.M., when the first dot was made (Fig. 7). During the 

Fig. 7. 




Brzssica oleracea : conjoint circumnutation of the hypocotyl and cotyledons, 
from 10.50 A.M. to 8 A.M. on the following morning. Tracing made 
on a vertical glass. 

next 12 hours the bead swept obliquely up and down 8 times 
and described 4 figures representing ellipses; so that it travelled 
at nearly the same rate as in the previous case. During the 
night it moved upwards, owing to the sleep-movement of the 
cotyledons, and continued to move in the same direction till 
9 A.M. on the following morning ; but this latter movement 
would not have occurred with seedlings under their natural 
conditions fully exposed to the light. 

By 9.25 A.M. on this second day the same cotyledon had 



18 



CIRCUMNUTATION OF SEEPLINGS. CHAP. 1 



Fig. 8. 




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 
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- 

mcnt f ^^ r S ans COQ - 
joined ; and W6 now 

wished to ascertain whe- 

thel ' to* circumnutated - 
Filaments were therefore 
fixed horizontally to two hypocotyls close beneath the petioles 
of their cotyledons. These seedlings had stood for two days 
in the same position before a north-east window. In the morn- 
ing, up to about 11 A.M., they moved in zigzag lines towards 
the light; and at night they again became almost upright 
through apogeotropism. After about 11 A.M. they moved a 
little back from the light, often crossing and recrossing their 
former path in zigzag lines. The sky on this day varied much 
in brightness, and these observations merely proved that the 
hypocotyls were continually moving in a manner resembling 
circumnutation. On a previous day which was uniformly 
cloudy, a hypocotyl was firmly secured to a little stick, and 
a filament was fixed to the larger of the two cotyledons, and its 
movement was traced on a vertical glass. It fell greatly from 
8.5'2 A.M., when the first dot was made, till 10.55 A.M. ; it then rose 
greatly until 12.17 P.M. Afterwards it fell a little and made a 
loop, but by 2.22 P.M. it had risen a little and continued rising 
till U.23 P.rf., when it made another loop, and at 10.30 P.M. was 
again rising. These observations show that the cotyledons move 



Bratsicaole.-acea: conjoint circnnmotation 
of the hypocotyl and cotyledons during 
8 hours. Figure here reduced to one- 



CHAP. I. 



BRASSiCA. 



vertically up and down all day long, and as there was somo 
Blight 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 1| inch in height. 
The figure traced was a very 
complex one, though the 
movement was not so great 
in extent as in the last case. 

The hypocotyl of another 
seedling of the same age was 
secured to a little stick, and 
a filament having been fixed 
to the midrib of one of the 
cotyledons, the movement of 
the bead was traced during 14 h. 15m. (see Fig. 10) in darkness. 
It should be noted that the chief movement of the cotyledons, 
namely, up and down, would be shown on a horizontal glass- 
plate only by the lines in the direction of the midrib (that is, 




Brassica oleracea : circuinnutation 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. ou the following morn- 
ing. Figure here reduced to one- 
half of original scale. 



20 



CIRCUMNUTATION OF SEEDLINGS. CHAP. L 



Fig. 10. 



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 
microscope with the stage removed, and 
with a micrometer eye-piece so adjusted 
that each division equalled -^ 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- 




\ 



Brassica oleracca-: cir- 
cumnutation of a 
cotyledon, the hypo- 
cotyl having been 
secured to a stick, 
traced on a horizon- 
tal glass, in dark- 
ness, from 8.15 A.M. 
to 10.30 P.M. Move- 

CViSiSt* iu s to observe how ra p idl y the circum - 

fied 13 times. nutating apex of a cotyledon passed across 

the divisions of the micrometer. Whilst 
travelling in any direction the apex generally oscillated back- 
wards and forwards to the extent of -^-^ and sometimes of nearly 
si-s of an inch. These oscillations were quite different from the 
trembling caused by any disturbance in the same room or by 
the shutting of a distant door. The first seedling observed was 
nearly two inches in height and had been etiolated by having 
been grown in darkness. The tip of the cotyledon passed across 
10 divisions of the micrometer, that is, -^ of an inch, in 6 m. 
40 s. Short glass filaments were then fixed vertically to the 
hypocotyls of several seedlings so as to project a little above the 
cotyledons, thus exaggerating the rate of movement ; but only a 
few of the observations thus made are worth giving. The most 
remarkable fact was the oscillatory movement above described, 
and the difference of rate at which the point crossed the divi- 
sions of the micrometer, after short intervals of time. For 
instance, a tall not-etiolated seedling had been kept for 14 h. 
in darkness ; it was exposed before a north-east window for only 



CHAP. 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 live divisions of the micrometer (i. e. -j^ inch) in 
1 m. 30 s. The seedling was then left in darkness for an hour, 
and now it required 3m. 6s. to cross one division, that is, 
15 m. 30 s. to have crossed five divisions. Another seedling, 
after being occasionally observed in the back part of a northern 
room with a very dull light, and left in complete darkness for 
intervals of half an hour, crossed five divisions in 5 m. in the 
direction of the window, so that we concluded that the move- 
ment was heliotropic. But this was probably not the case, for 
it was placed close to a north-east window and left there for 
25 m., after which time, instead of moving still more quickly 
towards the light, as might have been expected, it travelled 
only at the rate of 12 in. 30 s. for five divisions. It was then 
again left in complete darkness for 1 h., and the point now 
travelled in the eaiue direction as before, but at the rate of 
3 m. 18 s. for five divisions. 

We shall have to recur to the cotyledons of the cabbage in a 
future chapter, when we treat of their sleep-movements. The 
circumuutation, also, of the leaves of fully-developed plants 
will hereafter be described. 

Fig. 11. 




Bithago seqetum: circumnutcition of hypocotyl, traced on a horizontal 
glass, by means of a filament fixed transversely across its summit, from 
8.15 A.M. to 12.15 P.M. on the following day. Movement of bead of 
filament magnified about 13 times, here reduced to one-half the original 
scale. 

Githago sec/etiim (Caryophyllea?). A yotmg seedling was dimly 
Uluminated from above, and the circumuutation of tha hypo- 




22 CIRCUMNUTATION OF SEEDLINGS. CHAF. 1 

eotyl 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. 

\Ve 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 
Fig. 12. troublesome. We shall recur to the noc- 

turnal or sleep-movements of the cotyle- 
dons in a future chapter. 

Gossypium (var. Nankin cotton) (Mal- 
vaceae). The circumnutation of a hypo- 
cotyl was observed in the hot-house, but 
the movement was so much exaggerated 

that the bead twice ^ &SSed for a timC OUt f 

tal glnss, from 10.30 view. It was, however, manifest that two 

A.M. to 9.30 A.M. on somewhat irregular ellipses were nearly 

b^m'ea^s ofTfih' com P Ieted in 9 h - Another seedling, 

ment ^fixed across 1* m - * u height, was then observed during 

its summit. Move- 23h.; but the observations were not 

roent of bead of fila- ma de at sufficiently short intervals, as 

rwTceTStgnt sho ^ the *" dots in K* 12 > and the 

minated from above, tracing was not now sufficiently enlarged. 

Nevertheless there could be no doubt 

about the circumnutation of the hypocotyl, which described 

in 12 h. a figure representing three irregular ellipses of unequal 

sizes. 

The cotyledons are in constant movement up and down during 
the whole day, and as they offer the unusual case of moving 
downwards late iu the evening and in the early part of the 
night, many observations were made on them. A filament was 
fixed along the middle of one, and its movement traced on a 
vertical glass; but the tracing is not given, as the hypocotyl 
was not secured, so that it was impossible to distinguish clearly 
between its movement and that of the cotyledon. The coty- 
ledons rose from 10.30 A.M. to about 3 P.M. ; they then sank til] 
10 P.M., rising, however, greatly in the latter part of the nigLt 






CHAP 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 : 



Oct. 20 2.50 P.M. 
4.20 
5.20 
10.40 

Oct. 21 8.40 A.M. 
11.15 
9.11 P.M. 



25 abore horizon. 
22 



35 

10 below horizo 



The position of the two cotyledons was roughly sketched at 
various hours with the same general result. 

In the following summer, the hypocotyl of a fourth seedling 
was secured to a little stick, and a glass filament with triangles 
of paper having been fixed to one of the cotyledons, its move- 
ments were traced on a vertical glass under a double skylight in 
the house. The first dot was made at 4.20 P.M. June 20th ; and 
the cotyledon fell till 10.15 P.M. in a nearly straight line. Just 
past midnight it was found a little lower and somewhat to one 
side. By the early morning, at 3.45 A.M., it had risen greatly, 
but by 6.20 A.M. had fallen a little. During the whole of this 
day (21st) it fell in a slightly zigzag line, but its normal course 
was disturbed by the want of sufficient illumination, for during 
the night it rose only a little, and travelled irregularly during 
the whole of the following day and night of June 22nd. The 
ascending and descending lines traced during the three days 
did not coincide, so that the movement was one of circumnuta- 
tion. This seedling was then taken back to the hot-house, and 
after five days was inspected at 10 P.M., when the cotyledons 
were found hanging so nearly vertically down, that they might 
justly be said to have been asleep. On the following morning 
they had resumed their usual horizontal position. 

Oxalis rosea (Oxalidese). 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 irch 
in length. In this and the following species the end of the 
petiole, where united to the blade, is developed into a pulvinus. 
The apex of the cotyledon stood only 5 inches from the vertical 
glass, so that its movement was not greatly exaggerated as long 
as it remained nearly horizontal ; but in the course of the day it 
both rose considerably above and fell beneath a horizontal posi- 
tion, and then of course the movement was much exaggerated. 



21 



CIRCUMNUTATION OF SEEDLINGS. CHAP. I 



In Fig. 13 its course is shown from 6.45 A.M. on June 17th, to 
7.40 A.M. on the following morn- 
ing ; and we see that during the 
daytime, in the course of 11 h. 
15 m., it travelled thrice down 
and twice up. After 5.45 P.M. it 
moved rapidly downwards, and 
in an hour or two depended verti- 
cally ; it thus remained all night 
asleep. This position could not 
be represented on the vertical 
glass nor in the figure here given. 
By 6.40 A.M. on the following 
morning (18th) both cotyledons 
had risen greatly, and they con- 
tinued to rise until 8 A.M., when 
they stood almost horizontally. 
Their movement was traced dur- 
ing the whole of this day and 
until the next morning ; but a 
tracing is not given, as it was 
closely similar to 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. 

Another seedling was observed 
in a similar manner during nearly 
24 h., but with the difference that 
the hypocotyl was left free. The 
movement also was less magnified. 
Between 8.12 A.M. and 5 P.M. on 
the 18th, the apex of the cotyle- 
don moved 7 times upwards or 

rotea : circnmnutation of downwards (Fig. 14). The noo 
cotyledons, the hypocotyl being turnal sinking movement, which 
is merely a great increase of one 
of the diurnal oscillations, com- 
menced about 4 P.M. 
Oxalis Ycddiviana. This species is interesting, as the coty- 




secured to a stick ; illumina- 
ted from above. Figure here 
given one-half of original scale. 



CHAP. I. 



OXALIS. 



ledons rise perpendicularly upwards at night so as to come into 
close contact, instead of sinking vertically downwards, as in the 
case of 0. rosea. A glass filament was fixed to a cotyledon, 
C 17 of an inch in length, and the hypocotyl was left free. On 



Fig. 14. 



812'am. 



' Fig. 15. 




S4S' 




Oxalis rosea : conjoint circumnutation of 
the cotyledons and hypocotyl, traced 
from 8.12 A.M. on June 18th to 7.30 
A.M. 19th. The apex of the cotyledon 
stood only 3f inches from the vertical 
glass. Figure here given one-half of 
original scale. 



Oxalis Valdiviana i conjoint 
circumnutation of a cotyle- 
don and the hypocotyl, traced 
on vertical glass, during 24 
hours. Figure here given 
one-half of original scale; 
seedling illuminated from 



the first day the seedling was placed too far from the vertical 
glass ; s J that the tracing was enormously exaggerated and the 
movement could not be traced when the cotyledon either rose or 
sank much; but it was clearly seen that the cotyledons rose 
thrice and fell twice between 8.15 A.M. and 4.15 P.M. Early on 
jhe following morning (June 19th) the apex of a cotyltdon was 



26 CIRCUMNUTATION OF SEEDLINGS. CHAP. L 

placed only 1J inch from the vertical glass. At 6.40 A.M. it 
stood horizontally; it then fell till 8.35, and then rose. Al- 
together in the course of 12 h. it rose thrice and fell thrice, as 
may be seen in Fig. 15. The great nocturnal rise of the coty- 
ledons usually commences about 4 or 5 P.M., and on the following 
morning they are expanded or stand horizontally at about 6.30 
A.M. In the present instance, however, Ihe 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. 

Oxcdis corniculata (var. cuprta). The cotyledons rise at night 
to a variable degree above the horizon, generally about 45 : 
those on some seedlings between 2 and 5 days old were found 
to be in continued movement all day long; but the movements 
were more simple than in the last two species. This may have 
partly resulted from their not being sufficiently illuminated 
whilst being observed, as was shown by their not beginning to 
rise until very late in the evening. 

Oxalis (Biophytum) sensitiva. The cotyledons are highly re- 
markable from the amplitude and rapidity of their movements 
during tho day. The angles at which they stood above or 
beneath the horizon were measured at short intervals of time ; 
and we regret that their course was not traced during the whole 
day. We will give only a few of the measurements, which were 
made whilst the seedlings were exposed to a temperature of 22 
to 24i C. One cotyledon rose 70 in 11 m. : another, on a distinct 
seedling, fell 80 in 12 m. Immediately before this latter fall 
the same cotyledon had risen from a vertically downward to a 
vertically upward position in 1 h. 48 m., and had therefore passed 
through 180 in under 2 h. We have met with no other instance 
of a circumnutating movement of such great amplitude as 180 ; 
nor of such rapidity of movement as the passage through 80 in 
12 m. The cotyledons of this plant sleep at night by rising 



CHAP. I. 



TROP^EOLUM. 



27 



Fig. 16. 



vertically and coming into close contact. This upward move- 
ment differs from one of the great diurnal oscillations above 
described only by the position being permanent during the night 
and by its periodicity, as it always commences late in the 
evening. 

Tropxolum minus (?) (var. Tom Thumb) (Tropseolese). 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 11 h. After the 
last dot given in the figure, the 
bead moved to a great distance, 
and finally off the glass, in the 
direction indicated by the broken 
line. This great movement, due to 
increased growth along the con- 
cave surface of the arch, was caused 
by the basal leg bending back- 
wards from the upper part, that is 
in a direction opposite to the depen- 
dent tip, in the same manrer as 
occurred with the hypocotyl of 
the cabbage. Another buried and 

arched epicotyl was observed in the same manner, excepting 
that the two legs of the arch were tied together with fine silk 
for the sake 01 preventing the great movement just mentioned. 
It moved, however, in the evening in the same direction as 
before, but the line followed was not so straight. During the 
morning the tied arch moved in an irregularly circular, strongly 
zigzag course, and to a greater distance than in the previous 
case, as was shown in a tracing, magnified 18 times. The move- 
ments of a young plant bearing a few leaves and of a mature 
plant, will hereafter be described. 




Tropceolum minus (?) : cirrum- 
nutation of buried and arched 
epicotyl, traced on a horizon- 
tal glass, from 9.20 A.M. to 
8.15P.M. Movement of bead 
of filament magnified 27 
times. 



28 CIRCUMNUT ATION OF SEEDLINGS. CHAP. I. 

Citrus auraniiam (Orange) (Aurantiaceae). The cotyledons 
are hypogean. The circumuutation of an epicotyl, which at the 
close of our observations was '59 of an inch (15 mm.) in height 
alx)ve the ground, is shown in the annexed figure (Fig. 17), us 
observed during a period of 44 h. 40 m. 




Citrus cntrantium: circumnutation of epicotyl with a filament fix*d 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 
fiiament was at first magnified 21 times, or lO.j, in figure here given, 
and afterwards 36 times, or 18 as here given; seedling illuminated 
from above. 



hippocastanum (Hippocastaneae). Germinating seeds 
were placed in a tin box, kept moist internally, with a sloping 
bank of damp argillaceous sand, on which four smoked glass- 
plates rested, inclined at angles of 70 and 65 with the 
horizon. The tips of the radicles were placed so as just to 
touch the upper end of the glass-plates, and, as they grew 
downwards they pressed lightly, owing to geotropism, on the 
smoked surfaces, and left tracks of their course. In the middle 
part of each track the glass was swept clean, but the margins 
were much blurred and irregular. Copies of two of these tracks 
(all four being nearly alike) were made on tracing paper placed 
aver 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 dowii- 
w;ird course pressed with unequal force on the plates, aa 



CHAP. I. 



YICIA. 



29 



Fig. 18. 




the tracks varied in breadth. The more perfectly serpentine 
trades made by the radicles of Phaseolus multiflorus and Vicia 
faba (presently to be described), render 
it almost certain that the radicles of 
the present plant circumnutated. 

Pltasedus multiflorus (Leguminosse). 
Four smoked glass-plates were ar- 
ranged in the same manner as des- 
cribed under 2Esculus, and the tracks 
left by the tips of four radicles of the 
present plant, whilst growing down- 
wards, were photographed as trans- 
parent objects. Three of them are 
here exactly copied (Fig. 1 1 J). 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 
odginal plates than in the copies. 
These radicles therefore were continually moving in all direc- * 
tions that is, they circumnutated. The distance between the 
extreme right and left positions 
of the radicle A, in its lateral 
movement, was 2 mm., as ascer- 
tained by measurement with an 
eye-piece micrometer. 

Vicia faba (Common Bean) 
(Lcguminosse). h'adide. Some 
beans were allowed to germinate 
on bare sand, and after one had 
protruded its radicle to a length 
of '2 of an inch, it was turned 
upside down, so that the radicle, 
which was kept in damp air, 
now stood upright. A filament, 
nearly an inch in length, was 
affixed obliquely near its tip; and the movement of the 
terminal bead was traced from 8.30 A.M. to 10.30 P.M., as shown 
in Fig. 18. The radicle at first changed its course twice 



lines of tracks left on in- 
clined glass-plates by ti|>t 
of radicles. In A the plate 
was inclined at 70 with 
the horizon, and the radicle 
was 1 9 inch in length, and 
23 inch in diameter at base. 
In B the plate was inclined 
65 with the horizon, and 
the radicle was a trifle 
larger. 



Fig. 19. 



Phasco'ut multiflorus: tracks left 
on inclined smoked glass-plates 
by tips of radicles in growing 
downwards. A and C, plates 
inclined at 60, B inclined at 
68 with the horizon. 



30 



CIRCUMNUTATION OF SEEDLINGS. CHAP. I 



abruptly, then made a small loop and then a larger zigzag 
curve. During the night and till 11 A.M. on the following 



Fig. 20. 




ricia faba: circumnutation of a radicle, at first pointing verti/ally up- 
wards, kept in darkness, traced on a horizontal glass, during 14 hour?. 
Movement of bead of filament magnified 23 times, here reduced to 
one-half of original scale. 

morning, the 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. 




D. E. 

facia faba : tracks left on inclined smoked glass-plates, by tips of radicles 
in growing downwards. Plate C was inclined at 6:>, plates A and D 
at 71, plate B at 7, and plate E at a few degrees beneath the 
horizon. 



CHAP. I. VICIA. 31 

"We next experimented on nearly a score of radicles by allowing 
them to grow downwards over inclined plates of smoked glass, 
in exactly the same manner as with -ZEsculus and Phaseolus. 
Some of the plates were inclined only a few degrees beneath 
the horizon, but most of them between 60 and 70. In tho 
latter cases the radicles in growing downwards were deflected 
only a little from the direction which they had followed whilst 
germinating in sawdust, and they pressed lightly on the glass- 
plates (Fig. 21). Five of the most distinct tracks are here 
copied, and they are all slightly sinuous, showing circumnuta- 
tion. Moreover, a close examination of almost every one of the 
tracks clearly showed that the tips in their downward course 
had alternately pressed with greater or less force on the plates, 
and had sometimes risen up so as nearly to leave them for short 
intervals. The distance between the extreme right and left 
positions of the radicle A was 0*7 mm., ascertained in the same 
manner as in the case of Phaseolus. 

Epicotyl. At the point where the radicle had protruded from 
a bean laid on its side, a flattened solid lump projected '1 of an 
inch, in the same horizontal plane with the bean. This protuber- 
ance consisted of the convex summit of the arched epicotyl; 
and as it became developed the two legs of the arch curved 
themselves laterally upwards, owing to apogeotropism, at such 
a rate that the arch stood highly inclined after 14 h., and 
vertically in 48 h. A filament was fixed to the crown of 
the protuberance before any arch was visible, but the basal 
half grew so quickly that on the second morning the end of the 
filament was bowed greatly downwards. It was therefore re- 
moved and fixed lower down. The line traced during these two 
days extended in the same general direction, and was in parts 
nearly straight, and in others plainly zigzag, thus giving some 
evidence of circumnutation. 

As the arched epicotyl, in whatever position it may be placed, 
bends quickly upwards through apogeotropism, and as the two 
legs tend at a very early age to separate from one another, as 
soon as they are relieved from the pressure of the surrounding 
earth, it was difficult to ascertain positively whether the epicotyl, 
whilst remaining arched, circtramutated. 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 Tropjeolum and the hypocotyl of the Cabbage. The move- 
.ments of the tied arches were traced in the usual manner on 



32 CIRCUMNUTATIOX OF SEEDLINGS. CHAP I. 

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 wero occasionally formed. We may therefore con- 
clude that the epicotyl circumnutates whilst still arched and 
before it has grown tall enough to break through the surface 
of the ground. 

In order to observe the movements of the epicotyl at a some- 
what more advanced age, a filament was fixed near the base of 
one which was no longer arched, for its upper half now formed 
a right angle with the lower half. This bean had germinated 
on bare damp sand, and the epicotyl began to straighten itself 
much sooner than would have occurred if it had been properly 
planted. The course pursued during 50 h. (from 9 A.M. Dec. 
26th, to 11 A.M. 28th) is here shown (Fig. 22) ; and we sea 

Fig. 22. 




Vicia faba: circumnutation of young epicotyl, traced in darkness during 
50 hours on a horizontal glass. Movement of bead of filament mag- 
nified 20 times, here reduced to one-half of original scale. 

that the epicotyl circumnutated during the whole time. Its 
basal part grew so much during the 50 h. that the filament 
at the end of our observations was attached at the height of 
'4 inch above the upper surface of the bean, instead of close 
to it. If the bean had been properly planttd, this part of the 
epicotyl would still have been beneath the soil. 

Late in the evening of the 28th, some hours after the above 
observations were completed, the epicotyl had grown much 
Btraighter, for the upper part now formed a widely open angle 
with the lower part. A filament was fixed to the upright basal 
part, higher up than before, close beneath the lowest scale-like 
process or homologue of a leaf; and its movement was traced 



LATHYBU8. H3 

during 38 h. (Fig. 23). We here again have plain evidence of 
continued circumnutation. Had the bean been properly planted, 
the part of the epicotyl to which the filament was attached, the 




Yioia faba ; circumnutation of the same epicotyl as in Fig. 22, a little more 
>idvanced in age, traced under similar conditions as before, from 8.40 A.M. 
Dec. 28th, to 10.50 A.M. 30th. Movement of bead here magnified 
20 times. 

movement of which is here shown, would probably have just 
risen above the surface of the ground. 

Littkyrus nissolia (Leguminosse). This plant was selected for 
observation from being an abnormal form with grass-like leaves. 




Lathyrus nis*o!ia: circumnutation of stem of young seedling, tracei in 
darkness on a horizontal glass, from 6.45 A.M. Nov. 22nd, to 7 A.M. 
23rd. Movement of end of leaf magnified about 12 times, here re- 
duced to one-half of original scale. 

The cotyledons are hypogean, and the epicotyl breaks through 
the ground in an arched form. The movements of a stem, T2 
inch in height, consisting of three internodes, the lower one 
almost wholly subterranean, and the upper one bearing a short, 



34 



CIRCUMNUTATION OF SEEDLINGS. CHAP. I. 



narrow leaf, is shown during 24 h., in Fig. 24. No glass filament 
was employed, but a mark was placed beneath the apex of the 
leaf. The actual length of the longer of the two ellipses de- 
scribed by the stem was about "14 of an inch. On the previous 
day the chief lino of movement was nearly 8+ right angles to 
that shown in the present figure, and it was more simple. 

Cassia tora* (Leguminosse). A seedling was placed before a 

Fig. 25 




Uaaria tora : conjoint cimimnutation of cotyledons and hypocotyl, traced 
on vertical glass, from 7.10 A.M. Sept. 25th to 7.30 A.M. 26th. Figure 
here given reduced to one- half of original scale. 



* Seeds of this plant, which 
prew nc-iir the sen-side, were sent 
to us by Fritz Miiller from S 
liraziL Ti,e seedlings did not 



flourish or flower well with us; 
they were sent to Kew, and were 
pronounced not to be distinguish- 
able from C. tora. 



OHAP. I. LOTUS. 35 

north-east window ; it bent very little towards it, as the bypo- 
cotyl which was left free was rather old, and therefore not highly 
heliotropic. A filament had been fixed to the midrib of one of 
the cotyledons, and the movement of the whole seedling was 
traced during two days. The circumnutation of the hypocotyl 
is quite insignificant compared with that of the cotyledons. 
These rise up vertically at night and come into close contact ; so 
that they may be said to sleep. This seedling was so old that a 
very small true leaf had been developed, which at night was 
completely hidden by the closed cotyledons. On Sept. 24th, 
between 8 A.M. and 5 P.M., the cotyledons moved five times up 
and five times down ; they therefore described five irregular 
ellipses in the course of the 9 h. The great nocturnal rise com- 
menced about 4.30 P.M. 

On the following morning (Sept. 25th) the movement of 
the same cotyledon was again traced in the same manner 
during 24 h. ; and a copy of the tracing is here given (Fig. 25). 
The morning was cold, and the window had been accidentally 
left open for a short time, which must have chilled the plant ; 
and this probably prevented it from moving quite as freely as 
on the previous day ; for it rose only four and sank only four 
times during the day, one of the oscillations being very small. 
At 7.10 A.M., when the first dot was made, the cotyledons were 
not fully open or awake ; they continued to open till about 9 A.M., 
by which time they had sunk a little beneath the horizon : by 
9.30 A.M. they had risen, and then they oscillated up and down ; 
but the upward and downward lines never quite coincided. At 
about 4.30 P.M. the great nocturnal rise commenced. At 7 A.M. 
on the following morning (Sept. 26th) they occupied nearly 
the same level as on the previous morning, as shown in the 
diagram : they then began to open or sink in the usual manner. 
The diagram leads to the belief that the great periodical daily 
rise and fall does not differ essentially, excepting in amplitude, 
from the oscillations during the middle of the day. 

Lotus Jacobceus (Leguminosse). The cotyledons of this plant, 
after the few first days of their life, rise so as to stand almost, 
though rarely quite, vertically at night. They continue to act tn 
this manner for a long time even after the development of some 
of the true leaves. With seedlings, 3 inches in height, and bear- 
ing five or six leaves, they roso at night about 45. They con- 
tinued to act thus for about an additional fortnight. Subse- 
quently they remained horizontal at night, though still green, 



36 CIRCUMNUTATION OF SEEDLINGS. CUAP. 1. 

and at last dropped off. Their rising at night so as to stand 
almost vertically appears to depend largely on temperature; 
for when the seedlings were kept in a cool house, though they 
still continued to grow, the cotyledons did not become vertical 
at night. It is remarkable that the cotyledons do not generally 
rise at night to any conspicuous extent during the first four or 
five days after germination; but the period was extremely 
variable with seedlings kept under the same conditions; and 
many were observed. Glass filaments with minute triangles of 
paper were fixed to the cotyledons (1J mm. in breadth) of two 
seedlings, only 24 h. old, and the hypocotyl was secured to a 
stick ; their movements greatly magnified were traced, and they 
certainly cii'cumnutated the whole time on a small scale, but 
they did not exhibit any distinct nocturnal and diurnal move- 
ment. The hypocotyls, when left free, circumnutated over a 



Another and much older seedling, bearing a half-developed 
leaf, had its movements traced in a similar manner during the 
three first days and nights of June ; but seedlings at this age 
appear to be very sensitive to a deficiency of light ; they were 
observed under a rather dim skylight, at a temperature of 
between 16 to 17 i C. ; and apparently, in consequence of these 
conditions, the great daily movement of the cotyledons ceased 
on the third day. During the first two days they began rising 
in the early afternoon in a nearly straight line, until between 
6 and 7 P.M., when they stood vertically. During the latter 
part of the night, or more probably in the early morning, they 
began to fall or open, so that by 6.45 A.M. they stood fully 
expanded and horizontal. They continued to fall slowly for 
some time, and during the second day described a single 
small ellipse, between 9 A.M. and 2 P.M., in addition to the 
great diurnal movement. The course pursued during the 
whole 24 h. was far less complex than in the foregoing case of 
Cassia. On the third morning they fell very much, and then 
circumnutated on a small scale round the same spot ; by 8.20 
r.M. they showed no tendency to rise at night. Nor did the 
cotyledons of any of the many other seedlings in the same pot 
rise ; and so it was on the following night of June 5th. The 
pot was then taken back into the hot-house, where it was exposed 
to the sun, and on the succeeding night all the cotyledons rose 
again to a high angle, but did not stand quite vertically. On 
each of the above days the line representing the great nocturnal 



CHAT>. L . CYTISUS. 37 

rise did not coincide with that of the great diurnal fall, so that 
narrow ellipses were described, as is the usual rule with circum- 
nutating organs. The cotyledons are provided with a pulvinus, 
and its development will hereafter be described. 

Mimosa pudica (Leguminosa3). The cotyledons rise up verti- 
cally at night, so as to close together. Two 'seedlings were 
observed in the greenhouse (temp. 16 to 17 C. or 63 to 65 FA 
Their hypocotyls were secured to sticks, and glass filaments 
bearing little triangles of paper were affixed to the cotyledons of 
both. Their movements were traced on a vertical glass during 
24 h. on November 13th. The pot had stood for some time ia 
the same position, and they were chiefly illuminated through 
the glass-roof. The cotyledons of one of these seedlings moved 
downward in the morning till 11.30 A.M., and then rose, moving 
rapidly in the evening until they stood vertically, so that in this 
case there was simply a single great daily fall and rise. The 
other seedling behaved rather differently, for it fell in the morn- 
ing until 11.30 A.M., and then rose, but alter 12.10 P.M. again fell; 
and the great evening rise did not begin until 1.22 P.M. On tbe 
following morning this cotyledon had fallen greatly from its 
vertical position by 8.15 A.M. Two other seedlings (one seven 
and the other eight days old) had been previously observed 
under unfavourable circumstances, for they had been brought 
into a room and placed before a north-east window, where the 
temperature was between only 56 and 57 F. They had. more- 
over, to be protected from lateral light, and perhaps were not 
sufficiently illuminated. Under these circumstances the coty- 
ledons moved simply downwards from 7 A.M. till 2 P.M., after 
which hour and during a large part of the night they con- 
tinued to rise. Between 7 and 8 A.M. on the following morning 
they fell again ; but on this second and likewise on the third 
day the movements became irregular, and between 3 and 10.30 
P.M. they circumnutated to a small extent about the same spot; 
but they did not rise at night. Nevertheless, on the following 
night they rose as usual. 

Cytisus frag ran s (Leguminosse). Only a few observations were 
made on this plant. The hypocotyl circumnutated to a con- 
siderable extent, but in a simple manner namely, for two hours 
in one direction, and then much more slowly back again in 
a zigzag course, almost parallel to the first line, and beyond the 
starting-point. It moved in the same direction all night, but 
next morning began to return. The cotyledons continually 



38 CIRCUMNUTATION OF SEEDLINGS. CHAP. L 

move both np and down and laterally ; but they do not rise up 
at night in a conspicuous manner. 

Lupinus luteus (Leguminosaj). Seedlings of this plant were 
observed because the cotyledons are so thick (about '08 of an 
inch) that it seemed unlikely that they would move. Our 
observations were not very successful, as the seedlings are 
strongly heliotropic, and their circumnutation could not be 
accurately observed near a north-east window, although they 
had been kept during the previous day in the same position. 
A seedling was then placed in darkness with the hypocotyl 
secured to a stick; both cotyledons rose a little at first, and 
then fell during the rest of the day ; in the evening between 
5 and 6 P.M. they moved very slowly ; during the night one 
continued to fall and the other rose, though only a little. The 
tracing was not much magnified, and as the lines were plainly 
zigzag, the cotyledons must have moved a little laterally, that 
is, they must have circumnutated. 

The hypocotyl is rather thick, about '12' of inch; nevertheless 
it circumnutated in a complex course, though to a small extent. 
The movement of an old seedling with two true leaves partially 
developed, was observed in the dark. As the movement was 
magnified about 100 times it is not trustworthy and is not 
given ; but there could be no doubt that the hypocotyl moved 
in all directions during the day, changing its course 19 times. 
The extreme actual distance from side to side through which 
the upper part of the hypocotyl passed in the course of 14 hours 
was only ^ of an inch ; it sometimes travelled at the rate of 
fa of an inch in an hour. 

CucurUta ovi/era (Cucurbitacese). Radicle : a seed which had 

Fig. 26. 




Cucurbita ovlfera : course followed by a radicle in bending geotropically 
downwards, traced on a horizontal glass, between 11.25 A.M. and 10.25 
P.M. ; the direction during the night is indicated by the broken line. 
Movement of bead magnified 14 times. 

germinated on damp sand was fixed so that the slightly curved 
radicle, which was only -07 inch in length, stood almost vertically 



GEAR I. 



CUCURBITS 



upwards, in which position geotropism would act at first with 
little power. A filament was attached near to its base, and 
projected at about an angle of 45 above the horizon. The 
general course followed during the 11 hours of observation and 
during the following night, is shown in the accompanying 
diagram (Fig. 26), and was plainly due to geotropism ; but it 
was also clear that the radicle circumnutated. By the next 
morning the tip had curved so much downwards that the fila- 
ment, instead of projecting at 45 above the horizon, was nearly 
horizontal. Another germinating seed was turned upside down 
and covered with damp sand ; and a filament was fastened to 
the radicle so as to project at an angle of about 50 above the 
horizon ; this radicle was '35 of an inch in length and a little 
curved. The course pursued was mainly governed, as in the 
last case, by geotropism, but the line traced during 12 hours and 
magnified as before was more strongly zigzag, ayain showing 
circumnutation. 

Four radicles were allowed to grow downwards over plates 
of smoked glass, inclined at 70 to the horizon, under the 



Fig. 27. 



Fig. 28. 




A. B. 

Cucurbita orifera: tracks 
left by tips of radicles 
in growing downwards 
over smoked glass- 
plates, inclined at 70 
to the horizon. 




Cucurbita ovifi'ra : circumnuta- 
tion of arched hypocotyl at 
a very early age, traced in 
darkness on a horizontal glass, 
from 8 A.M. to 10.20 A.M. on 
the following day. The move- 
ment of the bead magnified 
20 times, here reduced to one- 
half of original scale. 



same conditions as in the cases of 2Esculus, Phaseolus, and 
Vicia. Facsimiles are here given (Fig. 27) of two of these 
tracks ; and a third short one was almost as plainly serpentine 
as that at A. It was also manifest by a greater or less amount 
nf soot having been swept off the glasses, that the tips had 
4 



CIRCUMNUTATION OF SEEDLINGS. CHAP. L 



pressed alternately with greater and less force on them. Thcro 
must, therefore, have been movement in at least two planes at 
right angles to one another. These radicles were so delicate that 
they rarely had the power to sweep the glasses quite clean. One 
of them had developed some lateral or secondary rootlets, which 
projected a few degrees beneath the horizon ; and it is an im- 
portant fact that three of them left distinctly serpentine tracks 
on the smoked surface, showing beyond doubt that they had 
circumnutated like the main or primary radicle. But the 
tracks were st> slight that they could not be traced and copied 
after the smoked surface had been varnished. 

Hypicotyl. A seed lying on damp sand was firmly fixed by 
two crossed wires and by its own growing radicle. The cotyle- 
dons were still enclosed within the seed-coats; and the short 
hypocotyl, between the summit of 
the radicle and the cotyledons, 
was as yet only slightly arched. A 
filament ('85 of inch in length) 

f' ^- was attached at an angle of 35 

I ^^ above the horizon to the side of 

J \^ the arch adjoining the cotyle- 

dons. This part would ultimately 
form the upper end of the hypo- 
cotyl, after it had grown straight 
and vertical. Had the seed been 
properly planted, the hypocotyl at 
this stage of growth would have 
been deeply buried beneath the 
surface. The course followed by 





ifera : circumnuta- 
tion of straight and verti- 

cal hypocotyl, with filament in Fig. 28. The chief lines of 

fastened transversely across movement from left to right in the 

its upper end, traced in dark- figure wero para ll e l to the plane 

from 830 A iT'lo" {fso^M* ^ * ne ^ wo un ited cotyledons and 

The movement ..f the terminal of the flattened seed; ard this 

bead originally magnified movement would aid in dragging 
about 18 times, here only 41 



jSIl are held down by a special struc- 

ture hereafter to be described. The movement at right angles 
to the above lines was due to the arched hypocotyl becoming 
more arched as it increased in height. The foregoing observa- 
tions apply to the leg of the arch next to the cotyledons, but 



CHAP. I. CUCURBIT A. 4] 

the other leg adjoining the radicle likewise circumnutated at an 
equally early age. 

The movement of the same hypocotyl after it had become 
straight and vertical, but with the cotyledons only partially 
expanded, is shown in Fig. 29. The course pursued during 12 h. 
apparently represents four and a half ellipses or ovals, with 
the longer axis of the first at nearly right angles to that of the 
others. The longer axes of all were oblique to a line joining 
the opposite cotyledons. The actual extreme distance from 
side to side over which the summit of the tall hypocotyl 
passed in the course of 12 h. was '28 of an inch. The original 
figure was traced on a large scale, and from the obliquity of 
the line of view the outer parts of the diagram are much 
exaggerated. 

Cotyledons. On two occasions the movements of the cotyle- 
dons were traced on a vertical glass, and as the ascending and 
descending lines did not quite coincide, very narrow ellipses 
were formed; they therefore circumnutated. Whilst young 
they rise vertically up at night, but their tips always remain 
reflexed ; on the following morning they sink down again. With 
a seedling kept in complete darkness they moved in the same 
manner, for they sank from 8.45 A.M. to 4.30P.M.; they then 
began to rise and remained close together until 10 P.M., when 
they were last observed. At 7 A.M. on the following morning 
they were as much expanded as at any hour on the previous 
day. The cotyledons of another young seedling, exposed to the 
light, were fully open for the first time on a certain day, but 
were found completely closed at 7 A.M. on the following morning. 
They soon began to expand again, and continued doing so till 
about 5 P.M. ; they then began to rise, and by 10.30 P.M. stood 
vertically and were almost closed. At 7 A.M. on the third morn- 
ing they were nearly vertical, and again expanded during the 
day; on the fourth morning they were not closed, yet they 
opened a little in the course of the day and rose a little on the 
following night. By this time a minute true leaf had become 
developed. Another seedling, still older, bearing a well-developed 
leaf, had a sharp rigid filament affixed to one of its cotyledons 
(85 mm. in length), which recorded its own movements on 
a revolving drum with smoked paper. The observations were 
made in the hot-house, where the plant had lived, so that there 
was no change in temperature or light. The record commenced 
at 11 A.M. on February 18th; and fioui this hour till 3 P.M. the 



2 CIRCUMNUTATION OF SEEDLINGS. CUAV. I. 

cotyledon fell; it then rose rapidly till 9 P.M., then very 
gradually till 3 A.M. February 19th, after which hour it sank 
gradually till 4.30 P.M.; but the downward movement was inter- 
rupted by one slight rise or oscillation about l.UO P.M. Aftoi 
4.30 P.M. (19th) the cotyledon rose till 1 A.M. (in the night of 
February 20th) and then sank very gradually till 9.30 A.M., 
when our observations ceased. The amount of movement was 
greater on the 18th than on the 19th or on the morning of 
the 20th. 

Cucurbita aurantia. An arched hypocotyl was found buried a 
little beneath the surface of the soil ; and in order to prevent it 
straightening itself quickly, when relieved from the surrounding 
pressure of the soil, the two legs of the arch were tied together. 
The seed was then lightly covered with loose clamp earth. A 
filament with a bead at the end was affixed to the basal leg, the 
movements of which were observed during two days in the 
usual manner. On the first day the arch moved in a zigzag line 
towards the side of the basal leg. On the next day, by which 
time the dependent cotyledons had been dragged above the sur- 
face of the soil, the tied arch changed its course greatly nine 
times in the course of 14J h. It swept a large, extremely irre- 
gular, circular figure, returning at night to nearly the same 
spot whence it had started early in the morning. The line was 
so strongly zigzag that it apparently represented five ellipses, with 
their longer axes pointing in various directions. With respect 
to the periodical movements of the cotyledons, those of several 
young seedlings formed together at 4 P.M. an angle of about 60, 
and at 10 P.M. their lower parts stood vertically and were in 
contact ; their tips, however, as is usual in the genus, were per- 
manently reflexed. These cotyledons, at 7 A.M. on the following 
morning, were again well expanded. 

Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbi- 
taceae). A seedling opened its cotyledons, the movements of 
which were alone observed, slightly on June 27th, and closed 
them at night: next day, at noon (28th), they included an 
angle of 53, and at 10 P.M. they were in close contact, so that 
each had risen 26 i. At noon, on the 29th, they included an 
angle of 118, and at 10 P.M. an angle of 54, so each had 
risen 32. On the following day they were still more open, and 
the nocturnal rise was greater, but the angles were not measured. 
Two other seedlings were observed, and behaved during threa 
days in a closely similar manner. The cotyledons, therefon. 



CHAP. I. 



CUCURLITA. 



43 



10-35 p.m 
' 



open more and more on each succeeding day, and rise each 
night about 30 ; consequently during the first two nights of 
their life they stand vertically and 
come into contact. 

In order to ascertain more ac- 
curately the nature of these move- 
ments, the hypocotyl of a seedling, 
with its cotyledons well expanded, 
was secured to a little stick, and a 
filament with triangles of paper 
was affixed to one of the cotyledons. 
The observations were made under 
a rather dim skylight, and the 
temperature during the whole' time 
was between 17 i to 18 C. (63 to 
65 F.), Had the temperature been 
higher and the light brighter, the 
movements would probably have 
been greater. On July IHh (see 
Fig. 30), the cotyledon fell from 
7.35 A.M. till 10 A.M. ; it then rose 
(rapidly after 4 P.M.) till it stood 
quite vertically at 8.40 P.M. During 
the early morning of the next day 
(12th) it fell, and continued to fall 
till 8 A.M., after which hour it rose, 
then fell, and again rose, so that by 
10.35 P.M. it stood much higher than 
it did in the morning, but was not 
vertical as on the preceding night. 
During the following early morn- 
ing and whole day (13th) it fell and 
circumnutated, but had not risen 
when observed late in the evening ; 
and this was probably due to the 
deficiency of heat or light, or of 
both. We thus see that the coty- 
ledons became more widely open at 
noon on each succeeding day ; and 
that they rose considerably each night, though not acquiring 
a vertical position, except during the first two nights. 

(Jucumis dudaim (Cucurbitaceze). Two seedlings had opened 




Lagcnaria vulgaris : cireumnn- 
tation of a cotyledon, l 
inch in length, apex only 4f 
inches from the vertical glass, 
on which its movements were 
traced from 7.35 A.M. July 
llth to 9.5 A.M. on the 14th. 
Figure here given reduced 
to one-third of original scale. 



34 CIRCUMNUTATION OF SEEDLINGS. CHAP. I. 

their cotyledons for the first time during the day, one to the 
extent of 90 and the other rather more; they remained in 
nearly the same position until 10.40 P.M. ; but by 7 A.M. on the 
following morning the one which had been previously open to 
the extent of 90 had its cotyledons vertical and completely 
shut; the other seedling had them nearly shut. Later in the 
morning they opened in the ordinary manner. It appears 
therefore that the cotyledons of this plant close and open at 
somewhat different periods from those of the foregoing species 
of the allied genera of Cucurbita and Lagenaiia. 

Opuntia basilaris (Cactete). A seedling was carefully ob- 
served, because considering its 
Fig. 31. appearance and the nature of the 

mature plant, it seemed very un- 
likely that either the hypocotyl or 
cotyledons would circumnutate to 
an appreciable extent. The coty- 
ledons were well developed, being 
"9 of an inch in length, "22 in 
breadth, and '15 in thickness. 
The almost cylindrical hypocotyl, 
now bearing a minute spinous bud 
on its summit, was only '45 of an 

/ inch in height, and '19 in dia- 

Ojmntia basilaris .- conjoint cir- meter - The tracing (Fig. 31) shows 

cumnutation of hypocotyl the combined movement of the 

and cotyledon ; filament hypocotyl and of one of the coty- 




during 66 h. on horizontal to 11 AM. on the 31 st. On the 29th 

glass. Movement of the ter- a nearly perfect ellipse was com- 

Sol^&'T * ] ' M ; On the 30th the hypocotyl 

third scale. Seedling kept in moved, from some unknown cause, 

hot-house, feebly illuminated in the same general direction in a 

zigzag line ; but between 4.30 and 

10 P.M. almost completed a second 

small ellipse. The cotyledons move only a little up and down : 
thus at 10.15 P.M. they stood only 10 higher than at noon. The 
chief seat of movement therefore, at least when the cotyledons 
are rather old as in the present case, lies in the hypocotyl. The 
ellipse described on the 29th had its longer axis directed at 
nearly right angles to a line joining the two cotyledons. The 
wtua! amount of movement of the bead at the end of the 



OHM-. L 



PRIMULA. 



46 




filament was, as far as could be ascertained, about "14 of an 
inch. 

Udianthus annuus (Composite). The upper part of the 
hypocotyl moved during the 

day-time in the course Fig- 32. 

shown in the annexed figure 
(Fig. 32). As the line runs 
in various directions, cross- 
ing itself several times, 
the movement may be con- 
sidered as one of circumnu- 
tation. The extreme actual 
distance travelled was at 
least '1 of an inch. The 
movements of the cotyle- 
dons of two seedlings were 
observed; one facing a north- 
east window, and the other 
so feebly illuminated from 
above as to be almost in 
darkness. They continued 
to sink till about noon, 

when they began to rise ; but between 5 and 7 or 8 P.M. 
they either sank a little, or moved laterally, and then again 
began to rise. At 7 A.M. on the following morning those on 
the plant before the north-east window had opened so little 
that they stood at an angle of 73 above the horizon, and were 
not observed any longer. Those on the seedling which had 
been kept in almost complete darkness, sank during the whole 
day, without rising about mid-day, but rose during the night. 
On the third and fourth days they continued sinking without 
any alternate ascending movement; and this, no doubt, was 
due to the absence of light. 

Primula sinmsis (Prinmlacere). A seedling was placed with 
the two cotyledons parallel to a north-east window on a day 
when the light was nearly uniform, and a filament was affixed 
to one of them. From observations subsequently made on 
another seedling with the stem secured to a stick, the greater 
part of the movement shown in the annexed figure (Fig. 33), 
must have been that of the hypocotyl, though the cotyledons 
certainly move up and down to a certain extent both during the 
day and night. The movements of the sa.i.c secdli ng were traced 



Ifelianfhus annuus : circumnutation of 
hypocotyl, with filament fixed across 
its summit, traced on a horizontal 
glass in darkness, from 8.45 A.M. to 
10.45 P.M., and for an hour on follow- 
ing morning. Movement of bead 
magnified 21 times, here reduced to 
one-half of original scale. 



tO CIBCUMNUTATION OF SEEDLINGS. CHAI>. 1 

on the following day with nearly the same result; and there 
can be no doubt about the cireumnutation of the hypocotyl. 




Primula Sinensis : conjoint eircumnutation of hypocotyl and cotyledon, 
traced on vertical glass, from 8.40 A.M. to 10.45 P.M. Movements of 
bead magnified about 26 times. 

Cyclamen Persicum (Primulaceao). This plant is generally sup- 
posed to produce only a single cotyledon, but Dr. H. Gressner * 
has shown that a second one is developed after a long interval 
of time. The hypocotyl is converted into a globular conn, even 
before the first cotyledon has broken through the ground with its 
blade closely enfolded and with its petiole in the form of an arch, 
like the arched hjpocotyl or epicotyl of any ordinary dicotyle- 
donous plant. A glass filament was affixed to a cotyledon, -55 
of an inch in height, the petiole of which had straightened itself 
and stood nearly vertical, but with the blade not as yet fully 
expanded. Its movements were traced during 24 2 h. on a 
horizontal glass, magnified 50 
times ; and in this interval it 



"V described two irregular small 

\ r' \ circles; it therefore circumnu- 

\ f ^tl \^ r~-/^h\ tates, though on an extremely 

" ' \T^^ small scale. 

*qf*. sarpcdon: circumnutation .^'^ "*!**" < AB ? 6 ' 

of hypocotyl, illuminated from piadero). - This plant, when 

above, traced on horizontal glass, mature, resembles a cactus. 

IT 28 4 fh A - M -i n p e . 2 230-r 4 o 8 - 4 c 5 The flattened hypocotyl is 

Movement of bead magnified 21 flesh y' enlarged in the upper 

times part, and bears two rudimen- 
tary cotyledons. It breaks 

through the ground in an arched form, with the rudimentary 

cotyledons closed or in contact. A filament was affixed almost 



* 'Bot. Zeitung,' 1874, p. 837. 



OHAF. L IPOMCEA. 47 

vertically to the hypocotyl of a seedling half an inch high ; and 
its movements were traced during 50 h. on a horizontal glass 
(Fig. 34). From some unknown cause it bowed itself to one 
side, anl as this was effected by a zigzag course, it probably 
circunmutated ; but with hardly any other seedling observed 
by us was this movement so obscurely shown. 

Jpomcea ccerulea vel Phurbitis nil (Convolvulacese). Seedlings 
of this plant were observed because it is a twiner, the upper 
internodes of which circumnutate conspicuously; but, like 
other twining plants, the first few internodes which rise above 
the ground are stiff enough to support themselves, and therefore 
do not circumnutate in any plainly recognisable manner.* In 
this particular instance the fifth internode (including the hypo- 
cotyl) was the first which plainly circumnutated and twined 
round a stick. We therefore wished to learn whether cireum- 
nutation could be observed in the hypocotyl if carefully observed 
in our usual manner. Two seedlings were kept in the dark 
with filaments fixed to the upper part of their hypocotyls ; but 
from circumstances not worth explaining their movements were 
traced for only a short time. One moved thrice forwards and 
twice backwards in nearly opposite directions, in the course of 
3h. 15m.; and the other twice forwards and twice backwards 
in 2h. 22m. The hypocotyl therefore circumnutated at a re- 
markably rapid rate. It may here be added that a filament was 
affixed transversely to the summit of the second internode above 
the cotyledons of a little plant 3J inches in height; and its 
movements were traced on a horizontal glass. It circumnutated, 
and the actual distance travelled from side to side was a quarter 
of an inch, which was too small an amount to be perceived with- 
out the aid of marks. 

The movements of the cotyledons are interesting from their 
complexity and rapidity, and in some other respects. The 
hypocotyl (2 inches high) of a vigorous seedling was secured to a 
stick, and a filament with triangles of paper was affixed to ouo 
of the cotyledons. The plant was kept all day in the hot-house, 
and at 4.20 P.M. (June 20th) was placed under a skylight in 
the house, and observed occasionally during the evening and 
night. It fell in a slightly zigzag line to a moderate extent 
from 4.20 P.M. till 10 15 P.M. When looked at shortly after mid- 
night (12.30 P.M.) it had risen a very little, and considerably by 



* 'Movements and Habits of Climbing Plants,' p. 33, 1875. 



ts 



CIBCUMNUTATION OF SEEDLINGS. CHAP. 1 



3.45 A M. When again looked 
Fig. 35. 




Ipomaea ccs/nlca : circumnutatton of 
cotyledon, traced on vertical glass, 
from 6.10 A.M. June 21st to 6.45 
A.M. 22nd. Cotyledon with petiole 
1'6 inch in length, apex of blade 
4--1 inch from the vertical glass; 
so movement not greatly mag- 
nified ; temp. 20 C. 



at, at 6.10 A.M. (21st), it had 
fallen largely. A new tracing 
was now begun (see Fig. 35), 
and soon afterwards, at 6.42 
A.M., the cotyledon had risen a 
little. During the forenoon it 
was' oleerved about every 
hour ; but between 12.30 and 
G P.M. every half-hour. If the 
observations had been made at 
these short intervals during the 
vrhole day, the figure would 
have been too intricate to have 
been copied. As it was, the 
cotyledon rnoved up and down 
in the course of 16 h. 20 rn. (i e. 
between 6.10 A.M. and 10.30 
P.M.) thirteen times. 

The cotyledons of this seed- 
ling sank downwards during 
both evenings and the early 
part of the night, but rose 
during the latter part. As this 
is an unusual movement, the 
cotyledons of twelve other seed- 
lings were observed ; they stood 
almost or quite horizontally at 
mid-day, and at 10 P.M. were 
all declined at various angles. 
The most usual angle was be- 
tween 30 and 35; but three 
stood at about 50 and one at 
even 70 beneath the horizon. 
The blades of all these cotyle- 
dons had attained almost their 
full size, viz. from 1 to la inches 
in length, measured along theix 
midribs. It is a remarkable 
fact that whilst young that 
is, when less than half tin inch 
in length, measured in the 
same manner they do not sink 



CHAP. T. CERINTHE. 49 

downwards in the evening. Therefore their weight, which is 
considerable when almost fully developed, probably came into 
play in originally determining the downward movement. The 
periodicity of this movement is much influenced by the degree 
of light to which the seedlings have been exposed during the 
day; for three kept in an obscure place began to sir.k about 
noon, instead of late in the evening ; and those of another seed- 
ling were almost paralysed by having been similarly kept during 
two whole days. The cotyledons of several other species (.f 
Ipomoea likewise sink downwards late in the evening. 

Cerinthe major (Boraginese). The circvimnutation of the 
hypocotyl of a young seedling with the cotyledons hardly 

Fig. 36. 




Cerinthe major: circumnut.ition of hypocotyl, with filament fixer! ncross its 
summit, illuminated from above, traced on horizontal glass, from 
9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead magnified 
30 times, here reduced to one-third of original scale. 

expanded, is shown in the annexed figure (Fig. 36), which 
apparently represents four or five irregular ellipses, described 
in the course of a little over 12 hours. Two older seedlings 
were similarly observed, excepting that one of then! was kept 
in the dark ; their hypocotyls also circumnutated, but in a more 
simple manner. The cotyledons on a seedling exposed to the 
light fell from the early morning Tintil a little after noon, and 
then continued to rise until 10.30 P.M. or later. The cotyledons 
of this same seedling acted i a the same general manner during 
the two following days. It had previously been tried in the 
dark, and after being thus kept for only 1 h. 40m. the cotyledons 
I'cgan at 4.30 P.M. to sink, instead of conticuing to rise till lata 
at night. 




W) CIBCUMNUTATION OF SEEDLINGS. Cn\p. I 

Nolana prostrata (Nolanese). The movements were not 
traced, but a pot with seedlings, which had been kept in the 
dark for an hour, was placed under the microscope, with the 
micrometer eye-piece so adjusted that each division equalled 
e-o-Q-th of an inch. The apex of one of the cotyledons crossed 
rather obliquely four divisions in 13 minutes ; it was also sink- 
ing, as shown by getting out of focus. The seedlings were 
again placed in darkness for another hour, and the apex now 
crossed two divisions in 6 m. 18 s. ; that is, at very nearly the 
same rate as before. After another interval of an hour in dark- 
ness, it crossed two divisions in 4 m. 15 s., there- 
Fig. 37. f ore a t a quicker rate. In the afternoon, after a 
/[ longer interval in the dark, the apex was motion- 
/ less, but after a time it recommenced moving, 
though slowly ; perhaps the room was too cold. 
Judging from previous cases, there can hardly 
be a doubt that this seedling was circumnuta- 
ting. 

,., Solatium lycopersicum (Solaneje) The move- 

V ments of the hypocotyls of two seedling to- 

8o'.anum lycoper- matoes were observed during seven hours, and 
sicum: ciruum- there could be no doubt that both circumnu- 

"ocot'T f Ji?h tated> They W6re illuminated from above > but 
filament fixed bv an accident a little light entered on one side, 
across its sum- and in the accompanying figure (Fig. 37) it 
mit, traced on may ^e seen that the hypocotyl moved to this 
fromTo* ^T' S ^ e (* ne u PP er one i n ^ ue n S ure )> making small 
5 P.M. Oct. 24th. loops and zigzagging in its course. The move- 
Illuminated ob- ments of the cotyledons were also traced both 
liquely fr m on vertical and horizontal glasses; their angles 
ment e ' of bead w i ta * be horizon were likewise measured at 
magnified about various hours. They fell from 8.30 A.M. (October 
35 times, here ijth) to about noon ; then moved laterally in a 
JhSorigS zi 8 za S line, and at about 4 P.M. began to rise; 
scale. they continued to do so until 10.30 P.M., by 

which hour they stood vertically and were asleep. 
At what hour of the night or early morning they began to fall 
was not ascertained. Owing to the lateral movement shortly 
after mid-day, the descending and ascending lines did not 
coincide, and irregular ellipses were described during each 24 h. 
The regular periodicity of these movements is destroyed, as we 
Bhall hereafter see, if the seedlings are kept in the dark. 



CHAP. L SOLANUM. 61 

Solanum palinacantltum. Several arched hypocotyls rising 
nearly '2 of an inch above the ground, but with the cotyledons 
still buried beneath the surface, were observed, and the tracings 
showed that they circumnutated. Moreover, in several cases 
little open circular spaces or cracks in the argillaceous sand 
which surrounded the arched hypocotyls were visible, and 
these appeared to have been made by the hypocotyls having 
bent first to one and then to another side whilst growing up- 
wards. In two instances the vertical arches were observed to 
move to a considerable distance backwards from the point where 
the cotyledons lay buried; this movement, which has been 
noticed in some other cases, and which seems to aid in extracting 
Hie cotyledons from the buried seed-coats, is duo to the com- 
mencement of the straightening of the hypocotyl. In order to 
prevent this latter movement, the two legs of an arch, the 

V Fig. 38. 




Solanum palinacantfium : circumnutation of an arched hypocotyl, just 
emerging from the ground, with the two legs tied together, traced in 
darkness on a horizontal glass, from 9.20 A.M. Dec. 17th to 8.30 A.M. 
19th. Movement of bead magnified 13 times; but the filament, which 
was affixed obliquely to the crown of the arch, was of unusual length. 

summit of which was on a level with the surface of the soil, 
were tied together ; the earth having been previously removed 
to a little depth all round. The movement of the arch during 
47 hours under these unnatural circumstances is exhibited 
in the annexed figure. 

The cotyledons of some seedlings in the hot-house were hori- 
zontal about noon on December 13th ; and at 10 P.M. had risen 
to an angle of 27 above the horizon ; at 7 A.M. on the following 



52 



CIRCUMNUTATION OF SEEDLINGS. CIJAK 1 



Fig. 39. 




morning, before it was light, they had risen to 59 above the 

horizon; in the afternoon of the same day they were found 

again horizontal. 

Beta vulf/aris (Chenopodese). The seedlings are excessively 

sensitive to light, so that although on the first day they 
were uncovered only during two or three 
minutes at each observation, they all moved 
steadily towards the side of the room 
whence the light proceeded, and the trac- 
ings consisted only of slightly zigzag lines 
directed towards the light. On the next 
day the plants were placed in a completely 
darkened room, and at each observation 
were illuminated as much as possible from 
vertically above by a small wax taper. The 
annexed figure (Fig. 39) shows the move- 
ment of the hypocotyl during 9 h. under 
these circumstances. A second seedling 
; tu'cjaris: circum- -^as similarly observed at the same time, 

cotyfwithfilamen ; &nd the **** h&d th0 Same ^^^ 

fixed obliquely a- character, due to the hypocotyl often mov- 

cross its summit, ing and returning in nearly parallel lines, 

traced in darkness The movement of a third hypocotyl differed 
on horizontal glass, ., 

from 8.'25 A.M. to g reatlv - 

5.30 P.M. Nov. 4th. We endeavoured to trace the movements 

Movement of bead of the cotyledons, and for this purpose 

her! ScritoTne' S me seedlin S s were ke P fc in the dark > but 
third of original they moved in an abnormal manner ; they 
scale. continued rising from 8.45 A.M. to 2 P.M., 

then moved laterally, and from 3 to 6 P.M. 
descended ; whereas cotyledons which have been exposed all 
the day to the light rise in the evening so as to stand verti- 
cally at night; but this statement applies only to young 
seedlings. For instance, six seedlings in the greenhouse had 
their cotyledons partially open for the first time on the morning 
of November 15th, and at 8.45 P.M. all were completely closed, 
BO that they might properly be said to be asleep. Again, on the 
morning of November 27th, the cotyledons of four other seedlings, 
which were surrounded by a collar of brown paper so that they 
received light only from above, were open to the extent of 
39; at 10 P.M. they were completely closed; next morning 
(November 28th) at 6.45 A.M., whilst it was still dark, two of them 



CHAP. I. RICINUS AND QUERCUS. 58 

were partially open and all opened in the course of the morning; 
but at 10.20 P.M. all four (not to mention nine others which 
had been open in the morning and six others on another occa- 
sion) were again completely closed. On the morning of the 
29th they were open, but at night only one of the four was 
closed, and this only partially ; the three others had their 
cotyledons much more raised than during the day. On the 
night of the 30th the cotyledons of the four were only slightly 
raised. 

Ricinus Borboniensis (Euphorbiaceae). Seeds were purchased 
under the above name probably a variety of the common castor- 
oil plant. As soou as an arched hypocotyl had risen clear above 
the ground, a filament was attached to the upper leg bearing the 
cotyledons which were still buried beneath Ihe surface, and the 
movement of the bead was traced on a horizontal glass during 
a period of 34 h. The lines traced were strongly zigzag, and 
as the bead twice returned nearly parallel to its former course 
in two different directions, there could be no doubt that the 
arched hypocotyl circunmutated. At the close of the 34 h. 
the upper part began to rise and straighten itself, dragging the 
cotyledons out of the ground, so that the movements of the 
bead could no longer be traced on the glass. 

Quercus (American sp.) (Cupuli ferae). Acorns of an American 
oak which had germinated at Kew were planted in a pot in 
the greenhouse. This transplantation checked theiv growth; 
but after a time one grew to a height of five inches, 
measured to the tips of the small partially unfolded leaves on 
the summit, and now looked vigorous. It consisted of six 
very thin internodes of unequal lengths. Considering these 
circumstances and the nature of the plant, we hardly expected 
that it would ciroumnutate ; but the annexed figure (Fig. 40) 
shows that it did so in a conspicuous manner, changing its 
course many times and travelling in all directions during the 
48 h. of observation. The figure seems to represent 5 or 6 
irregular ovals or ellipses. The actual amount of movement 
from side to side (excluding one great bend to the left) was 
about '2 of an inch ; but this was difficult to estimate, as owing 
to ihe rapid growth of the stem, the attached filament was 
much further from the mark beneath at the close than at the 
commencement of the observations. It deserves notice that the 
pot was placed in a north-east room within a deep box, the top 
oi which was not at first covered up, so that the inside facing 



54 CIllGUMNUTATION OF SEEDLINGS. CHAP. I 

tho windows was a little more illuminated than the opposite 
Bide; and during the first morning the stem travelled to a 
greater distance in this direction (to the left in the figure) than 
it did afterwards when the box was completely protected from 
light 

Fig. 40. 




Querctts (American sp.) : circumnutation of young stem, traced on hori- 
zontal glass, from 12.50 P.M. Feb. 22nd to 12.50P.M. 24th. Movement 
of bead greatly magnified at first, but slightly towards the close of the 
observations about 10 times on an average. 

Quercus robur. Observations were made only on the move- 
ments of the radicles from germinating acorns, which were allowed 
to grow downwards in the manner previously described, over 
plates of smoked glass, inclined at angles between 65 and 69 
to the horizon. In four cases the tracks left were almost straight, 
but the tips had pressed sometimes with more and sometimes 
with less force on the glass, as shown by the varying thickness 
of the tracks and by little bridges of soot left across them. 
In the fifth case the track was slightly serpentine, that is, the 
tip had moved a little from side to side. In the sixth case 
(Fig. 41, A) it was plainly serpentine, and the tip had pressed 
almost equably on the glass in its whole course. In the seventh 
case (B) the tip had moved both laterally and had pressed 



CHAP. L QUERCUS AND COEYLUS. 55 

alternately with unequal force on the glass ; so that it had 
moved a little in two planes at right angles to one another. In 
the eighth and last case (C) it had moved very little laterally, 
but had alternately left the glass and come into contact with it 
again. There can be no doubt that in the last four cases the 
radicle of the oak circumnutated whilst growing downwards. 

Fig. 41. 



Q'tercus rrf>ur: tracks left on inclined smoked glass-plates by tips of 
radicles in growing downwards. Plates A and C inclined at Gi> and 
plate B at 68 to the horizon. 

Corylus avellana (Corylaceae). The epicotyl breaks through 
the ground in an arched form ; but in the specimen which was 
first examined, the apex had become decayed, and the epicotyl 
grew to some distance through the soil, in a tortuous, almost 
1 orizpntal direction, like a root. In consequence of this injury 
it had emitted near the hypogean cotyledons two secondary 
shoots, and it was remarkable that both of these were arched, 
like the normal epicotyl in ordinary cases. The soil was removed 
from around one of these arched secondary shoots, and a glass 
filament was affixed to the basal leg. The whole was kept 
damp beneath a metal-box with a glass lid, and was thus illumi- 
nated only from above. Owing apparently to the lateral pressure 
of the earth being removed, the terminal and bowed-down part 
of the shoot began at once to move upwards, so that after 
'24 h. it formed a right angle with the lower part. This lower 
part, to which the filament was attached, also straightened 
itself, and moved a little backwards from the upper part. Con- 
sequently a long line was traced on the horizontal glass; and 
5 



CIRCUMNUTATION OF SEEDLINGS. CHAP. L 



this was in parts straight and in parts decidedly zigzag, 
indicating circumnutation. 

On the following day the other secondary shoot was observed ; 
it was a little more advanced in age, for the upper part, instead 
of depending vertically downwards, 
F 'g- 42 - stood at an angle of 45 above the 

horizon. The tip of the shoot pro- 
jected obliquely '4 of an inch above 
the ground, but by the close of our 
observations, which lasted 47 h., it 
had grown, chiefly towards its base_, 
to a height of -85 of an inch. Tho 
filament was fixed transversely to 
the basal and almost upright half 
of the shoot, close beneath the lowest 
scale-like appendage. The circum- 

\ nutating course pursued is shown 

avellana: circumnuta- in the accompanying figure (Fig. 




42). The actual distance traversed 
from side to side was about '04 of 
an inch. 

Finns pinaster (Conifers). A 
young hypocotyl, with the tips 
of the cotyledons still enclosed 
within the seed-coats, was at first 
only -35 of an inch in height; but the upper part grew so 
rapidly that at the end of our observations it was '6 in height, 



tion of a young shoot emitted 
from the epicotyl, the apex 
of which had been injured, 
traced en a horizontal gla^s, 
from 9 A.M. Feb. 2nd to 8 
A.M. 4th. Movement oJ 
bead magnified about 27 
times. 



Fig. 43. 




Pinrts pinaster : circumnutation of hypocotyl, with filament fixed across iti 
summit, trace 1 on horizontal glass, from 10 A.M. March 21st to 9 A.M. 
23r I. Seedling kept in darkness. Movement of bead magnified ab< u 4 . 
35 times. 



CHAT. I. PINUS AND CYCA8T 57 

and by this time the filament was attached some way down the 
little stem. From some unknown cause, the hypocotyl moved 
far towards the left, but there could be no doubt (Fig. 43) that 
it circumnutated. Another hypocotyl was similarly observed, 
and it likewise moved in a strongly zigzag line to the same side. 
This lateral movement was not caused by the attachment of 
the glass filaments, nor by the action of Light ; for no light was 
allowed to enter when each observation was made, except from 
vertically above. 

The hypocotyl of a seedling was secured to a little stick ; it 
bore nine in appearance distinct cotyledons, arranged in a circle. 
The movements of two nearly opposite ones were observed. The 
tip of one was painted white, with a mark placed below, and the 
figure described (Fig. 44, A) shows that it made an irregular 

Fig. 44. 





Pinus pinaster: circumnutation of two opposite cotyledons, traced on 
horizontal glass in darkness, from 8.45 A.M. to 8.35 P.M. Nov. 25th. 
Movement of tip in A magnified about 22 times, here reduced to one- 
half of original scale. 

circle in the course of about 8 h. During the night it 
travelled to a considerable distance in the direction indicated 
by the broken line. A glass filament was attached longitu- 
dinally to the other cotyledon, and this nearly completed 
(Fig. 44, B) an irregular circular figure in about 12 hours. 
During the night it also moved to a considerable distance, in 
the direction indicated by the broken line. The cotyledons 
therefore circumnutate independently of the movement of the 
hypocotyl. Although they moved much during the night, they 
did not approach each other so as to stand more vertically than 
during the day. 



58 CIRCUMNUTATION OF SEEDLINGS. CHAP t 

Cycas-pedinata (Cycadese). The large seeds of this plant in 
germinating first protrude a single leaf, which breaks through 
the ground with the petiole bowed into an arch and with the 
leaflets involuted. A leaf in this condition, which at the close 
of our observations was 2j inches in height, had its movements 
traced in a warm greenhouse by means of a glass filament 
bearing paper triangles attached across its tip. The tracing 
(Fig. 45) siiivws how large, complex, and rapid were the circum- 



Fig. 45. 




Cycas peotinata : circumnutation of young leaf whilst emerging from the 
ground, feebly illuminated from above, traced on vertical glass, from 
5 P.M. May 28th to 11 A.M. 31st. Movement magnified 7 times, here 
reduced to two-thirds of original scale. 

nutating movements. The extreme distance from side to side 
which it passed over amounted to between '6 and '7 of an 
inch. 

Cauiut, \7arscewiczii (Cannacese). A seedling with the plu- 
mule projecting one inch above the ground was observed, but 
not under fair conditions, as it was brought out of the hot- 
house and kept in a room not sufficiently warm. Nevertheless 
the tracing (Fig. 46) shows that it made two or three incom- 
plete irregular circles or ellipses in the course of 48 hours. The 
plumule is straight ; and this was the first instance observed 



CUAP. L 



A.LLIUM. 



59 



by us of the part that fust breaks through the ground not 
being arched. 

Fig 46. 




Camna Warscewkzii : circumnutation of plumule with filament affixed 
obliquely to outer sheath-like leaf, traced in darkness onhorizontal glass 
from 8.45 A.M. Nov. 9th to 8.10 A.M. llth. Movement of bead mag- 
nified 6 times. 

Alliiim cepa (Liliacese). The narrow green leaf, which pro- 
trudes from the seed of the common onion as a cotyledon,* 
breaks through the ground in the form of an arch, in the same 
manner as the hypoootyl or epicotyl of a dicotyledonous plant. 
Long after the arch has risen above the surface the apex 
remains within the seed-coats, evidently absorbing the still 
abundant contents. The summit or crown of the arch, when 
it first protrudes from the seed and is still buried beneath the 
ground, is simply rounded; but before it reaches the surface 
it is developed into a conical protuberance of a white colour 
(owing to the absence of chlorophyll), whilst the adjoining parts 
are green), with the epidermis apparently rather thicker and 
tougher than elsewhere. We may therefore conclude that this 
conical protuberance is a special adaptation for breaking through 
the ground,f and answers the same end as the knife-like white 
crest on the summit of the straight cotyledon of the Graminese. 



* This is the expression used 
by Sachs in his 'Text-book of 
Botany.' 

t Haberlandt has briefly de- 
scribed ('Dio Sehutneinrichtun- 
gen . . . Keimpflanze,' 1877, p. 77) 
this curious structure and the 



purpose which it subserves. He 
states that good figures of the 
cotyledon of the onion have been 
given by Tiltmann and by Sachs 
in hia ' Experimental Physiologie,' 
p. 93. 



50 



CIECUMNUTATICN OF SEEDLINGS. CHAP. L 



Fig. 47. 



After a lime the apex is drawn out of the empty seed-coats, 
and rises up, forming a right angle, or more commonly a still 
larger angle with the lower part, and occasionally the whole 
becomes nearly straight. The conical protuberance, which 
originally formed the crown of the arch, is now seated on one 
side, and appears like a joint or knee, which from acquiring 
chlorophyll becomes green, and increases in size. In rarely oi 
never becoming perfectly straight, these cotyledons differ remark- 
ably from the ultimate condition of the arched hypocotyls or 
epicotyls of dicotyledons. It is, also, a singular circumstance 
that the attenuated extremity of the upper bent portion 
invariably withers and dies. 

A filament, 1'7 inch in length, was affixed nearly upright 
beneath the knee to the basal and vertical portion of a 
cotyledon; and its movements were 
traced during 14 h. in the usual manner. 
The tracing here given (Fig. 47) indi- 
cates circumnutation. The movement of 
the upper part above the knee of the same 
cotyledon, which projected at about an 
angle of 45 above the horizon, was 
observed at the same time. A filament 
was not affixed to it, but a mark was 
placed beneath the apex, which was 
almost white from beginning to wither, 
and its movements were thus traced. The 
Alliwn cepa : circumnu- figure described resembled pretty closely 
tation of basal half that above given . an( J this shows that the 

traced in darknesson chief seat of movement is in the lower or 

horizontal glass, from basal part of the cotyledon. 

8.15 A.M. to 10 P.M. Asparagus officinalis (Asparagese). 

of* bead A mag e fied The tip of a strai S ht plumule or cotyledon 
about 17 times. (for we do not know which it should be 

called) was found at a depth of "1 inch 
beneath the surface, and the earth was then removed all round 
to the depth of -3 inch. A glass filament was affixed obliquely to 
it, and the movement of the bead, magnified 17 times, was traced 
in darkness. During the first 1 h. 15 m. the plumule moved to 
the right, and during the next two hours it returned in a roughly 
parallel but strongly zigzag course. From some unknown causa 
it had grown up through the soil in an inclined direction, and 
now through apogeotropism it moved during nearly 24 h. in 




CHAA 1 



ASPARAGUS. 



Gl 



Ihe same general direction, but in a slightly zigzag manner, 
until it became upright. On the following morning it changed 
its course completely. There can therefore hardly be a doubt 
that the plumule circumnutates, whilst buried beneath the 
ground, as much as the pressure of the surrounding earth will 
permit. The surface of the soil in the pot was now covered with 
a thin layer of very fine argillaceous sand, which was kept damp; 
and after the tapering seedlings had grown a few tenths of 
an inch in height, each was found surrounded by a little open 
space or circular crack ; and this could be accounted for only by 
their having circumnutated and thus pushed away the sand on 
all sides ; for there was no vestige of a, crack in any other part. 
In order to prove that there was circumnutation, the move- 
Fig. . 





Asparagus officinalis : circumnutation of plumules with tips whitened and 
marks placed beneath, traced on a norizontal glass. A, young plumule ; 
movement traced from 8.30A.M. Nov. 30th to 7.15 A.M. next morning ; 
magnified about 35 times. B, older plumule ; movement traced from 
10.15 A.M. to 8.10 P.M. Nov. 29th; magnified 9 times, but here reduced 
to one-half of original scale. 

ments of five seedlings, varying in height from '3 inch to 2 inches, 
were traced. They were placed within a box and illuminated 
from above ; but in all five cases the longer axes of the figures 
described were directed to nearly the same point ; so that more 
light seemed to have come through the glass roof of the green- 
house on one side than on any other. All five tracings re- 
sembled each other to a certain extent, and it will suffice to give 
two of them. In A (Fig. 48) the seedling was only '45 of an 



62 CmCUMNUTATION OF SEEDLINGS. CHAP. L 

inch in height, and consisted of a single internode bearing a 
bud on its summit. The apex described between 8.30 A..M. and 
10.20 P.M. (i.e. during nearly 14 hours) a figure which would 
probably have consisted of 3i ellipses, had not the stem been 
drawn to one side until 1 P.M., after which hour it moved back- 
wards. On the following morning it was not far distant from 
the point whence it had first started. The actual amount of 
movement of the apex from side to side was very small, viz. 
about y^h f an inch. The seedling of which the movements 
are shown in Fig. 48, B, was If inch in height, and consisted of 
three internodes besides the bud on the summit. The figure, 
which was described during 10 h., apparently represents two 
irregular and unequal ellipses or circles. The actual amount of 
movement of the apex, in the line not influenced by the light, was 
11 of an inch, and in that thus influenced '37 of an inch. With 
a seedling 2 inches in height it was obvious, even without the 
aid of any tracing, that the uppermost part of the stem bent 
successively to all points of the compass, like the stein of a 
twining plant. A little increase in the power of circumnutating 
and in the flexibility of the stem, would convert the common 
asparagus into a twining plant, as has occurred with one species 
in this genus, namely, A. scandens. 

Phalaris Canariensis (Gramineae). With the Graminese tho 
part which first rises above the ground has been called by some 
authors the pileole; and various views have been expressed on 
its homological nature. It is considered by some great authori- 
ties to be a cotyledon, which term we will use without venturing 
to express any opinion on the subject.* It consists in the 
present case of a slightly flattened reddish sheath, terminating 
upwards in a sharp white edge; it encloses a true green leaf, 
whijh protrudes from the sheath through a slit-like orifice, 
close beneath and at right angles to the sharp edge on the 
summit Tho sheath is not arched when it breaks through the 
ground. 

The movements of three rather old seedlings, about 1J inch 
in height, shortly before the protrusion of the leaves, were first 
traced. They were illuminated exclusively from above; for, as 
will hereafter be shown, they are excessively sensitive to tha 



We are indebted to the Rev. this subject, together with 
G. 11.,-nslow for an abstract of the ferencea. 
views which have been held oa 



CUAK I, 



PHALAEIS. 



Fig. 49 




notion of light ; and if any enters even temporarily on one side, 

they merely bend to this side in slightly zigzag lines. Of the three 

tracings one alone (Fig. 49) is here given. Had the observations 

been more frequent during the 12 h. 

two oval figures would have been 

described with their longer axes at 

right angles to one another. The 

actual amount of movement of the 

apex from side to side was about 

3 of an inch. The figures described 

by the other two seedlings resembled 

to a certain extent the one here 

given. 

A seedling which had just broken 
through the ground and projected 
only ^th of an inch above the 
surface, was next observed in the 
same manner as before. It was 
necessary to clear away the earth 
all round the seedling to a little 
depth in order to place a mark 

beneath the apex. The figure (Fig. 50) shows that the apex 
moved to one side, but changed its course ten times in the 
course of the ten hours of observa- 
tion ; so that there can be no doubt 
about its circumnutation. The 
cause of the general movement 
in one direction could hardly be 
attributed to the entrance of 
lateral light, as this was carefully 
guarded against ; and we suppose 
it was in some manner connected 
with the removal of the earth 
round the little seedling. 

Lastly, the soil in the same pot 
was searched with the aid of a 
lens, and the white knife-like apex 
of a seedling was found on an exact 
level with that of the surrounding 
surface. The soil was removed all round the apex to the depth 
of a quarter of an inch, the seed itself remaining covered. The 
pot, protected from lateral light, was placed under the micro- 



Pfw,laris Gmariensis: circumnn- 
tation of a cotyledon, with a 
mark placed below the apex, 
traced on a horizontal glass, 
from 8.35 A.M. Nov. 26th to 
8.45A.M. 27th. Movement of 
apex magnified 7 times, here 
reduced to one-half scale. 



Fig. 50. 




Phnlaris Canariensis : circumnu- 
tation of a very young coty- 
ledon, with a mark placed 
below the apex, traced on a 
horizontal glass, from 11.37 
A.M. to 9.30 P.M. Dec. 13th. 
Movement of apex greatly 
magnified, here reduced to 
one-fov.rth of original scale. 



54 



CIRCUMNUTATION OF SEEDLINGS. CHAP. L 



scope with a micrometer eye-piece, so arranged that each 
division equalled ^th of an inch. After an interval of 30 m. 
the apex was observed, and it was seen to cross a little obliquely 
two divisions of the micrometer in 9 m. 15 s. ; and after a few 
minutes it crossed the same space in 8 m. 50 s. The seedling 
was again observed after an intervalof three-quarters of an hour, 
and now the apex crossed rather obliquely two divisions in 10 m. 
We may therefore conclude that it was travelling at about the 
rate of g'gth of an inch in 45 minutes. We may also conclude 
from these and the previous observations, that the seedlings of 
Phalaris in breaking through the surface of the soil circum- 
nutate as much as the surrounding pressure will permit. This 
fact accounts (as in the case before given of the asparagus) for 
a circular, narrow, open space or crack being distinctly visible 
rflmd several seedlings which had risen through very fine 
argillaceous sand, kept uniformly damp. 

Zea mays (Gramineje). A glass filament was fixed obliquely 
to the summit of a cotyledon, 
rising '2 of an inch above the 
ground ; but by the third morn- 
ing it had grown to exactly 
thrice this height, so that the 
distance of the bead from the 
mark below was greatly in- 
creased, consequently the trac- 
ing (Fig. 51) was much more 
magnified on the first than on 
the second day. The upper 
part of the cotyledon changed 
> *" its course by at least as much 



Fig. 51. 




Zea mays : circumnutation of cotyle- 
don, traced on horizontal glass, from 
8.30 A.M. Feb. 4th to 8 A.M. 6th. 
Movement of bead magnified on an 
average about 25 times. 



as a rectangle six times on each 
of the two days. The plant 
was illuminated by an obscure 
light from vertically above. 
This was a necessary precau- 
tion, as on the previous day we had traced the movements of 
cotyledons placed in a deep box, the inner side of which was 
feebly illuminated on one side from a distant north-east window, 
and at each observation by a wax taper held for a minute or 
two on the same side ; and the result was that the cotyledon > 
travelled all daylong to this side, though making in their course 
Borne conspicuous flexures, from wnich fact alone we might have 



CRAP. I. PHALARIS 65 

concluded that they were circumnutating ; but wo thought it 
advisable to make the tracing above given. 

Radicles. Glass filaments were fixed to two short radicles, 
placed so as to stand almost upright, and whilst bending down- 
wards through geotropism their courses were strongly zigzag ; 
from this latter circumstance circumnutation might have been 
inferred, had not their tips become slightly withered after the 
first 24 h., though they were watered and the air kept very 
damp. Nine radicles were next arranged in the manner 
formerly described, so that in growing downwards they left 
tracks on smoked glass-plates, inclined at various angles between 
45 and 80 beneath the horizon. Almost every one of these 
tracks offered evidence in their greater or less breadth in dif- 
ferent parts, or in little bridges of soot being 
left, that the apex had come alternately into Fig. 52. 
more and less close contact with the glass. In 
the accompanying figure (Fig. 52) we have 
an accurate copy of one such track. In two 
instances alone (and in these the plates were 
highly inclined) there was some evidence of 
slight lateral movement. We presume therefore 
that the friction of the apex on the smoked 
surface, little as this could have been, sufficed 

to check the movement from side to side of these ,, 
, ,. ... . Zeamays; track 

delicate radicles. left on inclined 

Avena sativa (Graminese). A cotyledon, la smoked glass- 
inch in height, was placed in front -of a north- ^"^S *| P 
east window, and the movement of the apex growTn "down- 
was traced on a horizontal glass during two wards. 
days. It moved towards the light in a slightly 
zigzag line from 9 to 11.30 A.M. on October 15th ; it then moved 
a little backwards and zigzagged much until 5 P.M., after which 
hour, and during the night, it continued to move towards the 
window. On the following morning the same movement was 
continued in a nearly straight line until 12.40 P.M., when the sky 
remained until 2.35 extraordinarily dark from thunder-clouds. 
During this interval of 1 h. 55 m., whilst the light was obscure, 
it was interesting to observe how circumnutation overcame 
heliotropism, for the apex, instead of continuing to move towards 
the window in a slightly zigzag line, reversed its course four 
times, making two small narrow ellipses. A diagram of this case 
will be given in the chapter on Heliotropism. 



66 



C1KCUMNUTATION OF SEEDLINGS. CHAP. I 



A filament was next fixed to a cotyledon only i of an inch in 
height, which was illuminated exclusively from above, and aa 
it was kept in a warm greenhouse, it grew rapidly ; and now 
there could be no doubt about its circumnutation, for it described 
a figure of 8 as well as two small ellipses in 62 hours. 

Nephrodium molle (Filices). A seedling fern of this species 



Fig. 53. 




came up by chance in a flower- 
pot near its parent. The frond, 
as yet only slightly lobed, was 
only '16 of an inch in length and 
*2 in breadth, and was supported 
on a rachis as fine as a hair 
and '23 of an inch in height. A 
very thin glass filament, which 
projected for a length of -36 of 
an inch, was fixed to the end of 
the frond. The movement was 
ffephrodium molle: circumnuUtion SQ M w magnified that the 

faSs y r\Van?a^ figure (Fig. 53) cannot be fully 

from 9 A M. to 9. P.M. Oct. 30th. trusted ; but the frond was 

Movement of bead magnified 48 constantly moving in a complex 

manner, and the bead greatly 

changed its course eighteen times in the 12 hours of observation. 
Within half an hour it often returned in a line almost parallel 
to its former course. The greatest amount of movement occurred 
between 4 and 6 P.M. The circumnuta- 
tion of this plant is interesting, because 
the species in the genus Lygodium are 
well known to circumnutate conspicuously 
and to twine round any neighbouring 
object. 

Sclaginella Kraussii (?) : Selayinella Kraussii (?) (Lycopodiaceae). 
circumnutation of A very young plant, only *4 of an inch 
young plant, kept in j n height, had sprung up in a pot in the 
KSTS toW PA ^-bouse. An extremely fine glass fila- 
Oct. 31st. ' ment was fixed to the end of the frond- 

like stem, and the movement of the bead 
traced on a horizontal glass. It changed its course several 
times, as shown in Fig. 54, whilst observed during 13 h. 15 m.,. 
and returned at night to a point not far distant from that 
whence it had started in the morning. There can be no doubt 
that this little plant circumnutatedL 



Fig. 54. 




CHAP. II. OIECUMNUTATION OF SEEDLINGS. 67 



CHAPTER II. 

GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF 
SEEDLING PLANTS. 

Generality of the circumiratating movement Radicles, their cirouin- 
nutation of service Manner in which they penetrate the ground 
Manner in which hypocotyls and other organs break through the 
ground by being arched Singular manner of germination in Megar- 
rhiza, &c. Abortion of cotyledons Circumnutation of hypocotyla 
and epicotyls whilst still buried and arched Their power of 
straightening themselves Bursting of the seed-coats Inherited 
effect of the arching process in hypogean hypocotyls Circumnuti- 
tion of hypocotyls and epicotyls when erect Ciicumnutation of 
cotyledons Pulvini or joints of cotyledons, duration of their 
activity, rudimentary in Oxalis corniculata, their development 
Sensitiveness of cotyledons to light and consequent disturbance of 
their periodic movements Sensitiveness of cotyleddns to contact. 

THE circumnutating movements of the several parts 
or organs of a considerable number of seedling plants 
have been described in the last chapter. A list is here 
appended of the Families, Cohorts, Sub-classes, &c., 
to which they belong, arranged and numbered ac- 
cording to the classification adopted by Hooker.* 
Any one who will consider this list will see that tho 
young plants selected for observation, fairly represent 
the whole vegetable series excepting the lowest 
cryptogams, and the movements of some of the lattei 
when mature will hereafter be described. As all tho 
seedlings which were observed, including Conifers ; 
Cycads and Ferns, which belong to the most ancient 



* As given in the ' General System of Botany.' by Le Maout and 
recaisne, 1873. 



G8 CIBCUMNUTATION OF SEEDLINGS. flr.*T. II 

types amongst plants, were continually circumnu- 
tating, we may infer, that this kind of movement is 
common to every seedling species. 

SUB-KINGDOM I. Phsenogamous Plants. 

Class I. DICOTYLEDONS. 
Sub-class I. Angiosperms. 

Family. Cohort. 

14. Cruciferce. II. PARIETALES. 

26. Caryopliyllece. IV. CARYOPiiYLLALKii. 

36 Mcdvaceas. VI. MALVALES. 

41. QxalidecR. VII. GERANIALES. 

49. Tropceolece. DITTO 

52. A urant iaccce. DITTO 

70. Hippocastanece. X. SAPINDALES. 

75. Lcguminosce. XI. ROSALES. 

108. Cucurbit icece. XII. PASSIFLO RALES. 

109. CactecB. XIV. FICOIDALES. 
122. Composites. XVII. ASTRALES. 
135. PrimulacecB. XX. PRIMULALFS. 
145. Asclepiadeas. XXII. OEXTIANALES. 
151. Convohulaccee. XXIII. POLEMONIALES. 
154. Borraginece. DITTO 

156. Kolaneae. DITTO 

157. Solancai. XXIV. SOLANALES. 
181. Chenopoiiiece. XXVII. 

202. Euphorbiaccas. XXXII. 

211. Cupullferx. XXXVI. QUERNALI-S. 

212. Corylaceae. DITTO 

Sub-class II. Gymnosperms. 

223. ConifercB. 

224. Cycadece. 

Class II. MONOCOTYLEDONS. 
2. Cannaceas. II. AMOMALta. 

34, LUiaceas. XI. LILIALKS. 

41. Asparagece, DITTO 

55 Gramineae. XV. GLUMALEB. 



II. Cryptogamic Plants. 
1. FHkei. j. FILICALES. 

. Lycopodiaceat. DITTO 



CHAP. IL A.CTION OF THE RADICLE. 69 

Radicles. In all the germinating seeds observed 
by us, the first change is the protrusion of the 
radicle, which immediately bends downwards and 
endeavours to penetrate the ground. In order to 
effect this, it is almost necessary that the seed should 
be pressed down so as to offer some resistance, unless 
indeed the soil is extremely loose ; for otherwise the 
seed is lifted up, instead of the radicle penetrating 
the surface. But seeds often get covered by earth 
thrown up by burrowing quadrupeds or scratching 
birds, by the castings of earth-worms, by heaps of 
excrement, the decaying branches of trees, &c., and 
will thus be pressed down ; and they must often fall 
into cracks when the ground is dry, or into holes. 
Even with seeds lying on the bare surface, the first 
developed root-hairs, by becoming attached to stones 
or other objects on the surface, are able to hold down 
the upper part of the radicle, whilst the tip pene- 
trates the ground. Sachs has shown* how well and 
closely root-hairs adapt themselves by growth to the 
most irregular particles in the soil, and become firmly 
attached to them. This attachment seerns to be 
effected by the softening or liquefaction of the outer 
surface of. the wall of the hair and its subsequent 
consolidation, as will be on some future occasion 
more fully described. This intimate union plays an 
important part, according to Sachs, in the absorption 
of water and of the inorganic matter dissolved in it. 
The mechanical aid afforded by the root-hairs in pene- 
trating the ground is probably only a secondary 
service. 

The tip of the radicle, as soon as it protrudes from 
the seed-coats, begins to circurnnutate, and the whole 



1 Physiologie Vegetale,' 18G8, pp. 199, 205. 



70 ACTION OF THE RADICLE. CHAP. II 

growing part continues to do so, probably for as long 
as growth, continues. This movement of the radicle 
has been described in Brassica, .ZEsculus, Phaseolus, 
Vicia, Cucurbita, Quercus and Zea. The probability 
of its occurrence was inferred by Sachs,* from radicles 
placed vertically upwards being acted on by geotro- 
pism (which we likewise found to be the case), for if 
they had remained absolutely perpendicular, the attrac- 
tion of gravity could not have caused them to bend to 
any one side. Circumnutation was observed in the above 
specified cases, either by means of extremely fine fila- 
ments of glass affixed to the radicles in the manner 
previously described, or by their being allowed to 
grow downwards over inclined smoked glass-plates, on 
which they left their tracks. In the latter cases the 
serpentine course (see Figs. 19, 21, 27, 41) showed 
unequivocally that the apex had continually moved 
from side to side. This lateral movement was small 
in extent, being in the case of Phaseolus at most 
about 1 mm. from a medial line to both sides. But 
there was also movement in a vertical plane at right 
angles to the inclined glass-plates. This was shown 
by the tracks often being alternately a little broader 
and narrower, due to the radicles having alternately 
pressed with greater and less force on the plates. 
Occasionally little bridges of soot were left across the 
tracks, showing that the apex had at these spots been 
lifted up. This latter fact was especially apt to occur 



* 'Ueber das Wachsthum der had previously reraorked (' B. i 

Wurzeln : Arbeiten des bot. In- trage zur Pflanzecphysiologie, 

Btituts in Wurzburg, 1 Heft iii. 18G8, p. 81) on the fact of radicl a 

1873, p. 460. This memoir, be- placed vertically upwards being 

sides its intrinsic and great in- acttd on by geutropism, and ho 

rerest, deserves to be studied as a explained it ly the supposition 

model of cart-ful investigation, that their grt.wth was not equtd 
and we shall liave occasion to ou all sides. 
refer to it repeatedly. Dr. Frank 



CUAP. II. ACTION OF THE KADICLE. 71 

when the radicle instead of travelling straight down 
the glass made a semicircular bend ; but Fig. 52 
shows that this may occur when the track is rectilinear. 
The apex by thus rising, was in one instance able to 
surmount a bristle cemented across an inclined glass- 
plate ; but slips of wood only ^ of an inch in thickness 
always caused the radicles to bend rectangularly to 
one side, so that the apex did not rise to this small 
height in opposition to geotropisrn. 

In those cases in which radicles with attached fila- 
ments were placed so as to stand up almost vertically, 
they curved downwards through the action of geotro- 
pisrn, circumnutating at the same time, and their 
courses were consequently zigzag. Sometimes, how- 
ever, they made great circular sweeps, the lines being 
likewise zigzag. 

Radicles closely surrounded by earth, even when 
this is thoroughly soaked and softened, may perhaps 
be quite prevented from circumnutating. Yet we 
should remember that the circumnutating sheath-like 
cotyledons of Phalaris, the hypocotyls of Solanum, 
and the epicotyls of Asparagus formed round them- 
selves little circular cracks or furrows in a superficial 
layer of damp argillaceous sand. They were also 
able, as well as the hypocotyls of Brassica, to form 
straight furrows in damp sand, whilst circumnutating 
and bending towards a lateral light. In a future 
chapter it will be shown that the rocking or circum- 
nutating movement of the flower-heads of Trifolium 
suUerraneum aids them in burying themselves. It is 
therefore probable that the circumnutation of the tip 
of the radicle aids it slightly in penetrating the 
ground ; and it may be observed in several of the 
previously given diagrams, that the movement is 
more strongly pronounced in radicles when they first 



72 ACTION OF THE RADICLE. CIUF. II 

protrude from the seed than at a rather later period ; 
but whether this is an accidental or an adaptive 
coincidence we do not pretend to decide. Never- 
theless, when young radicles of Pliaseolus multiflorus 
were fixed vertically close over damp sand, in the 
expectation that as soon as they reached it they 
would form circular furrows, this did not occur, a 
fact which may be accounted for, as we believe, by 
the furrow being filled up as soon as formed by the 
rapid increase of thickness in the apex of the radicle. 
Whether or not a radicle, when surrounded by soft- 
ened earth, is aided in forming a passage for itself 
by circumnutating, this movement can hardly fail 
to be of high importance, by guiding the radicle 
along a line of least resistance, as will be seen in the 
next chapter when we treat of the sensibility of the 
tip to contact. If, however, a radicle in its down- 
ward growth breaks obliquely into any crevice, or a 
hole left by a decayed root, or one made by the 
larva of an insect, and more especially by worms, the 
circumnutating movement of the tip will materially 
aid it in following such open passage ; and we have 
observed that roots commonly run down the old 
burrows of worms.* 

When a radicle is'placed in a horizontal or inclined 
position, the terminal growing part, as is well known, 
bends down towards the centre of the earth; and 
Sachs f has shown that whilst thus bending, the growth 
of the lower surface is greatly retarded, whilst that 



* Sec, also, Prof. Hensen's state- rov,-s made by worms, 
menta (' Zeitschrift fur Wissen, f ' Arbeiten des bot. Inst. 

Zool.,'. B. xxviii. p. 354, 1877) to Wiirzburg,' vol. i. 1873, p. 401. 

the same effect. He goes so far See also p. 397 for the length of 

as to believe that roots are able the growing part, and p. 451 ou 

to penetrate the ground to a great the force of gcotropism. 
depth only by means of the bur- 



Giui'.IL ACTION OF THE RADICLE. 73 

of the upper surface continues at the normal rate, 
or may be even somewhat increased. He has further 
shown by attaching a thread, running over a pulley, 
to a horizontal radicle of large size, namely, that 
of the common bean, that it was able to pull up a 
weight of only one gramme, or 15'4: grains. We may 
therefore conclude that geotropism does not give a 
radicle force sufficient to penetrate the ground, but 
merely tells it (if such an expression may be used) 
which course to pursue. Before we knew of Sachs' 
more precise observations we covered a flat surface of 
damp sand with the thinnest tin-foil which we could 
procure (-02 to '03 mm., or -00012 to '00079 of an inch 
in thickness), and placed a radicle close above, in such 
a position that it grew almost perpendicularly down- 
wards. When the apex came into contact with the 
polished level surface it turned at right angles and 
glided over it without leaving any impression ; yet 
the tin-foil was so flexible, that a little stick of soft 
wood, pointed to the same degree as the end of the 
radicle and gently loaded with a weight of only a 
quarter of an ounce (120 grains) plainly indented the 
tin-foil. 

Eadicles are able to penetrate the ground by the 
force due to their longitudinal and transverse growth ; 
the seeds themselves being held down by the weight 
of the superincumbent soil. In the case of the bean 
the apex, protected by the root-cap, is sharp, and 
the growing part, from 8 to 10 mm. in length, is 
much more rigid, as Sachs has proved, than the part 
immediately above, which has ceased to increase in 
length. We endeavoured to ascertain the downward 
pressure of the growing part, by placing germinating 
beans between two small metal plates, the upper one 
of which was loaded with a known weight; and the 



74 ACTION OF THE RADICLE. CHAP, rt 

radicle was then allowed to grow into a narrow hole in 
wood, 2 or 3 tenths of an inch in depth, and closed at 
the bottom. The wood was so cut that the short space 
of radicle between the mouth of the hole and the 
bean could not bend laterally on three sides ; but it 
was impossible to protect the fourth side, close to 
the bean. Consequently, as long as the radicle con- 
tinued to increase in length and remained straight, 
the weighted bean would be lifted up after the tip 
had reached the bottom of the shallow hole. Beans 
thus arranged, surrounded by damp sand, lifted up a 
quarter of a pound in 24 h. after the tip of the 
radicle had entered the hole. With a greater weight 
the radicles themselves always became bent on the one 
unguarded side ; but this probably would not have 
occurred if they had been closely surrounded on all 
sides by compact earth. There was, however, a 
possible, but not probable, source of error in these 
trials, for it was not ascertained whether the beans 
themselves go on swelling for several days after they 
have germinated, and after having been treated in 
the manner in which ours had been ; 
namely, being first left for 24 h. in 
water, then allowed to germinate in 
very damp air, afterwards placed over 
Outline of piece of the hole and almost surrounded by 
?ne C - k h^ du u C a C t d urd ^P Band in a closed box. 
size) with a hole We succeeded better in ascertaining 

through which .-i / , ^ 111 

the radicle of a * ne i rce exerted transversely by these 

nes S ng of W ' rhick " rac ^ c l es - ^ W0 were so placed as to 

narrow end -08 penetrate small holes made in little 

VJe 1 at dqlth 6n d f sticks > one of which was cut into the 

hoie'-i inch. shape here exactly copied (Fig. 55). 

The short end of the stick beyond 

the hole was purposely split, but not the opposite 



CUAP. II. 



ACTION OF THE KADICLE. 



7fi 



Fig. 56. 



end. As the wood was highly elastic, the split or 
fissure closed immediately after being made. After 
six days the stick and bean were dug out of the damp 
sand, and the radicle was found to be much enlarged 
above and beneath the hole. The fissure, which was 
at first quite closed, was now open to a width of 
4 mm. ; as soon as the radicle was extracted, it imme- 
diately closed to a width of 2 mm. The stick was 
then suspended horizontally by 
a fine wire passing through the 
hole lately filled by the radicle, 
and a little saucer was sus- 
pended beneath to receive the 
weights ; and it required 8 Ibs. 
8 ozs. to open the fissure to the 
width of 4 mm. that is, the 
width before the root was ex- 
tracted. But the part of the 
radicle (only -1 of an inch in 
length) which was embedded in 
the hole, probably exerted a 
greater transverse strain even 
than 8 Ibs. 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 P incers > ke P* closed b 7 

, . - . , . , a spiral brass spring, with a 

A second stick was tried in the 
same manner with almost ex- 
actly the same result. 

We then followed a better 
plan. Holes were bored near 
the narrow end of two wooden clips or pincers (Fig. 56), 
kept closed by brass spiral springs. Two radicles in damp 
sand were allowed to grow througlj these holes. The 




hole (-14 inch in diameter 
and 6 inch in depth) bored 
through the narrow closed 
part, through which a radicle 
of a bean was allowed to 
grow. Temp. 50-60 F. 



76 ACTION OF THE RADICLE. CHAP. IT. 

pincers rested on glass-plates to lessen the friction from 
the sand. The holes were a little larger (viz. '14 inch) 
and considerably deeper (viz. *6 inch) than in the 
trials with the sticks; so that a greater length of a 
rather thicker radicle exerted a transverse strain. 
After 13 days they were taken up. The distance of 
two dots (see the figure) on the longer ends of the 
pincers was now carefully measured ; the radicles were 
then extracted from the holes, and the pincers of 
course closed. They were then suspended horizontally 
in the same manner as were the bits of sticks, and a 
weight of 1500 grams (or 3 Ibs. 4 ozs.) was necessary 
with one of the pincers to open them to the same 
extent as had been effected by the transverse growth 
of the radicle. As soon as this radicle had slightly 
opened the pincers, it had grown into a flattened form 
and had escaped a little beyond the hole; its diameter 
in one direction being 4*2 mm., and at right angles 
3-5 mm. If this escape and flattening could have 
been prevented, the radicle would probably have 
exerted a greater strain than the 3 Ibs. 4 ozs. With 
the other pincers the radicle escaped still further 
out of the hole ; and the weight required to open 
them to the same extent as had been effected by the 
radicle, was only 600 grams. 

With these facts before us, there seems little diffi- 
culty in understanding how a radicle penetrates the 
ground. The apex is pointed and is protected by 
the root-cap ; the terminal growing part is rigid, and 
increases in length with a force equal, as far as our 
observations can be trusted, to the pressure of at least 
a quarter of a pound, probably with a much greater 
force when prevented from^ bending to any side by the 
surrounding earth. Whilst thus increasing in length 
it increases in thickness, pushing away the damp 



CHAP II. HYPOCOTYLS AND EPICOTYLS. 77 

earth on all sides, with a force of above 8 pounds in 
one case, of 3 pounds in another case. It was impos- 
sible to decide whether the actual apex exerts, relatively 
to its diameter, the same transverse strain as the parts 
a little higher up ; but there seems no reason to doubt 
that this would be the case. The growing part there- 
fore does not act like a nail when hammered into a 
board, but more like a wedge of wood, which whilst 
slowly driven into a crevice continually expands at 
the same time by the absorption of water; and a 
wedge thus acting will split even a mass of rock. 

Manner in which Hypocotyls, Epicotyls, &c., rise up 
and break through the ground. After the radicle has 
penetrated the ground and fixed the seed, the hypo- 
cotyls of all the dicotyledonous seedlings observed by 
us, which lift their cotyledons above the surface, break 
through the ground in the form of an arch. When 
the cotyledons are hypogean, that is, remain buried in 
the soil, the hypocotyl is hardly developed, and the 
epicotyl or plumule rises in like manner as an arch 
through the ground. In all, or at least in most of such 
cases, the downwardly bent apex remains for a time 
enclosed within the seed-coats. With Conjlus avel- 
lena the cotyledons are hypogean, and the epicotyl 
is arched ; but in the particular case described in 
the last chapter its apex had been injured, and it 
grew laterally through the soil like a root; and in 
consequence of this it had emitted two secondary 
shoots, which likewise broke through the ground as 
arches. 

Cyclamen does not produce any distinct stem, and 
only a single cotyledon appears at first ; * its petiole 



* This is the conclusion arrived considered by other botanists as 

at by Dr. H. Gressner ('Bot. the first true leaf is really tlio 

Xeitung,' 1874, p. 837), who second cotyledon, which is gieatlj 

maintains that what has been delayed in its development. 



78 



HYPOCOTYLS, EPICOTYLS, ETC., CHAP. ft. 



breaks through the ground as an arch (Fig. 57). 
Abronia also has only a single fully 
developed cotyledon, but in thia 
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 
, g Wade e of i ts a P ex upturned and parallel to 
not yet the descending leg of the arched 




Persicum 



Cyclamen 
seedling, 
larged : ( 
cotyledon, 

expanded, with arched 
petiole beginning to 
straighten itself; A, 
hypocotyl developed 
into a corm ; r, second- 
ary radicles. 



Fig. 58. 



hypocotyl ; but it is dragged 
out of the ground by the con- 
tinued growth of the hypocotyl, 
with the apex pointing downward. 
With Cycas pectinata the cotyledons are hypogean, 
and a true leaf first breaks 
through the ground with 
its petiole forming an 
arch. 

In the genus Acanthus 
the cotyledons are likewise 
hypogean. In A. mollis, 
a single leaf first breaks 
through the ground with 
its petiole arched, and with 
the opposite leaf much less 
developed, short, straight, 
of a yellowish colour, and 
with the petiole at first not 
half as thick as that of the 
other. The undeveloped 
leaf is protected by stand- 
ing beneath its arched fel- 
low; and it is an instruc- 




A'lanihus mollis ; seedling, with the 
hypogean cotyledon on the near 
side removed and the radicles cut 
off: a, blade of first leaf begin- 
ning to expand, with petiole still 
partially arched ; 6, second and 
opposite leaf, as yet very imper- 
fectly developed ; c, hypogean 
cotyledon on the opposite side. 



CHAP. II. BREAKING THROUGH THE GROUND. 79 

tive fact that it is not arched, as it has not to force 
for itself a passage through the ground. In the accom- 
panying sketch (Fig. 58) the petiole of the first leaf 
has already partially straightened itself, and the blade 
is beginning to unfold. The small second leaf ulti- 
mately grows to an equal size with the first, but this 
process is effected at very different rates in different 
individuals : in one instance the second leaf did not 
appear fully above the ground until six weeks after the 
first leaf. As the leaves in the whole family of the 
Acanthacese stand either opposite one another or in 
whorls, and as these are of equal size, the great in- 
equality between the first two leaves is a singular fact. 
We can see how this inequality of development and 
the arching of the petiole could have been gradually 
acquired, if they were beneficial to the seedlings by 
favouring their emergence ; for with A. candelabrum, 
spinosus, and latifolius there was great variability in the 
inequality between the two first leaves and in the 
arching of their petioles. In one seedling of A. can- 
delabrum the first leaf was arched and nine times as 
long as the second, which latter consisted of a mere 
little, yellowish-white, straight, hairy style. In other 
seedlings the difference in length between the two 
leaves was as 3 to 2, or as 4 to 3, or as only '76 to 
62 inch. In these latter cases the first and taller leaf 
was not properly arched. Lastly, in another seedling 
there was not the least difference in size between the 
two first leaves, and both of them had their petioles 
straight ; their laminae were enfolded and pressed 
against each other, forming a lance or wedge, by 
which means they had broken through the ground. 
^Therefore in different individuals of this same species 
of Acanthus the first pair of leaves breaks through 
the ground by two widely different methods ; and if 



80 HYPOCOTYLS, EPICOTTLS, ETC., CHAP. H 

either had proved decidedly advantageous or disad- 
vantageous, one of them no doubt would soon have 
prevailed. 

Asa Gray has described * the peculiar manner of ger- 
mination of three widely different plants, in which the 
hypocotyl is hardly at all developed. These were there- 
fore observed by us in relation to our present subject. 

Delphinium nudicaule. The elongated petioles of 
the two cotyledons, are confluent (as are sometimes 
their blades at the base), and they break through the 
ground as an arch. They thus resemble in a most 
deceptive manner a hypocotyl. At first they are 
solid, but after a time become tubular ; and the basal 
part beneath the ground is enlarged into a hollow 
chamber, within which the young leaves are developed 
without any prominent plumule. Externally root- 
hairs are formed on the confluent petioles, either a 
little above, or on a level with, the plumule. The 
first leaf at an early period of its growth and whilst 
within the chamber is quite straight, but the petiole 
soon becomes arched ; and the swelling of this part 
(and probably of the blade) splits open one side of 
the chamber, and the leaf then emerges. The slit 
was found in one case to be 3-2 mm. in length, and 
it is seated on the line of confluence of the two 
petioles. The leaf when it first escapes from the 
chamber is buried beneath the ground, and now an 
upper part of the petiole near the blade becomes 
arched in the usual manner. The second leaf comes 
out of the slit either straight or somewhat arched, but 
afterwards the upper part of the petiole, certainly in 
some, and we believe in all cases, arches itself whilst 
forcing a passage through the soil. 



* Botanical Text- Bock,' 1879, p. 22. 



OHAP. II. BREAKING THROUGH THE GROUND. 81 

Megarrliiza Californica. The cotyledons of this 
Gourd never free themselves from the seed-coats and 
are hypogeau. 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 2^ 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 1\ inches, and the 
radicle by this time had also become well developed. 
"The plumule, still enclosed within the tube, was now 



American Journal of Science,' vol. xiv. 1877, p. 21 



82 



HYPOCOTYLS, EPICOTYLS, ETC., CHAP. TI 



Fig. 58, A. 



''6 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 
g. ....... ..". _J3|j^l___ Q 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 2^ inches of superin- 
cumbent soil, still retaining its 
arched form. After reaching the 
surface it straightened itself in the 
usual manner. In the accompany- 
ing figure (Fig. 58, A) we have a 
sketch of a seedling in this ad- 
vanced state of development; the 
surface of the ground being re- 
: presented by the line G ........ G. 

The germination of the seeds in 
reduced to one-half their native Californian home pro- 

wfthYn seed-coats ; 
the two confluent 

petioles ; h and r, hy- , . . ,, -. r ,-> , 

pocotyl and radicle; letter from Mr. Kattan, sent to us 
ty p ro f. Asa Gray. The petioles 

' J 

protrude from the seeds soon after 
the autumnal rains, and penetrate the ground, generally 
in a vertical direction, to a depth of from 4 to even 
6 inches. They were found in this state by Mr. 
Rattan during the Christmas vacation, with the plu- 




Megan-hiza Calif 



ceeds in a rather different manner, 



as we infer from an interesting 



tyl 
/./.plumule; G ...... G, 

surface of soil. 



CHAP. II. BREAKING THEOUGH 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. Yv^e may, how- 
ever, believe that the extraordinary manner of germi- 
nation of Megarrhiza has another and secondary 
advantage. The radicle begins in a few weeks to 
enlarge into a little tuber, which then abounds with 
starch and is only slightly bitter. It would therefore 
be very liable to be devoured by animals, were it not 
protected by being buried whilst young and tender, at a 
depth of some inches beneath the surface. Ultimately 
it grows to a huge size. 

Ipomcea leptopliylla. 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 iin- 
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 



84 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. 

were placed in a horizontal position in damp air in the 
dark, and in the course of 4 hours they again became 
curved vertically downwards, having passed through 
90 in this time. But their sensitiveness to geotropism 
lasts for only 2 or 3 days; and the terminal part 
alone, for a length of between '2 and '4 inch, is thus 
sensitive. Although the petioles of our specimens 
did not penetrate the ground to -a greater depth than 
about \ inch, yet they continued for some time to grow 
rapidly, and finally attained the great length of about 
3 inches. The upper part is apogeotropic, and there- 
fore grows vertically upwards, excepting a short 
portion close to the blades, which at an early period 
bends downwards and becomes arched, and thus 
breaks through the ground. Afterwards this portion 
straightens itself, and the cotyledons then free them- 
selves from the seed-coats. Thus we here have in 
different parts of the same organ widely different kinds 
of movement and of sensitiveness ; for the basal part 
is geotropic, the upper part apogeotropic, and a portion 
near the blades temporarily and spontaneously arches 
itself. The plumule is not developed for some little 
time ; and as it rises between the bases of the parallel 
and closely approximate petioles of the cotyledons, 
which in breaking through the ground have formed an 
almost open passage, it does not require to be arched and 
is consequently always straight. Whether the plumule 
remains buried and dormant for a time in its native 
country, and is thus protected from the cold of winter, 
we do not know. The radicle, like that of the Megar- 
rhiza, grows into a tuber-like mass, which ultimately 
attains a great size. So it is with Ipomasa pandurata, 
the germination of which, as Asa Gray informs us, 
resembles that of I. leptopliylla. 
The following case is interesting in conneetion with 



CHAP. II BREAKING THROUGH THE GROUND. 85 

the root-like nature of the petioles. The radicle of a 
seedling was cut off, as it was completely decayed, 
and the two now separated cotyledons were planted. 
They emitted roots from their bases, and continued 
green and healthy for two months. The Wades 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 
like roots under the guidance of geotropisrn. 

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 Lathrtea 
squamaria, which is destitute of true leaves, breaks 
through the ground as an arch ;t so does the flower- 

* ' Transact. St. Louis Acad. ground cannot fail to be great]} 

Science,' vol. iv. p. 190. facilitated by the extraordinary 

t The passage of the flower- quantity of water secreted afc thia 

etnm of the Lathrsea through the period of the year by the subter- 



HYPOCOTYLS. EPICOTYLS, ETC., CIIAP. II. 



stein of the parasitic and leafless Monotropa hypopitys. 
With Helleborus niger, the flower-stems, which rise up 
independently of the leaves, likewise break through 
the ground as arches. This is also the case with the 
greatly elongated flower-stems, as well as with the 
petioles of Epimedium pinnatum. So it is with the 
petioles of Ranunculus ficaria, when they have to break 
through the ground, but when they arise from the 
summit of the bulb above ground, they are from the 
first quite straight ; and this is a fact which deserves 
notice. The r.achis of the bracken fern (Pteris aqui- 
lina), and of some, probably many, other ferns, like- 
wise rises above ground under the form of an arch. 
No doubt other analogous instances could be found by 
careful search. In all ordinary cases of bulbs, rhizomes, 



ranean scale-like leaves : not that 
there is any reason to suppose 
that the secretion is a special 
adaptation for this purpose : it 
probably follows from the great 
quantity of sap absorbed in the 
early spring by the parasitic roots. 
Alter a long period without any 
rain, the earth had become light- 
coloured and very dry, but it was 
dark coloured and damp, even in 
parts quite wet, for a distance of 
al least six inches all round each 
flower-stem. The water is secreted 
by glands (described by Cohn, 
Bericht. Bot. Sect, der Schle- 
sischen Gesell.,' 1876, p. 113) 
which" line the longitudinal 
channels running through o.tch 
scale-like leaf. A large plant was 
dug up, washed so as to remove 
tlie earth, left for some time to 
drain, and then placed in the 
evening on a dry glass-plate, 
covered with a bell-glass, and by 
next morning it had secreted a 
Jarge pool of water. The pl:te 
was wiped dry, nnd in the course 
of the succeeding 7 or 8 hours 



another little pool was secreted, 
and after 16 additional hours 
several large drops. A smaller 
plant was washed and placed in a 
large jar, which was left inclined 
for an hour, by which time no 
more water drained off. The jar 
was then placed upright and 
closed : after 23 hours twodrachms 
of water were collected from the 
bottom, and a little more after 25 
additional hours. The flower- 
stems were now cut off, for they 
do not secrete, and the subter- 
ranean part of the plant was found 
to weigh 106-8 grams (1611 
grains), and the water secreted 
during the 48 . hours weighed 
11-9 grams (1>3 grains). that is, 
one-ninth of the whole weight of 
the plant, excluding the flower- 
stems. We should remember that 
plants in a state of nature would 
probably secrete in 48 hours much 
more than the above large amount, 
for their roots would continue all 
the time absorbing sap from the 
plant on which they were para* 
sitic. 



CIIAP. II. BREAKING THROUGH THE GROUND. 87 

root-stocks, &c., buried beneath the ground, the surface 
is broken by a cone formed by the young imbricated 
leaves, the combined growth of which gives them force 
sufficient for the purpose. 

With germinating monocotyledonous seeds, of 
which, however, we did not observe a large number, 
the plumules, for instance, those of Asparagus and 
Canna, are straight whilst breaking through the ground. 
With the Gramineee, the sheath-like cotyledons are 
likewise straight ; they, however, terminate in a sharp 
crest, which is white and somewhat indurated ; and this 
structure obviously facilitates their emergence from 
the soil : the first true leaves escape from the sheath 
through a slit beneath the chisel-like apex and at 
right angles to it. In the case of the onion (Allium 
cepa) we again meet with an arch ; the leaf-like coty- 
ledon being abruptly bowed, when it breaks through 
the ground, with the apex still enclosed within the 
seed-coats. The crown of the arch, as previously 
described, is developed into a white conical pro- 
tuberance, which we may safely believe to be a 
special adaptation for this office. 

The fact of so many organs of different kinds 
hypocotyls and epicotyls, the petioles of some coty- 
ledons and of some first leaves, the cotyledons of 
the onion, the rachis of some ferns, and some flower- 
sterns being all arched whilst they break through 
the ground, shows how just are Dr. Haberlandt's * 
remarks on the importance of the arch to seedling 
plants. He attributes its chief importance to the 
upper, young, and more tender parts of the hypocotyl 



* ' Die Schutzeinrichtungen in though our observations lead 

ACT Entwickelung der Keim- to differ on some points from 1 

pllanze,' 1877. Wo have learned author, 
much i'rom this interesting essay, 



88 HYP9COTYLS, EPICOTYLS, ETC., CHAP II. 

Dr epicotyl, being thus saved from abrasion and 
pressure whilst breaking through the ground. But 
we think that some importance may be attributed to 
the increased force gained by the hypocotyl, epicotyl, 
or other organ by being at first arched ; for both legs of 
the arch increase in length, and both have points of 
resistance as long as the tip remains enclosed within 
the seed-coats; and thus the crown of the arch is 
pushed up through the earth with twice as much force 
as that which a straight hypocotyl, &c., could exert. 
As soon, however, as the upper end has freed itself, 
all the work has to be done by the basal leg. In 
the case of the epicotyl of the common bean, the 
basal leg (the apex having freed itself from the seed- 
coats) grew upwards with a force sufficient to lift a 
thin plate of zinc, loaded with 12 ounces. Two more 
ounces were added, and the 14 ounces were lifted up 
to a very little height, and then the epicotyl yielded 
and bent to one side. 

With respect to the primary cause of the arching 
process, we long thought in the case of many seedlings 
that this might be attributed to the manner in which 
the hypocotyl or epicotyl was packed and curved 
within the seed-coats ; and that the arched shape thus 
acquired was merely retained until the parts in question 
reached the surface of the ground. But it is doubtful 
whether this is the whole of the truth in any case. 
For instance, with the common bean, the epicotyl or 
plumule is bowed into an arch whilst breaking through 
the seed-coats as shown in Fig. 59 (p. 92). The 
plumule first protrudes as a solid knob (e in A), which 
after twenty-four hours' growth is seen (e in B) to be 
the crown of an arch. Nevertheless, with several 
beans which germinated in damp air, and had other- 
wise been treated in an unnatural manner, little 



CHAP. II, BREAKING THROUGH THE GROUND. 89 

plumules were developed in the axils of the petioles 
of both cotyledons, and these were as perfectly arched 
as the normal plumule ; yet they had not been sub- 
jected to any confinement or pressure, for the seed- 
coats were completely ruptured, and they grew in the 
open air. This proves that the plumule has an innate 
or spontaneous tendency to arch itself. 

In some other cases the hypocotyl or epicotyl pro- 
trudes from the seed at first only slightly bowed ; but 
the bowing afterwards increases independently of any 
constraint. The arch is thus made narrow, with the 
two legs, which are sometimes much elongated, parallel 
and close together, and thus it becomes- well fitted 
for breaking through the ground. 

With many kinds of plants, the radicle, whilst still 
enclosed within the seed and likewise after its first pro- 
trusion, lies in a straight line with the future hypocotyl 
and with the longitudinal axis of the cotyledons. This 
is the case with Cucurbita ovifera; nevertheless, in 
whatever position the seeds were buried, the hypocotyl 
always came up arched in one particular direction. 
Seeds were planted in friable peat at a depth of about 
an inch in a vertical position, with the end from which 
the radicle protrudes downwards. Therefore all the 
parts occupied the same relative positions which 
they would ultimately hold after the seedlings had 
risen clear above the surface. Notwithstanding this 
fact, the hypocotyl arched itself; and as the arch 
grew upwards through the peat, the buried seeds were 
turned either upside down, or were laid horizontally, 
being afterwards dragged above the ground. Ulti- 
mately the hypocotyl straightened itself in the usual 
manner; and now after all these movements the 
several parts occupied the same position relatively to 
one another and to the centre of the earth, which they 



90 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. It 

had done when the seeds were first buried. But it may 
be argued in this and other such cases that, as the 
hypocotyl grows up through the soil, the seed will 
almost certainly be tilted to one side ; and then 
from the resistance which it must offer during its 
further elevation, the upper part of the hypocotyl will 
be doubled down and thus become arched. This view 
seems the more probable, because with Ranunculus 
ficaria only the petioles of the leaves which forced 
a passage through the earth were arched ; and not 
those which arose from the summits of the bulbs above 
the ground. Nevertheless, this explanation does not 
apply to the Cucurbita, for when germinating seeds 
were suspended in damp air in various positions by 
pins passing through the cotyledons, fixed to the 
inside of the lids of jars, in which case the hypo- 
cotyls were not subjected to any friction or constraint, 
yet the upper part became spontaneously arched. 
This fact, moreover, proves that it is not the weight 
of the cotyledons which causes the arching. Seeds 
of Helianthus annuus and of two species of Ipomoea 
(those of J. bona nox being for the genus large 
and heavy) were pinned in the same manner, 
and the hypocotyls became spontaneously arched ; 
the radicles, which had been vertically dependent, 
assumed in consequence a horizontal position. In 
the case of Ipomoea leptophylla it is the pdtioles of the 
cotyledons which become arched whilst rising through 
the ground ; and this occurred spontaneously when 
the seeds were fixed to the lids of jars. 

It may, however, be suggested with some degree of 
probability that the arching was aboriginally caused 
by mechanical compulsion, owing to the confinement 
of the parts in question within the seed-coats, or to 
friction whilst they were being dragged upwards. But 



CHAP. II. BREAKING THROUGH THE GROUND. 91 

if Iliis is so, we must admit from the cases just given, 
that a tendency in the upper part of the several 
specified organs to bend downwards and thus to be- 
come arched, has now become with many plants firmly 
inherited. The arching, to whatever cause it may be 
due, is the result of modified circumnutation, through 
increased growth along the convex side of the part ; 
such growth being only temporary, for the part always 
straightens itself subsequently by increased growth 
along the concave side, as will hereafter be described. 

It is a curious fact that the hypocotyls of some 
plants, which are but little developed and which 
never raise their cotyledons above the ground, never- 
theless inherit a slight tendency to arch themselves, 
although this movement is not of the least use to 
them. We refer to a movement observed by Sachs 
in the hypocotyls of the bean and some other Legumi- 
nosae, and which is shown in the accompanying figure 
(Fig. 59), copied from his Essay.* The hypocotyl 
and radicle at first grow perpendicularly downwards, 
as at A, and then bend, often in the course of 24 hours, 
into the position shown at B. As we shall here- 
after often have to recur to this movement, we will, for 
brevity sake, call it " Sachs' curvature." At first sight 
it might be thought that the altered position of the 
radicle in B was wholly due to the outgrowth of the 
epicotyl (e), the petiole (p) serving as a hinge ; and 
it is probable that this is partly the cause ; but the 
hypocotyl and upper part of the radicle themselves 
become slightly curved. 

The above movement in the bean was repeatedly 
seen by us ; but our observations were made chiefly on 
Phaseolus muttiflorus, the cotyledons of which are like- 



* ' Arbeiten dos bot. Instit. Wurzburg,' vol. i. 1873, p. 403. 



92 



HYPOCOTYLS, EPICOTYLS, ETC., CHAP. TC. 



wise hypogean. Some seedlings with well-developed 
radicles were first immersed in a solution of perman- 
ganate of potassium ; and, judging from the changes 
of colour (though these were not very clearly defined), 
the hypocotyl is about '3 inch in length. Straight, 
thin, black lines of this length were now drawn from 
the bases of the short petioles along the hypocotyls 




Vicin faba: germinating seeds, suspended in damp air: A, with radicle 
growing perpendicularly downwafrds ; B, the same beau after 24 hours 
and after the radicle has curved itself; r, radicle; A, short hypocotyl ; 
e, epicotyi appearing as a knoh in A and as an arch in B ; p, petiole of 
the cotyledon, the latter enclosed within the seed-coats. 

of 23 germinating seeds, which were pinned to the 
lids of jars, generally with the hilum downwards, and 
with their radicles pointing to the centre of the 
earth. After an interval of from 24 to 48 hours the 
black lines on the hypocotyls of 16 out of the 23 
seedlings became distinctly curved, but in very 
various degrees (namely, with radii between 20 and 



CHAP. II. BREAKING THROUGH THE GROUND. 03 

80 mm. on Sachs' cyclometer) in the same relative 
direction as shown at B in Fig. 59. As geotropism 
will obviously tend to check this curvature, seven 
seeds were allowed to germinate with proper pre- 
cautions for their growth in a klinostat,* by which 
means geotropism was eliminated. The position of the 
hypocotyls was observed during four successive days, 
and they continued to bend towards the hilum and 
lower surface of the seed. On the fourth day the/ 
were deflected by an average angle of 63 from a lino 
perpendicular to the lower surface, and were therefore 
considerably more curved than the hypocotyl and 
radicle in the bean at B (Fig. 59), though in the same 
relative direction. 

It will, we presume, be admitted that all leguminous 
plants with hypogean cotyledons are descended from 
forms which once raised their cotyledons above the 
ground in the ordinary manner ; and in doing so, it is 
certain that their hypocotyls would have been abruptly 
arched, as in the case of every other dicotyledonous 
plant. This is especially clear in the case of Phaseolus, 
for out of five species, the seedlings of which we 
observed, namely, P. multiflorus, caracalla, vulgaris, 
Hernandesii and Eoxburgliii (inhabitants of the Old 
and New Worlds), the three last-named species have 
well-developed hypocotyls which break through the 
ground as arches. Now, if we imagine a seedling of 
the common bean or of P. multiflorus, to behave as its 
progenitors once did, the hypocotyl (h, Fig. 59), in 
whatever position the seed may have been buried; 
would become so much arched that the upper part 
would be doubled down parallel to the lower part ; and 



* An instrument devised by on which the plant under obsrrvn- 
Sachs, consisting essentially of a tion is supported: see ' Wiirzbiirg 
slowly revolving horizontal axis. Arbeiteu,' 1870, p, 209. 



94 



RUDIMENTARY COTYLEDONS. 



CHAP. II 



this is exactly the kind of curvature which actually 
occurs in these two plants, though to a much less 
degree. Therefore we can hardly doubt that their 
short hypocotyls have retained by inheritance a ten- 
dency to curve themselves in the same manner as they 
did at a former period, when this movement was highly 
important to them for breaking through the ground, 
though now rendered useless by the cotyledons being 
hypogean. Rudimentary structures are in most cases 
highly variable, and we might expect that rudimentary 
or obsolete actions would be equally so ; and Sachs' 
curvature varies extremely in amount, and sometimes 
altogether fails. This is the sole instance known to 
us of the inheritance, though in a feeble degree, of 
movements which have become superfluous from 
changes which the species has undergone. 

Eudimentary Cotyledons. A few remarks on this 
subject may be here interpolated. It is well known 
y 60 that some dicotyle- 

donous plants produce 
only a single cotyle- 
don ; for instance, cer- 
tain species of Ranun- 
culus, Corydalis, Cliao- 
rophyllurn ; and we 
will here endeavour to 
show that the loss of 
one or both cotyle- 
dons is apparently due 

/ mtrantium: two young seedlings: to a S t re of liutri- 
c, larger cotyledon ; c', smaller cotyle- , . , . , 

don ; A, thickened hypocotyl ; r, radicle, ment being laid Up in 





the hypocotyl or one 
of the two cotyledons, or one of the secondary radicles. 



CHAP. II. 



RUDIMENTARY COTYLEDONS 



95 



Fig. 61. 



With the orange (Citrus aurantium) the cotyledons are 
liypogean, and one is larger than the other, as may 
be seen in A (Fig. 60). In B the inequality is rather 
greater, and the stem has grown between the points 
of insertion of the two petioles, so that they do not 
stand opposite to one another; in another case the 
separation amounted to one-fifth of an inch. The 
smaller cotyledon of one seedling 
was extremely thin, and not half 
the length of the larger one, so that 
it was clearly becoming rudimen- 
tary.* In all these seedlings the 
liypocotyl was enlarged or swollen. 
With Abronia unibellata one of 
the cotyledons is quite rudimen- 
tary, as may be seen (c) in Fig. 61. 
In this specimen it consisted of a 
little green "flap, ^th inch in 
length, destitute of a petiole and 
covered with glands like those on 
the fully developed cotyledon (c). 

At first it Stood opposite to the Abronia umbcllata : seed- 
larger cotyledon ; but as the petiole lin s tw . ic ? natui ; al si f : 

f. , , . 1-1 c i cotyledon ; c , rudi- 

ot the latter increased in length mentary cotyledon ; k, 
and grew in the same line with ^AJy^ 
the hypocotyl (&), the rudiment tion (/') at the lower 
appeared in older seedlings as if end ; r ' radicle ' 
seated some way down the hypocotyl. With Abronia 
arenaria there is a similar rudiment, which in one 




* In Fachira aquatica, as de- 
ecribed by Mr. R. I. Lynch 
('Journal Linn. Snc. Bot.' vol. 
xvii. 1878, p. 147), one of the 
liypogean cotyledons is of im- 
mense size ; the other is small 
and soon falls off; the pair do not 
always stand opposite. In another 



and very different water-plant, 
Trapa natans, one of the cotyle- 
dons, filled with farinaceous 
matter, is much larger than the 
other, which is scarcely visible, 
as is stated by Aug. de Cundolle, 
' Physiologic Ve'g.' torn. ii. p. 83\ 
1832 



06 EUDIMENTAEY COTYLEDONS. CHAP II 



specimen was only -^oth and in another -^th inch in 
length; it ultimately appeared as if seated halfway 
down the hypocotyl. In both these species the hypo- 
cotyl is so much enlarged, especially at a very early 
age, that it might almost be called a corm. The lower 
end forms a heel or projection, the use of which will 
hereafter be described. 

In Cyclamen Persicum the hypocotyl, even whilst still 
within the seed, is enlarged into a regular corm,* and 
only a single cotyledon is at first developed (see former 
Fig. 57.) With Ranunculus ficaria two cotyledons are 
never produced, and here one of the secondary radicles 
is developed at an early age into a so-called bulb.f 
Again, certain species of Chserophyllum and Corydalis 
produce only a single cotyledon;! in the former the 
hypocotyl, and in the latter the radicle is enlarged, 
according to Irmisch, into a bulb. 

In the several foregoing cases one of the cotyledons 
is delayed in its development, or reduced in size, or 
rendered rudimentary, or quite aborted ; but in other 
cases both cotyledons are represented by mere rudi- 
ments. With Opuntia basilaris this is not the case, 
for both cotyledons are thick and large, and the 
hypocotyl shows at first ho signs of enlargement ; but 
afterwards, when the cotyledons have withered and dis- 
articulated themselves, it becomes thickened, and from 
its tapering form, together with its smooth, tough, 
brown skin, appears, when ultimately drawn down to 
some depth into the soil, like a root. On the other 



* Dr. H. Gressner, 'Dot. Zei- Vaucl er's account ('Hist. Phys. 

tung,' 1874, p. 824. desPlantosd'Europe,'tom i. 1841, 

t Irmisch, 'Beitrage zur TMor- p. 149) of the germinntion of the 

phologie der Pflanzen,' 1854, pp. seeds of several species of Cory- 

II, 12; 'Bot. Zeitung,' 1874, p. dalts, that (he fculb or tnbercule 

805- be<rins to be formed at au ea> 

t Delpino, 'Bivista Botanica,' tremely early age. 
1877, p. 21. It is evident from 



CHAP. II. RUDIMENTARY COTYLEDONS. 97 

hand, with several other Cacteae, the hypocotyl is from 
the first much enlarged, and both cotyledons are 
almost or quite rudimentary. Thus with Cereus Land 
leckii two little triangular projections, representing the 
cotyledons, are narrower than the hypocotyl, which is 
pear-shaped, with the point downwards. In RhipsaUs 
cassytha the cotyledons are represented by mere points 
on the enlarged hypocotyl. In Ecliinocactus viridescens 
the hypocotyl is globular, with two little prominences 
on its summit. In Pilocereus Iloulletii the hypocotyl, 
much swollen in the upper part, is merely notched on 
the summit ; and each side of the notch evidently repre- 
sents a cotyledon. Stapelia sarpedon, a member of the 
very distinct family of the Asclepiadeoe, is fleshy like 
a cactus ; and here again the upper part of the flattened 
hypocotyl is much thickened and bears two minute coty- 
ledons, which, measured internally, were only *15 inch 
in length, and in breadth not equal to one-fourth of the 
diameter of the hypocotyl in its narrow axis ; yet these 
minute cotyledons are probably not quite useless, for 
when the hypocotyl breaks through the ground in the 
form of an arch, they are closed or pressed against one 
another, and thus protect the plumule. They after- 
wards open. 

From the several cases now given, which refer to 
widely distinct plants, we may infer that there is some 
close connection between the reduced size of one or 
both cotyledons and the formation, by the enlargement 
of the hypocotyl or of the radicle, of a so-called bulb. 
But it may be asked, did the cotyledons first tend to 
abort, or did a bulb first begin to be formed? As 
all dicotyledons naturally produce two well-developed 
cotyledons, whilst the thickness of the hypocotyl and 
of the radicle differs much in different plants, it seems 
probable that these latter organs first became from 



98 CIKCUMXUTATING MOVEMENTS OF CHAP. II 

Borne cause thickened in several instances apparently 
in correlation with the fleshy nature of the mature 
plant so as to contain a store of nutriment sufficient 
for the seedling, and then that one or both cotyledons, 
from being superfluous, decreased in size. It is not 
surprising that one cotyledon alone should sometimes 
have been thus affected, for with certain plants, for 
instance the cabbage, the cotyledons are at first of 
unequal size, owing apparently to the manner in which 
they are packed within the seed. It does not, how- 
ever, follow from the above connection, that whenever 
a bulb is formed at an early age, one or both coty- 
ledons will necessarily become superfluous, and conse- 
quently more or less rudimentary. Finally, these 
cases offer a good illustration of the principle of com- 
pensation or balancement of growth, or, as Goethe 
expresses it, " in order to spend on one side, Nature 
is forced to economise on the other side." 

Circummitation and other movements of Hijpocotyls 
and Epicotyls, whilst still arched and buried beneath 
the ground, and whilst breaking through it. According 
to the position in Avhich a seed may chance to 
have been buried, the arched hypocotyl or epicotyl 
will begin to protrude in a horizontal, a more or 
less inclined, or in a vertical plane. Except when 
already standing vertically upwards, both legs of the 
arch are acted on from the earliest period by apo- 
geotropisni. Consequently they both bend upwards, 
until the arch becomes vertical. During the whole of 
this process, even before the arch has broken through 
the ground, it is continually trying to circumnutate 
to a slight extent ; as it likewise does if it happens at 
first to stand vertically up, all which cases have 
been observed and described, more or less fully, in 
the last chapter. After the arch has grown to some 



CHAP. II HYPOCOTYLS, ETC., WHILST AECHED. 09 

height upwards, the basal part ceases to circumnutate, 
whilst the upper part continues to do so. 

That an arched hypocotyl or epicotyl, with the two 
legs fixed in the ground, should be able to cir- 
cumnutate, seemed to us, until we had read Prof. 
Wiesner's observations, an inexplicable fact. He has 
shown* in the case of certain seedlings, whose tips 
are bent downwards (or which nutate), that whilst the 
posterior side of the upper or dependent portion grows 
quickest, the anterior and opposite side of the basal 
portion of the same internode grows quickest ; these 
two portions being separated by an indifferent zone, 
where the growth is equal on all sides. There may 
even be more than one indifferent zone in the same 
internode ; and the opposite sides .of the parts above 
and below each such zone grow quickest. This pecu- 
liar manner of growth is called by Wiesner "un- 
dulatory nutation." Circumnutation depends on one 
side of an organ growing quickest (probably preceded 
by increased turgescence), and then another side, 
generally almost the opposite one, growing quickest. 
Now if we look at an arch like this fj and suppose 
the whole of one side we will say the whole convex 
side of both legs to increase in length, this would 
not cause the arch to bend to either side. But if the 
outer side or surface of the left leg were to increase 
in length the arch would be pushed over to the right, 
and this would be aided by the inner side of the 
right leg increasing in length. If afterwards the 
process were reversed, the arch would be pushed over 
to the opposite or left side, and so on alternately, 
that is, it would circumnutate. As an arched hypo 



* 'Die undulirende Nutation Also published separately se 
dcr Interned ien,' Akad. der Wis- p. 32. 
sewh. (Vienna), Jan. 17th, 1878. 



100 CIECUMNUTATING MOVEMENTS OF CHAP. II 

cotyl, with the two legs fixed in the ground, certainly 
circumnutates, and as it consists of a single internode, 
we may conclude that it grows in the manner de- 
scribed by Wiesner. It may be added, that the crown 
of the arch does not grow, or grows very slowly, for 
it does not increase much in breadth, whilst the arch 
itself increases greatly in height. 

The circumnutating movements of arched hypo- 
cotyls and epicotyls can hardly fail to aid them in 
breaking through the ground, if this be damp and 
soft; though no doubt their emergence depends 
mainly on the force exerted by their longitudinal 
growth. Although the arch circumnutates only to a 
slight extent and probably with little force, yet it is 
able to move the soil near the surface, though it may 
not be able to do so at a moderate depth. A pot with 
seeds of Solanum palinacanthum, the tall arched hypo- 
cotyls of which had emerged and were growing rather 
slowly, was covered with fine argillaceous sand kept 
damp, and this at first closely surrounded the bases of 
the arches ; but soon a narrow open crack was formed 
round each of them, which could be accounted for 
only by their having pushed away the sand on all 
sides ; for no such cracks surrounded some little sticks 
and pins which had been driven into the sand. It 
has already been stated that the cotyledons of Phalaris 
and Avena, the plumules of Asparagus and the hypo- 
cotyls of Brassica, were likewise able to displace the 
same kind of sand, either whilst simply circumnu- 
tating or whilst bending towards a lateral light. 

As long as an arched hypocotyl or epicotyl remains 
buried beneath the ground, the two legs cannot sepa- 
rate from one another, except to a slight extent from 
the yielding of the soil; but as soon as the arch 
ises above the ground, or at an earlier period if 



CEAP. II. IIITOCOTYLS, ETC., WHILST AKCHED. 101 

the pressure of the surrounding earth be artificially 
removed, the arch immediately begins to straighten 
itself. This no doubt is due to growth along the 
whole inner surface of both legs of the arch ; such 
growth being checked or prevented, as long as the two 
legs of the arch are firmly pressed together. When the 
earth is removed all round an arch and the two legs 
are tied together at their bases, the growth on the 
under side of the crown causes it after a time to 
become much flatter and broader than naturally 
occurs. The straightening process consists of a mo- 
dified form of circumnutation, for the lines described 
during this process (as with the hypocotyl of Brassica, 
and the epicotyls of Vicia and Corylus) were often 
plainly zigzag and sometimes looped. After hypo- 
cotyls or epicotyls have emerged from the ground, 
they quickly become perfectly straight. No trace is 
left of their former abrupt curvature, excepting in the 
case of Allium cepa, in which the cotyledon rarely 
becomes quite straight, owing to the protuberance 
developed on the crown of the arch. 

The increased growth along the inner surface of the 
arch which renders it straight, apparently begins in 
the basal leg or that which is united to the radicle ; 
for this leg, as we often observed, is first bowed back- 
wards from the other leg. This movement facilitates 
the withdrawal of the tip of the epicotyl or of the 
cotyledons, as the case may be, from within the seed- 
coats and from the ground. But the cotyledons often 
emerge from the ground still tightly enclosed within 
the seed-coats, which apparently serve to protect them. 
The seed-coats are afterwards ruptured and cast off by 
the swelling of the closely conjoined cotyledons, and not 
by any movement or their separation from one another. 

Nevertheless, in some few cases, especially with the 



102 KUPTURE OF THE SEED-COATS. CTIAP. IL 

Cucurbitaceae, the seed-coats are ruptured by a curious 
contrivance, described by M. Flahault.* A heel or 
peg is developed on one side of the summit of the 
radicle or base of the hypocotyl ; and this holds down 
the lower half of the seed-coats (the radicle being 
fixed into the ground) whilst the continued growth of 
the arched hypocotyl forces up- 
wards the upper half, and tears 
asunder the seed-coats at one end, 
and the cotyledons are then easily 
withdrawn. The accompanying 
figure (Fig. 62) will render this 
description intelligible. Forty- 
one seeds of Gucurbita ovifera 
were laid on friable peat and were 
covered by a layer about an inch 
in thickness, not much pressed 
down, so that the cotyledons in 
being dragged up were subjected 
to very little friction, yet forty of 
them came up naked, the seed- 
coats being left buried in the peat. 
heel or peg projecting This was certainly due to the action 

on one side from summit _ o i 

of radicle and holding of the peg, for when it was pre- 
down lower tip of seed- vente d from acting, the cotyledons, 

coats, which have been i -n -, 

partially ruptured by as we shall presently see, were 



lifted U P Sti11 enclosed in tlieir 

seed-coats. They were, however, 
cast off in tne course of two or three days by the 
swelling of the cotyledons. Until this occurs light is 
excluded, and the cotyledons cannot decompose car- 
bonic acid ; but no one probably would have thought 
that the advantage thus gained by a little earlier cask 



* 'Bull. Soc. T?ot. de France,' torn. xxiv. 1877, p. 201. 



CHAP. II. EUPTUEE OF THE SEED-COATS. 103 

ing off of the seed-coats would be sufficient to account 
for the development of the peg. Yet, according to 
M. Flahault, seedlings which have been prevented 
from casting their seed-coats whilst beneath the 
ground, are inferior to those which have emerged with 
their cotyledons naked and ready to act. 

The peg is developed with extraordinary rapidity ; 
for it could only just be distinguished in two seed- 
lings, having radicles "35 inch in length, but after an 
interval of only 24 hours was well developed in 
both. It is formed, according to Flahault, by the 
enlargement of the layers of the cortical parenchyma 
at the base of the hypocotyl. If, however, we judge 
by the effects of a solution of permanganate of 
potassium, it is developed on the exact line of 
junction between the hypocotyl and radicle; for 
the flat lower surface, as well as the edges, were 
coloured brown like the radicle : whilst the upper 
slightly inclined surface was left uncoloured like the" 
hypocotyl, excepting indeed in one out of 33 im- 
mersed seedlings in which a large part of the upper sur- 
face was coloured brown. Secondary roots sometimes 
spring from the lower surface of the peg, which thus 
seems in all respects to partake of the nature of the 
radicle. The peg is always developed on the side which 
becomes concave by the arching of the hypocotyl; 
and it would be of no service if it were formed on any 
other side. It is also always developed with the flat 
lower side, which, as just stated, forms a part of the 
radicle, at right angles to it, and in a horizontal plane. 
This fact was clearly shown by burying some of the 
thin flat seeds in the same position as in Fig. 62, 
excepting that they were not laid on their flat broad 
sides, but with one edge downwards. Nine seeds 
were thus planted, and the peg was developed in th 



104 RUPTURE OF THE SEED-COATS. CiiAt. IL 

same position, relatively to the radicle, as in the 
figure; consequently it did not rest on the flat tip 
of the lower half of the seed-coats, but was inserted 
like a wedge between the two tips. As the arched 
hypocotyl grew upwards it tended to draw up the 
whole seed, and the peg necessarily rubbed against 
both tips, but did not hold either down. The result 
was, that the cotyledons of five out of the nine seeds 
thus placed were raised above the ground still enclosed 
within their seed-coats. Four seeds were buried with 
the end from which the radicle protrudes pointing 
vertically downwards, and owing to the peg being 
always developed in the same position, its apex alone 
came into contact with, and rubbed against the tip on 
one side ; the result was, that the cotyledons of all 
four emerged still within their seed-coats. These cases 
show us how the peg acts in co-ordination with the 
position which the flat, thin, broad seeds would almost 
always occupy when naturally sown. When the tip 
of the lower half of the seed-coats was cut off, Flahault 
found (as we did likewise) that the peg could not act, 
since it had nothing to press on, and the cotyledons 
were raised above the. ground with their seed-coats not 
cast off. Lastly, nature shows us the use of the peg ; 
for in the one Cucurbitaceous genus known to us, in 
which the cotyledons are hypogean and do not cast 
their seed-coats, namely, Megarrhiza, there is no 
vestige of a peg. This structure seems to be present 
in most of the other genera in the family, judging from 
Flahault's statements ; we found it well-developed and 
properly acting in Trichosanthes anguina, in which we 
hardly expected to find it, as the cotyledons are some- 
what thick and fleshy. Few cases can be advanced 
of a structure better adapted for a special purpose 
than the present one. 



CHAP. II. KUPTURE OF THE SEED-COATS. 105 

With Mimosa pudica the radicle protrudes from a 
small hole in the sharp edge of the seed ; and on its 
summit, where united with the hypocotyl, a transverse 
ridge is developed at an early age, which clearly aids 
in splitting the tough seed-coats ; but it does not aid 
in casting them off, as this is subsequently effected by 
the swelling of the cotyledons after they have been 
raised above the ground. The ridge or heel therefore 
acts rather differently from that of Cucurbita. Its 
lower surface and the edges were coloured brown by 
the permanganate of potassium, but not the upper 
surface. It is a singular fact that after the ridge has 
done its work and has escaped from the seed-coats, 
it is developed into a frill all round the summit of the 
radicle.* 

At the base of the enlarged hypocotyl of Abronia 
unibellata, where it blends into the radicle, there is a 
projection or heel which varies in shape, but its out- 
line is too angular in our former figure (Fig. 61). The 
radicle first protrudes from a small hole at one end of 
the tough, leathery, winged fruit. At this period the 
upper part of the radicle is packed within the fruit 
parallel to the hypocotyl, and the single cotyledon is 
doubled back parallel to the latter. The swelling of 
these three parts, and especially the rapid development 
of the thick heel between the hypocotyl and radicle 
at the point where they are doubled, ruptures the 
tough fruit at the upper end and allows the arched 
hypocotyl to emerge ; and this seems to be the function 
of the heel. A seed was cut out of the fruit and 



* Our attention was called to at the junction of the radicle and 

this case by a brief statement by hypocotyl. This seed possesses a 

Nobbe in his ' Handbuch der very hard and tough coat, and 

Samenkunde,' 1876. p. 215, where would be likely to require aid in 

a figure is also given of a seedling bursting and freeing the cotyle- 

of Martynia with a heel or ridge dons. 



106 RUPTURE OF THE SEED-COATS. CHAP. TL 

allowed to germinate in damp air, and now a thin 
flat disc was developed all round the base of the 
hypocotyl and grew to an extraordinary breadth, like 
the frill described under Mimosa, but somewhat broader. 
Flahault says that with Mirabilis, a member of the 
same family with Abronia, a heel or collar is developed 
all round the base of the hypocotyl, but more on one 
side than on the other; and that it frees the coty- 
ledons from their seed-coats. We observed only old 
seeds, and these were ruptured by the absorption of 
moisture, independently of any aid from the heel and 
before the protrusion of the radicle ; but it does not 
follow from our experience that fresh and tough fruits 
would behave in a like manner. 

In concluding this section of the present chapter it 
may be convenient to summarise, under the form of an 
illustration, the usual movements of the hypocotyls 
and epicotyls of seedlings, whilst breaking through the 
ground and immediately afterwards. We may suppose 
a man to be thrown down on his hands and knees, and 
at the same time to one side, by a load of hay falling 
on him. He would first endeavour to get his arched 
back upright, wriggling at the same time in all 
directions to free himself a little from the surrounding 
pressure ; and this may represent the combined effects 
of apogeotropism and circumnutation, when a seed is so 
buried that the arched hypocotyl or epicotyl protrudes 
at first in a horizontal or inclined plane. The man, 
still wriggling, would then raise his arched back as 
high as he could ; and this may represent the growth 
and continued circumnutation of an arched hypocotyl 
or epicotyl, before it has reached the surface of the 
ground. As soon as the man felt himself at all free, he 
would raise the upper part of his body, whilst still on 



CHAP. II. CIRCUMNUTATION OF HYPOCOTYLS, ETC. 107 

his knees and still wriggling ; and this may represent, 
the bowing backwards of the basal leg of the arch, 
which in most cases aids in the withdrawal of the 
cotyledons from the buried and ruptured seed-coats, 
and the subsequent straightening of the whole hypo- 
cotyl or epicotyl circumnutation still continuing. 

Oircumnutation of Hypocotyls and Epicotyls, ivlien 
erect. The hypocotyls, epicotyls, and first shoots of the 
many seedlings observed by us, after they had become 
straight and erect, circumnutated continuously. The 
diversified figures described by them, often during two 
successive days, have been shown in the woodcuts in 
the last chapter. It should be recollected that the 
dots were joined by straight lines, so that the figures 
are angular; but if the observations had been made 
every few minutes the lines would have been more 
or less curvilinear, and irregular ellipses or ovals, or 
perhaps occasionally circles, would have been formed. 
The direction of the longer axes of the ellipses made 
during the same day or on successive days generally 
changed completely, so as to stand at right angles to 
one another. The number of irregular ellipses or 
circles made within a given time differs much with 
different species. Thus with Brassica oleracea, Cerintlie 
major, and Cucurbita ovifera about four such figures 
were completed in 12 h. ; whereas with Solatium palina- 
canthum and Opuntia basilaris, scarcely more than one. 
The figures likewise differ greatly in size ; thus they 
were very small and in some degree doubtful in Stapelia, 
and large in Brassica, &c. The ellipses described by 
Lathyrus nissolia and Brassica were narrow, whilst 
those made by the Oak were broad. The figures are 
often complicated by small loops and zigzag lines. 

As most seedling plants before the development 
of true leaves are of low, sometimes very low stature, 



108 CIKCUMNUTATION OF HYPOCOTYLS, ETC. CHAP. li- 
the extreme amount of movement from side to side 
of their circumnutating stems was small; that of 
the hypocotyl of Githago segetum was about '2 of an 
inch, and that of Cucurbita ovifera about '28. A 
very young shoot of Lathyrus nissolia moved about 
14, that of an American oak '2, that of the common 
nut only '04, and a rather tall shoot of the Asparagus 
11 of an inch. The extreme amount of movement 
of the sheath-like cotyledon of Phdlaris Canariensis 
was '3 of an inch ; but it did not move very quickly, 
the tip crossing on one occasion five divisions of the 
micrometer, that is, y^th of an inch, in 22 m. 5 s. A 
seedling Nolana prostrata travelled the same distance 
in 10 m. 38 s. Seedling cabbages circumutated much 
more quickly, for the tip of a cotyledon crossed 
1 ^ -th of an inch on the micrometer in 3 m. 20 s. ; and 
this rapid movement, accompanied by incessant oscil- 
lations, was a wonderful spectacle when beheld under 
the microscope. 

The absence of light, for at least a day, does not 
interfere in the least with the circumnutation of the 
hypocotyls, epicotyls, or young shoots of the various 
dicotyledonous seedlings observed by us ; nor with that 
of the young shoots of some monocotyledons. The 
circumnutation was indeed much plainer in darkness 
than in light, for if the light was at all lateral the 
stem bent towards it in a more or less zigzag course. 

Finally, the hypocotyls of many seedlings are drawn 
during the winter into the ground, or even beneath it 
BO that they disappear. This remarkable process, 
which apparently serves for their protection, has 
been fully described by De Vries.* He shows that 

* ' Bot. Zeitung,' 1879, p. G49. burg,' Jahrg.xvi. p. 16, as quoted 
See also Winkler in 'Verhandl. by Haberlandt, ' Schutzeinrichun- 
des Bot Vereins cler P. Branden- gen der Keimpflanze,' 1877, p. 52, 



CHAP. II. CIRCUMNUTATION OF COTYLEDONS. 109 

it is effected by the contraction of the parenchyma- 
cells of the root. But the hypocotyl itself in some 
cases contracts greatly, and although at first smooth 
becomes covered with zigzag ridges, as we observed 
with Gitliago segetum. How much of the drawing 
down and burying of the hypocotyl of Opuniia lasilaris 
was due to the contraction of this part and how much 
to that of the radicle, we did not observe. 

Circumnutation of Cotyledons. With all the dico- 
tyledonous seedlings described in the last chapter, the 
cotyledons were in constant movement, chiefly in a ver- 
tical plane, and commonly once up and once down in 
the course of the 24 hours. But there were many excep- 
tions to such simplicity of movement ; thus the cotyle- 
dons of Ipomoea cserulea moved 13 times either upwards 
or downwards in the course of 10 h. 18 m. Those of 
Oxalis rosea moved in the same manner 7 times in the 
course of 24 h. ; and those of Cassia tora described 5 
irregular ellipses in 9 h. The cotyledons of some 
individuals of Mimosa pudica and of Lotus Jacobteus 
moved only once up and down in 24 h., whilst those of 
others performed within the same period an additional 
small oscillation. Thus with different species, and 
with different individuals of the same species, there 
were many gradations from a single diurnal move- 
ment to oscillations as complex as those of the 
Ipomoea and Cassia. The opposite cotyledons on the 
same seedling move to a certain extent independently 
of one another. This was conspicuous with those of 
Oxalis sensitiva, in which one cotyledon might be 
seen during the daytime rising up until it stood 
vertically, whilst the opposite one was sinking down. 

Although the movements of cotyledons were gene- 
rally in nearly the same vertical plane, yet their 
upward and downward courses never exactly coin- 



110 CIRCUMNUTATION OF COTYLEDONS. CHAP. II 

cided; so that ellipses, more or less narrow, were 
described, and the cotyledons may safely be said to 
have circummitated. Nor could this fact be accounted 
for by the mere increase in length of the cotyledons 
through growth, for this by itself would not induce 
any lateral movement. That there was lateral move- 
ment in some instances,- as with the cotyledons of the 
cabbage, was evident ; for these, besides moving up 
and down, changed their course from right to left 12 
times in 14 h. 15 m. With Solatium lycopersicum the 
cotyledons, after falling in the forenoon, zigzagged 
from side to side between 12 and 4 P.M., and then 
commenced rising. The cotyledons of Lupinus luteus 
are so thick (about '08 of an inch) and fleshy,* that 
they seemed little likely to move, and were there- 
fore observed with especial interest ; they certainly 
moved largely up and down, and as the line traced was 
zigzag there was some lateral movement. The nine 
cotyledons of a seedling Pinus pinaster plainly circum- 
nutated ; and the figures described approached more 
nearly to irregular circles than to irregular ovals or 
ellipses. The sheath-like cotyledons of the Gra- 
minese circumnutate, that is, move to all sides, as 
plainly as do the hypocotyls or epicotyls of any dico- 
tyledonous plants. Lastly, the very young fronds of 
a Fern and of a Selaginella circumnutated. 

In a large majority of the cases which were care- 
fully observed, the cotyledons sink a little downwards 
in the forenoon, and rise a little in the afternoon or 
evening. They thus stand rather more highly inclined 
during the night than during the mid-day, at which 

* The cotyledons, though bright &c , 1877, p. 95), on the gradations 
green, resemble to a certain ex- in the Leguminosae between sub- 
tent hypogean ones; see the in- aerial and subterranean cotvlo- 
teresting discussion by Haber- dons. 
Jandt ('Die Schutzcinrichtungen,' 



CAHP. II. CIRCUMNUTATION OF COTYLEDONS. Ill 

time they are expanded almost horizontally. The 
circunmutating movement is thus at least partially 
periodic, no doubt in connection, as we shall hereafter 
see, with the daily alternations of light and darkness. 
The cotyledons of several plants move up so much at 
night as to stand nearly or quite vertically ; and in 
this latter case they come into close contact with one 
another. On the other hand, the cotyledons of a 
few plants sink almost or quite vertically down at 
night ; and in this latter case they clasp the upper 
part of the hypocotyl. In the same genus Oxalis the 
cotyledons of certain species stand vertically up, and 
those of other species vertically down, at night. In 
all such cases the cotyledons may be said to sleep, 
for they act in the same manner as do the leaves of 
many sleeping plants. This is a movement for a 
special purpose, and will therefore be considered in a 
future chapter devoted to this subject. 

In order to gain some rude notion of the proportional 
number of cases in which the cotyledons of dico- 
tyledonous plants (hypogean ones being of course 
excluded) changed their position in a conspicuous 
manner at night, one or more species in several 
genera were cursorily observed, besides those described 
in the last chapter. Altogether 1 53 genera, included 
in as many families as could be procured, were thus 
observed by us. The cotyledons were looked at in 
the middle of the day and again at night ; and those 
were noted as sleeping which stood either vertically 
or at an angle of at least 60 ; above or beneath the 
horizon. Of such genera there were 26 ; and in 21 of 
them the cotyledons of some of the species rose, and 
in only 6 sank at night; and some of these latter 
cases are rather doubtful from causes to be explained 
in the chapter on the sleep of cotyledons. When 



112 PULVINI OF COTYLEDONS. CHAP. II. 

cotyledons which at noon were nearly horizontal, stood 
at night at more than 20 and less than 60 above the 
horizon, they were recorded as " plainly raised ;" and 
of such genera there were 38. We did not meet with 
any distinct instances of cotyledons periodically sink- 
ing only a few degrees at night, although no doubt 
such occur. We have now accounted for 64 genera 
out of the 153, and there .remain 89 in which the 
cotyledons did not change their position at night by 
as much as 20 that is, in a conspicuous manner 
which could easily be detected by the unaided eye and 
by memory; but it must not be inferred from this 
statement that these cotyledons did not move at all, 
for in several cases a rise of a few degrees was re- 
corded, when they were carefully observed. The 
number 89 might have been a little increased, for the 
cotyledons remained almost horizontal at night in 
some species in a few genera, for instance, Trifo- 
lium and Geranium, which are included amongst the 
sleepers, such genera might therefore have been added 
to the 89. Again, one species of Oxalis generally 
raised its cotyledons at night more than 20 and less 
than 60 above the horizon ; so that this genus might 
have been included under two heads. But as several 
species in the same genus were not often observed, 
such double entries have been avoided. 

In a future chapter it will be shown that the leaves 
of many plants which do not sleep, rise a few degrees in 
the evening and during the early part of the night ; 
and it will be convenient to defer until then the 
consideration of the periodicity of the movements of 
cotyledons. 

On the Pulvini or Joints of Cotyledons. With several 
of the seedlings described in this and the last chapter, 
the summit of the petiole is developed into a pulvinus, 



CHAP. II. 



PULVIXI OF COTYLEDONS. 



113 



cushion, or joint (as this organ has been variously 
called), like that with which many leaves are provided. 
It consists of a mass of small cells usually of a pale 
colour from the absence of chlorophyll, and with its 
outline more or less convex, as shown in the annexed 
figure. In the case of Oxalis 
sensitiva two-thirds of the 
petiole, and in that of Mi- 
mosa pudica, apparently the 
whole of the short sub- 
petioles of the leaflets have 
been converted into pulvini. 
With pulvinated leaves (i.e. 
those provided with a pul- 
vinus) their periodical move- 
ments depend, according to 
PfefTer,* on the cells of the 
pulvinus alternately expand- 
ing more quickly on one side 
than on the other; whereas 
the similar movements of 
leaves not provided with pul- 
vini, depend On their growth Oxalis rosea: longitudinal section 
1 oi a pulvinus on the summit 

being alternately more rapid 




of the petiole of a cotyledon, 
drawn with the camera lucida, 
magnified 75 times : p, />, pe- 
tiole ; /, fibro-vascular bundle ; 
6, 6, commencement of blade of 
cotyledon. 



on one side than on the 
other.! As long as a leaf 
provided with a pulvinus is 
young and continues to grow, 
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. 



t Batalin, 'Flora,' Oct. 1st, 1873 
J Pfeffer, ibid. p. 5. 



114 rULVINT OF COTYLEDONS. CHAP. It 

without such aid, is reduced to the expansion of the 
cells not being followed by growth in the first case, 
and being so followed in the second case. 

Dots were made with Indian ink along the midrib 
of both pulvinated cotyledons of a rather old seedling 
of Oxalis Valdiviana ; their distances were repeatedly 
measured with an eye-piece micrometer during 8 f days, 
and they did not exhibit the least trace of increase. 
It is therefore almost certain that the pulvinus itself 
was not then growing. Nevertheless, during this 
whole time and for ten days afterwards, these coty- 
ledons rose vertically every night. In the case of 
some seedlings raised from seeds purchased under the 
name of Oxalis 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 
growth of the petiole were probably combined in 
causing their prolonged periodic movements. It was 
equally evident that the cotyledons of many plants, 
not provided with pulvini, increased rapidly in length ; 
and their periodic movements no doubt were exclu- 
sively due to growth. 

In accordance with the view that the periodic 
movements of all cotyledons depend primarily on the 
expansion of the cells, whether or not followed by 
growth, we can understand the fact that there is but 
little difference in the kind or form of movement 
in the two sets of cases. This may be seen by corn- 



CHAP. II. PULVINI OF COTYLEDONS. 115 

paring the diagrams given in the last chapter. Thus 
the movements of the cotyledons of Brassica oleracea 
and of Ipomoea cserulea, which are not provided with 
pulvini, are as complex as those of Oxalis and Cassia 
which are thus provided. The pulvinated cotyledons 
of some individuals of Mimosa pudica and Lotus 
Jacobseus made only a single oscillation, whilst those 
of other individuals moved twice up and down in the 
course of 24 hours ; so it was occasionally with the 
cotyledons of Cucurbita ovifera, which are destitute of 
a pulvinus. The movements of pulvinated cotyledons 
are generally larger in extent than those without a 
pulvinus; nevertheless some of the latter moved 
through an angle of 90. There is, however, one 
important difference in the two sets of cases; the 
nocturnal movements of cotyledons without pulvini, 
for instance, those in the Cruciferae, Cucurbitaceae, 
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 Valdiuiana. 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 oi 
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 tlie horizon, but at night were raised 4 S 



110 PULVINI OF COTYLEDONS. CHAP. IL 

above it. After 24 days from the first observation (begun after 
a true leaf had been developed) the cotyledons ceased to rise at 
night. 

Oaalis (B tophi/turn") 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 corniculata. The cotyledons do not stand vertical at 
night, but generally rise to an angle of about 45 above the 
horizon. They continued thus to act for 23 days after their 
first expansion, by which time two leaves had been formed ; 
even after 29 days they still rose moderately above their 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. 

Tr (folium subterrancum.A seedling, 8 days old, had its coty- 
ledons horizontal at 10.30 A.M. and vertical at 9.15 P.M. After an 
interval of two montlis, 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 1 

Trifolium strictum. After 17 days the cotyledons still rose at 
night, but were not afterwards observed. 

Lotus Jacobceus. The cotyledons of some seedlings having 
well-developed leaves rose to an angle of about 45 at night; 
and even after 3 or 4 whorls of leaves had been formed, the co- 
tyledons rose at night considerably above their diurnal hori- 
zontal position. 

Cassia mimosoides. The cotyledons of this Indian species, 
U dajs after their first expansion, and when a leaf had been 
formed, stood during the day horizontal, and at night vertical. 

Cassia sp ? (a large S. Brazilian tree raised from seeds sent us 



CHAP. II. PULVINI OF COTYLEDONS. 117 

by F. Mtiller). The cotyledons, after 16 days from their first 
expansion, had increased greatly in size with two leaves just 
formed. They stood horizontally during the day and vertically 
at night, but were not afterwards observed. 

Cassia neglecta (likewise a S. Brazilian species). A seedling, 
34 days after the first expansion of its cotyledons, was between 3 
and 4 inches in height, with 3 well-developed leaves ; and the 
cotyledons, which during the day were nearly horizontal, at night 
stood vertical, closely embracing the young stem. The cotyle- 
dons of another seedling of the same age, 5 inches in height, 
with 4 well-developed leaves, behaved at night in exactly the 
same manner. 

It is known * that there is no difference in structure 
between the upper and lower halves of the pulvini of 
leaves, sufficient to account for their upward or down- 
ward movements. In this respect cotyledons offer an 
unusally good opportunity for comparing the structure 
of the two halves ; for the cotyledons of Oxalis Valdi- 
viana rise vertically at night, whilst those of 0. rosea 
sink vertically ; yet when sections of their pulvini were 
made, no clear difference could be detected between the 
corresponding halves of this organ in the two species 
which move so differently. With 0. rosea, however, 
there were rather more cells in the lower than in the 
upper half, but this was likewise the case in one speci- 
men of 0. Valdiviana. The cotyledons of both species 
(o mm. in length) were examined in the morning 
Avhilst extended horizontally, and the upper surface of 
the pulvinus of 0. rosea was then wrinkled transversely, 
showing that it was in a state of compression, and this 
might have been expected as the cotyledons sink at 
night; with 0. Valdiviana it was the lower surface 
which was wrinkled, and its cotyledons rise at night. 

Trifolium is a natural genus, and the leaves of all 



* Pfeffer, ' Die Period. Bewegungcn,' 1875, p. 157. 



J 18 PULVINI OF COTYLEDONS. CHAP. H. 

the species seen by us are pulvinated ; so it is with 
the cotyledons of T. suUerraneum and strictum, which 
stand vertically at night ; whereas those of T. resupi- 
natum exhibit not a trace of a pulvinus, nor of any 
nocturnal movement. This was ascertained by mea- 
suring the distance between the tips of the cotyledons 
of four seedlings at mid-day and at night. In this 
species, however, as in the others, the first-formed leaf, 
which is simple or not trifoliate, rises up and sleeps 
like the terminal leaflet on a mature plant. 

In another natural genus, Oxalis, the cotyledons of 
0. Valdiviana, rosea, floribunda, articulata, and sensitiva 
are pulvinated, and all move at night into an upward 
or downward vertical position. In these several species 
the pulvinus is seated close to the blade of the coty- 
ledon, as is the usual rule with most plants. Oxalis cor- 
niculata (var. Atro-purpurea) differs in several respects ; 
the cotyledons rise at night to a very variable amount, 
rarely more than 45; and in one lot of seedlings 
(purchased under the name of 0. tropeeoloides, but 
certainly belonging to the above variety) they rose 
only from 5 to L5 above the horizon. The pulvinus 
is developed imperfectly and to an extremely variable 
degree, so that apparently it is tending towards abor- 
tion. No such case has hitherto, we 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. Firm these several reasons 
and from our having partially traced the develop- 
ment of the pulvinus from an early age, th*e caso 
seems worth describing in some detail. 



CHAP. II. 



PULVINI OF COTYLEDONS. 



119 



When the cotyledons of 0. corniculata were dissected out of a 
seed from which they would soon have naturally emerged, no 
trace of a pulvinus could be detected ; and all the cells forming 
the short petiole, 7 in number in a longitudinal row, were of nearly 
equal size. In seedlings one or two days old, the pulvinus was 
so indistinct that we thought at first that it did not exist ; but 
in the middle of the petiole an ill-defined transverse zone of cells 
could be seen, which were much shorter than those both above 
and below, although of the same breadth with them. They 
presented the appearance of having been just formed by the 
transverse division of longer cells ; and there can be little doubt 
that this had occurred, for the cells in the petiole which had 

Fig. 64. 




A. B. 

Oxalis corniculata: A nnd B the almost rudimentary pulvini of the coty- 
ledons of two rather old seedlings, viewed as transparent objects. 
Magnified 50 times. 

been dissected out of the seed averaged in length 7 divisions 
of the micrometer (each division equalling '003 mm ), and were 
a little longer than those forming a well-developed pulvinus, 
which varied between 4 and 6 of these same divisions. After a 
faw additional days the ill-defined zone of cells becomes distinct, 
and although it does not extend across the whole width of the 
petiole, and although the cells are of a green colour from contain- 
ing chlorophyll, yet they certainly constitute a pulvinus, which, 
as we shall presently see, acts as one. These small cells were 
arranged in longitudinal rows, and varied from 4 to 7 in number ; 
and the cells themselves varied in length in different i arts of the 



120 PULVINI OF COTYLEDONS. CHAP. 11. 

same pulvinus and in different individuals. In the accompany- 
ing figures, A and B (Fig. 64), we have views of the epidermis * 
in the middle part of the petioles of two seedlings, in which, the 
pulvinus was for this species well developed. They offer a 
striking contrast with the pulvinus of 0. rosea (see former 
Fig. 63), or of 0. Valdivwna. With the seedlings, falsely called 
0. tropceoloi/les, the cotyledons of which rise very little at night, 
the small cells were still fewer in number and in parts formed 
a single transverse row, and in other parts short longitudinal 
rows of only two or three. Nevertheless they sufficed to attract 
the eye, when the whole petiole was viewed as a transparent 
object beneath the microscope. In these seedlings there could 
hardly be a doubt that the pulvinus was becoming rudimentary 
and tending to disappear; and this accounts for its great 
variability in structure and function. 

In the following Table some measurements of the cells in 
fairly well-developed pulvini of 0. corniculata are given : 

Seedling 1 day old, with cotyledon 2'3 mm. in length. 

Divisions of 
Micrometer.f 

Average length of cells of pulvinus 6 to 7 

Length of longest cell below the pulvinus 13 

Length of longest cell above the pulvinus 20 

Seedlimj 5 diys old, cotyledon :-M mm. in length, with the pulmnus 
quite distinct. 

Average length of cells of pulvinus 6 

Length of longest cell below the pulvinus 22 

Leng;h of longest cell above the pulvinus 40 

Seedling 8 days old, coti/ledon b mm. in length, with a true leaf 
formed but not i/et expanded. 

Average length of cells of pulvinus 9 

Length of longest cell below the pulviuus 44 

Length of longest cell above the pulvinus 70 

Seedling 13 days old, coti/ledon 4'5 mm. in length, with a smill 
true leaf fully developed. 

Average length of cells of pulvinus 7 

Length of longest cell below the pulvinus 30 

Length of longest cell above the pulviuus tO 



* Longitudinal sections show pulvinus. 

that the forms of the epidermic f Each division equalled '008 

cells raay ho taken as a fair repre- mm. 
entutiDii of those constituting the 



CHAP. II. PULVINI OF COTYLEDONS 121 

"We here see that the cells of the pulvinas increase but little 
iu 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 night to a higher inclination than whilst young, yet they have 
to pass through a larger angle (in one instance amounting to 
63) to gain their nocturnal position, as they are generally 
deflected beneath the horizon during the day. Even with the 
11-days' old seedling the movement did not depend exclusively 
on the pulvimis, for the blade where joined to the petiole was 
curved upwards, and this must be attributed to unequal growth. 
Therefore the periodic movements of the cotyledons of 0. corni- 
culata depend on two distinct but conjoint actions, namely, the 
expansion of the cells of the pulvinus and on the growth of 
the upper part of the petiole, including the base of the blade. 

Lotus JacobcKiis. 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 en* 
almost vertically up at night. There is, however, some degree of 
variability in this respect, apparently dependent on the season 
anil on the degree to which they have been illuminated during 



122 PULVINI OF COTYLEDONS. CIIAP. IL 

the day. With older seedlings, having cotyledons 4 mm. ID 
length, which rise considerably at night, there is a well -deve- 
loped pulvinus close to the blade, colourless, and rather nar- 
rower than the rest of the petiole, from which it is abruptly 
separated. It is formed of a mass of small cells of an average 
length of '021 mm. ; whereas the cells in the lower part of the 
petiole are about '06 mm., and those in the blade from '034 to 
04 mm. in length. The epidermic cells in the lower part of the 
petiole project conically, and thus differ in shape from those 
over the pulvinus. 

Turning now to very young seedlings, the cotyledons of which 
do not rise at night and are only from 2 to 2 mm. in length, 
their petioles do not exhibit any denned zone of small cells, 
destitute of chlorophyll and differing in shape exteriorly from 
the lower ones. Nevertheless, the cells at the place where a 
pulvinus will afterwards be developed are smaller (being on an 
average -015 mm. in length) than thoss in the lower parts of 
the same petiole, which gradually become larger in proceeding 
downwards, the largest being -030 mm. in length. At this 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, tiose 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 denned space of the 
petiole being almost arrested at an early age. With 
Lotus Jacdbseus the cells at first increase a little in 
length ; in Oxalis corniculata they decrease a little, 
owing to seli-division. A mass of such small cells 
forming a pulvinus, might therefore be either acquired 
or lost without any special difficulty, by different 
species in the same natural genus : and we know that 



CHAP. II. DISTURBED PERIODIC MOVEMENTS. 123 

with seedlings of 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 light ; that 
is, their daily periodic movements are greatly and 
quickly disturbed by changes in its intensity or by 
its absence. It is not that they cease to circumnutate 
in darkness, for in all the many cases observed by us 
they continued to do so ; but the normal order of 
their movements in relation to the alternations of day 
and night is much disturbed or quite annulled. This 
holds good with species the cotyledons of which rise 
or sink so much at night that they may be said to 
sleep, as well as with others which rise only a little. 
But different species are affected in very different 
degrees by changes in the light. 



124 DISTURBED PERIODIC MOVEMENTS. CHAI. It. 

For instance, the cotyledons of Beta, vulyaris, Solanum lycoper- 
ticum, Cerinthe major, and Lvpinus luteus, when placed in dark- 
ness, moved down during the afternoon and early night, instead 
of rising as they would have done if they had heen exposed to 
the light. All the individuals of the Solanum did not behave 
in the same manner, for the cotyledons of one circumnutated 
about the same epot 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 tho 
normal manner, but on the second morning it moved upwards. 
The cotyledons of Lotus Jacobceus were not affected by 4 h. of 
complete darkness, but when placed under a double skylight 
and thus feebly illuminated, they quite lost their periodical 
movements on the third morning. On the other hand, the 
cotyledons of Ccurbita ovifera moved in the normal manner 
during a whole day in darkness. 

Seedlings of Githago seyetum were feebly illuminated from 
above in the morning before their cotyledons had expanded, and 
they remained closed for the next 40 h. Other seedlings were 
placed in the dark after their cotyledons had opened in the 
morning and these did not begin to close until about 4 h. had 
elapsed. The cotyledons of Oxalis rosca sank vertically down- 
wards after being left for 1 h. 20 m. in darkness ; but those of 
some other species of Oxalis were not affected by several hours 
of darkness. The cotyledons of several species of Cassia are 
eminently susceptible to changes in the degree of light to which 
they are exposed : thus seedlings of an unnamed S. Brazilian 
species (a large and beautiful tree) were brought out of the hot- 
house and placed on a table in the middle of a room with two 
north-east and one north-west window, so that they were fairly 
well illuminated, though of course less so than in the hot-house, 
the day being moderately bright ; and after 36 m. the cotyledons 
which had been horizontal rose up vertically and closed together 
as when asleep ; after thus remaining on the table for 1 h. 13 m. 
they began to open. The cotyledons of young seedlings of another 
Brazilian species and of C. neglecta, treated in the same manner, 
behaved similarly, excepting that they did not rise up quite so 
much : they again became horizontal after about an hour. 

Here is a more interesting case : seedlings of Cassia fora in 
two pots, which had stood for some time on the table in the 
room just described, had their cotyledons horizontal. Ore pot 
was now exposed for 2 h. to dull sunshine, and the cotyledons 



CHAP. IT. 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. Those two 
pots with seedlings of the same age stood close together, and 
were exposed to exactly the same amount of light, yet the coty- 
ledons in the one pot were rising, whilst those in the other 
pot were at the same time sinking. This fact illustrates in a 
striking manner that their movements are not governed by the 
actual amount, but by a change in the intensity or degree of 
the light. A similar experiment was tried with two sets of seed- 
lings, both exposed to a dull light, but different in degree, and 
the result was the same. The movements of the cotyledons of this 
Cassia are, however, determined (as in many other cases) largely 
by habit or inheritance, independently of light; for seedlings 
which had been moderately illuminated during the day, were 
kept all night and on the following morning in complete dark- 
ness; yet the cotyledons were partially open in the morning 
and remained open in the dark for about 6 h. The cotyledons 
in another pot, similarly treated on another occasion, were open 
at 7 A.M. and remained open in the dark for 4 h. 30 m., after 
which time they began to close. Yet these same seedlings, when 
brought in the middle of the day from a moderately bright 
into only a moderately dull light raised, as we have seen, their 
cotyledons high above the horizon. 

Sensitiveness of Cotyledons to contact. This subject does not 
possess much interest, as it is not known that sensitiveness of this 
kind is of any service to seedling plants. We have observed cases 
in only four genera, though we have vainly observed the coty- 
ledons of many others. The genus Cassia seems to be pre-eminent 
in this respect : thus, the cotyledons of 0. torn, when extended 
horizontally, were both lightly tapped with a very thin twig for 
8 m., and in the course of a few minutes they formed together 
an angle of 90, so that each had risen 45. A single cotyledon 
of another seedling was tapped in a like manner for 1 ra., 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 a 



T26 COTYLEDONS SENSITIVE CHAP. IX 

pin in this part, they rose up vertically ; but the blade was found 
also to be sensitive, care having been taken that the pulvinua 
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 in. 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 
80 s. or for 1 m. ; but when the pulvinus was rubbed and slightly 
pricked with a pin, the cotyledons rose in the course of a few 
minutes through an angle of 60. Several cotyledons of 
C, neglecta (likewise from S. Brazil) rose in from 5 m. to 15 m. to 
various angles between 16 and 34, after being rubbed during 
1 m. with a twig. Their sensitiveness is retained to a somewhat 
advanced age, for the cotyledons of a little plant of O. i<eglecta, 
34 days old and bearing three true leaves, rose when lightly 
pinched between the finger and thumb. Some seedlings were 
exposed for 30 m. to a wind (temp. 50 F.) sufficiently strong to 
keep the cotyledons vibrating, but this to our surprise did not 
cause any movement. The cotyledons of four seedlings of the 
Indian C. gluucawevQ either rubbed with a thin twig for 2m. or 
were lightly pinched : one rose 34 ; a second only 6 ; a third 
13; and a fourth 17. A cotyledon of C. florida similarly 
treated rose 9 ; one of C. corymkosa rose 7, 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. 
in dosa, but these latter are rather thick and fleshy, and do not 
rise at night or go to sleep. 

Smithia sensitiua.TMs plant belongs to a distinct sub-order of 
the Leguminosre from Cassia. Both cotyledons of an oldish 
seedling, with the first true leaf partially unfolded, were rubbed 
for 1 m. with a fine twig, and in 5 m. each rose 32; the; 



CHAP. II. TO CONTACT. x 127 

remained in this position for 15 m., but when looked at again 
40-m. after the rubbing, each had fallen 14. Both cotyledons of 
another and younger seedling were lightly rubbed in the same 
manner for 1m., and after an interval of 32 m. each had risen 
30. They were hardly at all sensitive to a fine jet of water. 
The cotyledons of S. Pfundii, an African water plant, are thick 
aud fleshy ; they are not sensitive and do not go to sleep. 

Mimosa pudica and albida. The blades of several cotyledons 
of both these plants were rubbed or slightly scratched with a 
needle during 1 m. or 2 m. ; but they did not move in the least. 
When, however, the pulvini of six cotyledons of M. pudica were 
thus scratched, two of them were slightly raised. In these two 
cases perhaps the pulvinus was accidentally pricked, for on 
pricking the pulvinus of another cotyledon it rose a little. It 
thus appears that the cotyledons of Mimosa are less sensitive 
than those of the previously mentioned plants.* 

Oxulis sensitiva. The blades and pulvini of two cotyledons, 
standing horizontally, were rubbed or rather tickled for 30 s. 
with a fine split bristle, and in 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 11 m. A pot was carried a little distance on a tray and thus 
jolted; and the cotyledons of four seedlings were all raised in 
10 m. ; after 17 m. one had risen 56, a second 45, a third almost 
90, and a fourth 90. After an additional interval of 40 m. three 
of them had re-expanded to a considerable extent. These obser- 
vations were made before we were aware at what an extraordi- 
narily rapid rate the cotyledons circumnutate, and are therefore 
liable to error. Nevertheless it is extremely improbable that the 
cotyledons in the eight cases given, should all have been rising 
at the time when they were irritated. The cotyledons of Oxalis 
Valdiviana and rosca were rubbed and did not exhibit any 
sensitiveness. 

Finally, there seems to exist some relation between 



* The solo notice which wo p. 865), " les cotyledons du M 

have met with on the sensitive- pudica tendent a se raproclier pal 

ness of cotyledons, relates to Mi- leurs faces superieuros lorsqu'on 

mosa ; for Auir. P. Do Candolle les irrite." 
ays ('Pliys. Ve'g.,' 1832, torn. ii. 



128 SENSITIVENESS OF COTYLEDONS. CHAP II 

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 1 h., and on a second occasion 
for nearly 4 h., in a dark closet. Some other coty- 
ledons, as those of Githago segetum, are much affected 
by a feeble light, but do not move when scratched by 
a needle. That with the same plant there is some 
relation between the sensitiveness of its cotyledons 
and leaves seems highly probable, for the above de- 
scribed Smithia and Oxalis have been called sensitiva y 
owing to their leaves beirig sensitive ; and though the 
leaves of the several species of Cassia are not sensitive 
to a touch, yet if a branch be shaken or syringed 
with water, they partially assume their nocturnal de- 
pendent position. But the relation between the sen- 
sitiveness to contact of the cotyledons and of the 
leaves of the same plant is not very close, as may be 
inferred from the cotyledons of Mimosa pudica being 
only slightly sensitive, whilst the leaves are well 
known to be so in the highest degree. Again, the 
leaves of Neptunia oleracea are very sensitive to a 
touch, whilst the cotyledons do not appear to be so iu 
any degree. 



CKAP. IIL SENSITIVENESS OF KADICLES. 



CHAPTER III. 

SENSITIVENESS or 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 st-nsitive 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- 
pacolum Gossypium Cucurbits Raphanus ^Esculus, lip riot 
sensitive to slight contact, highly sensitive to caustic Quercus, 
tip highly sensitive to contact Power of discrimination Zea 
tip highly sensitive, secondary radicles Sensitiveness of radicles 
to moist air Summary of chapter. 

IN order to see how the radicles of seedlings would 
pass over stones, roots, and other obstacles, which they 
must incessantly encounter in the soil, germinating 
beans (Vicia faba) were so placed that the tips of the 
radicles came into contact, almost rectangularly or 
at a high angle, with underlying plates of glass. In 
other cases the beans were turned about whilst their 
radicles were growing, so that they descended nearly 
vertically on their own smooth, almost flat, broad upper 
surfaces. The delicate root-cap, when it first touched 
any directly opposing surface, was a little flattened 
transversely ; the flattening soon became oblique, and 
in a few hours quite disappeared, the apex now 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 



J 30 SENSITIVENESS OF KADICLES. CHAP. IIL 

it, pressing on it with very little force. How far such 
abrupt changes in its former course are aided by the 
eircum nutation of the tip must be left doubtful. Thin 
slips of wood were cemented on more or less steeply 
inclined glass-plates, at right angles to the radicles 
which were gliding down them. Straight lines had 
been painted along the growing terminal part of some 
of these radicles, before they met the opposing slip 
of wood ; and the lines became sensibly curved in 2 h. 
after the apex had come into contact with the slips. 
In one case of a radicle, which was growing rather 
slowly, the root-cap, after encountering a rough slip 
of wood at right angles, was at first slightly flat- 
tened transversely : after an interval of 2 h. 30 m. 
the flattening became oblique; and after an addi- 
tional 3 hours the flattening had wholly disappeared, 
and the apex now pointed at right angles to its former 
course. It then continued to grow in its new direc- 
tion alongside the slip of wood, until it came to the 
end of it, round which it bent rectangularly. Soon 
afterwards when coming to the edge of the plate of 
glass, it was again bent at a large angle, and de- 
scended perpendicularly into the damp 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 fot 



CIIAP. III. SENSITIVENESS OF RADICLES. 131 

tiicir curvature ; and Sachs has shown * that the 
growing part is more rigid than the part immediately 
above which has ceased to grow, so that the latter 
might have been expected to yield and become curved 
as soon as the apex encountered an unyielding object ; 
whereas it was the stiff growing part which became 
curved. Moreover, an object which yields with the 
greatest ease will deflect a radicle : thus, as we have 
seen, when the apex of the radicle of the 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. Wiir/burg,' Heft i'ii. 1873, p. 398. 



132 SENSITIVENESS OF THE APEX CHAP. Ill 

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 
Dionaea, &c. ; and many organs, such as tendrils, whe- 
ther modified leaves or flower-peduncles, and some few 
stems, bend towards a touching object ; but no case, 
we believe, is known of an organ bending away from 
a touching object. 

Sensitiveness of the Apex of the Radicle of Vicia faba. 
Common beans, after being soaked in water for 24 h., 
were pinned with the hilum downwards (in the manner 
followed by Sachs), inside the cork lids of glass- vessels, 
which were half filled with water; the sides and the 
cork were well moistened, and light was excluded. 
As soon as the beans had protruded radicles, some to a 
length of less than a tenth of an inch, and others to 
a length of several tenths, little squares or oblongs of 
card were affixed to the short sloping sides of their 
conical tips. The squares therefore adhered obliquely 
with reference to the longitudinal axis of the radicle ; 
and this is a very necessary precaution, for if the bits 
of card accidentally became displaced, or were drawn 
by the viscid matter employed, so as to adhere parallel 
to the side of the radicle, although only a little way 
above the conical apex, the radicle did not bend in 
the peculiar manner which we are here considering. 
Squares of about the ^th of an inch (i.e. about 1^ mm.), 
or oblong bits of nearly the same size, were found to 



CHAP. III. OF THE RADICLE OF THE BEAN. 133 

be the most convenient and effective. We employed 
at first ordinary thin card, such as visiting cards, or 
bits of very thin glass, and various other objects ; but 
afterwards sand-paper was chiefly employed, for it was 
almost as stiff as thin card, and the roughened surface 
favoured its adhesion. At first we generally used very 
thick gum-water ; and this of course, under the cir- 
cumstances, never dried in the least ; on the contrary, 
it sometimes seemed to absorb vapour, so that the bits 
of card became separated by a layer of fluid from the 
tip. When there was no such absorption and the card 
was not displaced, it acted well and caused the radicle 
to bend to the opposite side. I should state that 
thick gum-water by itself induces no action. In most 
cases the bits of card were touched with an extremely 
small quantity of a solution of shellac in spirits of 
wine, which had been left to evaporate until it was 
thick ; it then set hard in a few seconds, and fixed the 
bits of card well. When small drops of the shellac 
were placed on the tips without any card, they set into 
hard little beads, and these acted like any other hard 
object, causing the radicles to bend to the opposite 
side. Even extremely minute beads of the shellac 
occasionally acted in a slight degree, as will hereafter 
be described. But that it was the cards which chiefly 
acted in our many trials, was proved by coating one 
side of the tip with a little bit of goldbeaters' skin 
(which by itself hardly acts), and then fixing a bit of 
card to the skin with shellac which never came into 
contact with the radicle : nevertheless the radicle bent 
away from the attached card in the ordinary manner. 

Some preliminary trials were made, presently to. 
be described, by which the proper temperature was 
determined, and then the following experiments were 
made. It should be premised that the beans were 



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 




Vilia faba : A, radicle beginning to bend from the attached little square 
of card ; B, bent at a rectangle ; C, bent into a circle or loop, with the 
tip beginning to bend downwards through the action of geotropis>m. 

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 



CIIAP. III. OF THE RADICLE OF THE BEAN. 135 

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, &c., as fixed in front, cr 
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, 
18 mm. in length, and 1-5 or only O9 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, 23 h. 
from the time of attachment. 

(2.) Eadicle 5'5 mm. in length, card fixed behind: after 9h. 
deflected in the plane of the bean 20 C from the perpendicular 
and from the card, and in opposition to Sachs' curvature : after 
23 h. no change. 
10 



L36 SENSITIVENESS OF THE APEX Cuxr. Ill 

(3.) Eadicle 11 mm. in length, card fixed behind: after 9 h. 
deflected in the plane of the bean 40 from the perpendicular 
and from the card, and in opposition to Sachs' curvature. The 
tip of the radicle more curved than the upper part, but in the 
same plane. After 23 h. the extreme tip was slightly bent to- 
wards the card ; the general curvature of the radicle remaining 
the same. 

(4.) Kadicle 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 card, in 
opposition to Sachs' curvature. There was in addition a slight 
lateral curvature directed partly from the card. After 23 h. no 
change. 

(5.) Eadicle 8 mm. long, card affixed almost laterally : after 
9h. deflected 30 from the perpendicular, in the plane of the 
bean and in opposition to Sachs' curvature ; also deflected in a 
plane at right angles to the above one, 20 from the perpen- 
dicular : after 23 h. no change. 

(6.) Eadicle 9 mm. long, card affixed in front : after 9 h. de- 
flected in the plane of the bean about 40 from the vertical, 
away from the card and in the direction of Sachs' curvature. 
Here therefore we have no evidence of the card being the 
cause of the deflection, except that a radicle never moves 
spontaneously, as far as we have seen, as much as 40 in the 
course of 9 h. After 23 h. no change. 

(7.) Eadicle 7 mm. long, card affixed to the back : after 9 h. 
the terminal part of the radicle deflected in the plane of the 
bean 20 from the vertical, away from the card and in opposition 
to Sachs' curvature. After 22 h. 30 m. this part of the radicle 
had become straight. 

(8.) Eadicle 12 mm. long, card affixed almost laterally : after 
9 h. deflected laterally in a plane at right angles to that of the 
bean between 40 and 50 from the vertical and from the card. 
In the plane of the bean itself the deflection amounted to 8 or 
9 from the vertical and from the card, in opposition to Sachs' 
curvature. After 22 h. 30 in. the extreme tip had become 
slightly curved towards the card. 

(9.) Card fixed laterally: after 11 h. 30m, no effect, the 
radicle being still almost vertical. 

(10.) Card fixed almost laterally: after 11 h. 30m. deflected 
90 from the vertical and from the card, in a plane inter- 
mediate between that of the bean itself and one at rigb.t 



CHAP. III. " OF THE RADICLE OF THE BEAN. 137 

angles to it. Radicle consequently partially deflected from 
Sachs' curvature. 

(11.) Tip of radicle protected with goldbeaters' skin, with a 
square of card of the usual dimensions affixed with shellac : 
after 11 h. greatly deflected in the plane of the bean, in the 
direction of Sachs' curvature, but to a much greater degree and 
in less time than ever occurs spontaneously. 

(12.) Tip of radicle protected as in last case : after 11 h. no 
eftcct, but after 24 h. 40m. radicle clearly deflected from the 
card. This slow action was probably due to a portion of the 
goldbeaters' skin having curled round and lightly touched the 
opposite side of the tip and thus irritated it. 

(13.) A radicle of considerable length had a small square of 
card fixed with shellac to its apex laterally : after only 7 h. 15 m. 
a length of -4 of an inch from the apex, measured along the 
middle, was considerably curved from the side bearing the card. 

(14.) Case like the last in all respects, except that a length of 
only -25 of an inch of the radicle was thus deflected. 

(15.) A small square of card fixed with shellac to the apex of 
a young radicle ; after 9 h. 15 m. deflected through 90 from the 
perpendicular and from the card. After 24 h. deflection much 
decreased, and after an additional day, reduced to 23 from the 
perpendicular. 

(10.) Square of card fixed with shellac behind the apex of a 
radicle, which from its position having been changed during 
growth had become very crooked; but the terminal portion 
was straight, and this became deflected to about 45 from 
the perpendicular and from the card, in opposition to Sachs' 
curvature. 

(17.) Square of card affixed with shellac : after 8 h. radicle 
curved at right angles from the perpendicular and from the 
card After 15 additional hours curvature much decreased. 

(18.) Square of card affixed with shellac : after 8 h. no effect ; 
after 23 h. 3 m. from time of affixing, radicle much curved from 
the square. 

(19.) Square of card affixed with shellac : after 24 h. no effect, 
but the radicle had not grown well and seemed sickly. 

(20.) Square of card affixed with shellac : after 24 h. no effect. 

(21, 22.) Squares of card affixed with shellac : after 24 h. 
radicles of both curved at about 45 from the perpendicular and 
from the cards. 

(23.) Square of card fixed with shellac to young radicle : after 



138 SENSITIVENESS OP THE APEX CHAP. 711 

3 h. very slightly curved from the card ; after 24 h. tip curved 
towards card. Kefixed new square laterally, alter 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 to 
apex : after 24 h. no effect, but the card was found not to be 
touching the apex. A small square was now refixed with 
shellac ; after 16 h. slight deflection from the perpendicular 
and from the card. After an additional day the radicle became 
almost straight. 

( 25.) Square of card fixed laterally to apex of young radicle ; 
after 9 h. deflection from the perpendicular considerable ; after 
24 h. deflection reduced. Kefixed a fresh square with shellac : 
after 24 h. deflection about 40 from the perpendicular and from 
the card. 

(26.) A very small square of card fixed with shellac to apex of 
young radicle : after 9 h. the deflection from the perpendicular 
and from the card amounted to nearly a right angle ; after 24 h. 
deflection much reduced ; after an additional 24 h. radicle almost 
straight. 

(27.) Square of cai'd 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. Kefixed 
a square laterally with shellac ; after 9 h. a little deflection, 
which after 24 h. increased to nearly 20 from the perpendicular 
and from the card. 

(28.) Square of card fixed with shellac; after 9 h. some 
deflection ; next morning the card dropped off; refixed it with 
shellac; it again became loose and was refixed; and now on the 
third trial the radicle was deflected after 14 h. at right angles 
from the card. 

(29.) A small square of card was first fixed with thick gum- 
water to the apex. It produced a slight effect but soon fell 
off. A similar square was now affixed laterally with shellac : 
after 9 h. the radicle was deflected nearly 45 from the perpen- 
dicular and from the card. After 36 additional hours angle of 
deflection reduced to about 30. 

(30.) A very small piece, less than ^th of an inch square, of 
thin tin-foil fixed with shellac to the apex of a young radicle ; 
after 24 h. no effect. Tin- foil removed, and a small square of 
sanded card fixed with shellac; after 9h. deflection at nearlj 
right angles from the perpei^dicular and from the card. Next 



CIIAP. III. OF THE EADIC1E 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 shellac 
to it, and after 9 h. radicle greatly deflected from the card. 
After two additional days the deflection had decreased and was 
only 35 from the perpendicular. 

(32.) Small square of sanded card, attached with thick gum- 
water laterally 1o the apex of a long straight radicle: after 9 h. 
greatly deflected from the perpendicular and from the card. 
Curvature extended for a length of '22 of an inch from the 
apex. After 3 additional hours terminal portion deflected at 
right angles from the perpendicular. Next morning the curved 
portion was '36 in length. 

(33.) Square of card gummed to apex : after 15 h. deflected at 
nearly 90 from the perpendicular and from the card. 

(34.) Small oblong of sanded card gummed to apex: after 
15 h. deflected 90 from the perpendicular and from the card : 
in the course of the three following days the terminal portion 
became much contorted and ultimately coiled into a helix. 

(35.) Square of card gummed to apex: after 9 h. deflected from 
card: after 24 h. from time of attachment greatly deflected 
obliquely and partly in opposition to Sachs' curvature. 

(36.) Small piece of card, rather less than -^th of an inch 
square, gummed to apex : in 9 h. considerably deflected from 
card and in opposition to Sachs' curvature ; after 24 h. greatly 
deflected in the same direction. After an additional day tho 
extreme tip was curved towards the card. 

(37.) Square of card, gummed to apex in front, caused after 
8 h. 30 m. hardly any effect ; refixed fresh square laterally, after 
15 h. deflected almost 90 from the perpendicular and from the 
card. After 2 additional days deflection much reduced. 

(38.) Square of card gummed to apex : after 9 h. much deflec- 
tion, which after 24 h. from time of fixing increased to nearly 
90, After an additional day terminal portion was curled into 
a loop, and on the following day into a helix. 

(39.) Small oblong piece of card gummed to apex, nearly in 
front, but a little to one side; in 9 h. slightly deflected in the 
direction of Sachs' curvature, but rather obliquely, and to 
side opposite to card. Next day more curved in the sanio 
direction, and after 2 additional days ceiled into a ring. 



110 SENSITIVENESS OF THE APEX CHAP. III. 

(40.) Square of card gummed to apex: after 9 h. slightl.v 
curved from card; next morning radicle straight, and apex had 
grown beyond the card. Eefixed another square laterally with 
shellac ; in 9 h. deflected laterally, but also in the direction of 
Sachs' curvature. After 2 additional days' curvature consider- 
ably increased in the same direction. 

(41.) Little square of tin-foil fixed with gum to one side of 
apex of a young and short radicle : after 15 h. no effect, but 
tin-foil had become displaced. A little square of card was now 
gummed to one side of apex, which after 8 h. 40 m. was slightly 
deflected ; in 24 h. from the time of attachment deflected at 90 
from the perpendicular and from the card ; after 9 additional 
hours became hooked, with the apex pointing to the zenith. In 
3 days from the time of attachment the terminal portion of the 
radicle formed a ring or circle. 

(42.) A little square of thick letter-paper gummed to the 
apex of a radicle, which after 9 h. was deflected from it. In 
24 h. from time when the paper was affixed the deflection much 
increased, and after 2 additional days it amounted to 50 from 
the perpendicular and from the paper. 

(43.) A narrow chip of a quill was fixed with shellac to the 
apex of a radicle. After 9 h. no effect; after 24 h. moderate 
deflection, but now the quill had ceased to touch the apex. 
Eemoved quill and gummed a little square of card to apex, 
which after 8 h. caused slight deflection. On the fourth day 
from the first attachment of any object, the extreme tip was 
curved towards the card. 

(44.) A rather long and narrow splinter of extremely thin 
glass, fixed with shellac to apex, it caused in 9 h. slight 
deflection, which disappeared in 24 h. ; the splinter was then 
found not touching the apex. It was twice refixed, with nearly 
similar results, that is, it caused slight deflection, which soon 
disappeared. On the fourth day from the time of first attach- 
ment the tip was bent towards the splinter. 

From these experiments it is clear that the apex of 
the radicle of the bean is sensitive to contact, and 
that it causes the upper part to bend away from the 
touching object. But before giving a summary of the 
results, it will be convenient briefly to give a few other 
observations. Bits of very thin glass and little square? 



CHAP. 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 to a 
temperature of between 69 and 72 F. ; some, how- 
ever, were placed in the hot-house where the tempera- 
ture was rather higher. Above two dozen beans were 
thus tried ; and when a square of glass or card did 
not act, it was removed, and a fresh one affixed, this 
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 
deflection. We were astonished at this result, and 
concluded that we had made some inexplicable mis- 
take in the first six experiments. But before finally 
relinquishing the subject, we resolved to niako oiiG 



L4-2 SENSITIVENESS OF THE APEX CHAP. III. 

other trial, for it occurred to us that sensitiveness is 
easily affected by external conditions, and that radicles 
growing naturally in the earth in the early spring 
would not be subjected to a temperature nearly so 
high as 70 F. We therefore allowed the radicles 
of 12 beans to grow at a temperature of between 
55 and 60 F. The result was that in every one of 
these cases (included in the above-described experi- 
ments) the radicle was deflected in the course of a few 
hours from the attached object. All the above re- 
corded successful trials, and some others presently to 
be given, were made in a sitting-room at the tempera- 
tures just specified. It therefore appears that a tem- 
perature of about, or rather above, 70 F. destroys 
the sensitiveness of the radicles, either directly, or 
indirectly through abnormally accelerated growth ; 
and this curious fact probably explains why Sachs, 
who expressly states that his beans were kept at a 
high temperature, failed to detect the sensitiveness of 
the apex of the radicle. 

But other causes interfere with this sensibility. 
Eighteen radicles were tried with little squares of 
sanded card, some affixed with shellac and some with 
gum-water, during the few last days of 1878, and few 
first days of the next year. They were kept in a room 
at the proper temperature during the day, but were 
probably too cold at night, as there was a hard frost at 
the time. The radicles looked healthy but grew very 
slowly. The result was that only 6 out of the 18 
were deflected from the attached cards, and this only 
to a slight degree and at a very slow rate. These 
radicles therefore presented a striking contrast with 
the 44 above described. On March 6th and 7th, when 
the temperature of the room varied between 53 and 
59 3 F., eleven germinating beans were tried in the 



(IHAP. Ill OF THE RADICLE OF THE BEAN. 143 

same manner, and now every one of the radicles 
became curved away from the cards, though one was 
only slightly deflected. Some horticulturists believe 
that certain kinds of seeds will not germinate pro- 
perly in the middle of the winter, although kept at a 
right temperature. If there really is any proper period 
for the germination of the bean, the feeble degree of 
sensibility of the above radicles may have resulted 
from the trial having been made in the middle of the 
winter, and not simply from the nights being too cold. 
Lastly, the radicles of four beans, which from some 
innate cause germinated later than all the others of 
the same lot, and which grew slowly though appearing 
healthy, were similarly tried, and even after 24 h. they 
were hardly at all deflected from the attached cards 
We may therefore infer that any cause which renders 
the growth of the radicles either slower or more rapid 
than the normal rate, lessens or annuls the sensibility 
of their tips to contact. It deserves particular atten- 
tion that when the attached objects failed to act, there 
was no bending of any kind, excepting Sachs' curva-j 
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 g^yth of an inch), generally of sanded paper 
as stiff as thin card (between '15 and *20 mm. in 
thickness), sometimes of ordinary card, <jr little frag- 



144 SENSITIVENESS OF THE APEX CHAP. Ill 

ments of very thin glass, &c., were affixed at different 
times to one side of the conical tips of 55 radicles. 
The 11 last-mentioned cases, but not the preliminary 
ones, are here included. The squares, &c., were most 
commonly affixed with shellac, but in 19 cases with 
thick gum-water. When the latter was used, the 
squares were sometimes found, as previously stated, 
to be separated from the apex by a layer of thick 
fluid, so that there was no contact, and conse- 
quently no bending of the radicle ; and such few 
cases were not recorded. But in every instance in 
which shellac was employed, unless the square fell 
off very soon, the result was recorded. In several 
instances when the squares became displaced, so as 
to stand parallel to the radicle, or were separated by 
fluid from the apex, or soon fell off, fresh squares 
were attached, and these cases (described under the 
numbered paragraphs) are here included. Out of 
55 radicles experimented on under the proper tempe- 
rature, 52 became bent, generally to a considerable 
extent from the perpendicular, and away from the 
side to which the object was attached. Of the three 
failures, one can be accounted for, as the radicle 
became sickly on the following day ; and a second 
was observed only during 11 h. 30 m. As in several 
cases the terminal growing part of the radicle continued 
for some time to bend from the attached object, it 
formed itself into a hook, with the apex pointing to 
the zenith, or even into a ring, and occasionally into a 
spire or helix. It is remarkable that these latter cases 
occurred more frequently when objects were attached 
with thick gum-water, which never became dry, than 
when shellac was employed. The curvature was often 
well-marked in from 7 h. to 11 h. ; and in one instance 
a semicircle was formed in 6 h. 10 m. from the tiino 



CHAP. III. 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 in one instance (No. 27) a radicle which in 9 h. 
had been deflected about 90 from the perpendicular, 
became quite straight in 24 h. from the period of 
attachment. With No. 26, the radicle was almost 
straight in 48 h. We at first attributed the straighten- 
ing process to the radicles becoming accustomed to a 
slight stimulus, in the same manner as a tendril or 
sensitive petiole becomes accustomed to a very light 
loop of thread, and unbends itself though the loop 
remains still suspended; but Sachs states* that radicles 
of the bean placed horizontally in damp air after 
curving downwards through geotropism, straighten 
themselves a little by growth along theft lower or 
concave sides. Why this should occur is not clear ; 
but perhaps it likewise occurred' in the above ten 
cases. There is another occasional movement which 
must not be passed over : the tip of the radicle, for a 
length of from 2 to 3 mm., was found in six instances,. 



Arbeitcn Bot. Instit., Wurzburg,' Heft iii. p. 456. 



146 SENSITIVENESS OF THE APEX CHAP. Ill 

after an interval of about 24 or more hours, bent 
towards the bit of still attached card, that is, in a 
direction exactly opposite to the previously induced 
curvature of the whole growing part for a length of 
from 7 to 8 mm. This occurred chiefly when the first 
curvature was small, and when an object had been 
affixed more than once to the apex of the same radicle. 
The attachment of a bit of card by shellac to one 
side of the tender apex may sometimes mechanically 
prevent its growth ; or the application of thick gum- 
water more than once to the same side may injure it ; 
and then checked growth on this side with continued 
growth on the opposite and unaffected side would 
account for the reversed curvature of the apex. 

Various trials were made for ascertaining, as far 
as we could, the nature and degree of irritation to 
which the apex must be subjected, in order that the 
terminal growing part should bend away, as if to 
avoid the cause of irritation. We have seen in the 
numbered experiments, that a little square of rather 
thick letter-paper gummed to the apex induced, 
though slowly, considerable deflection. Judging from 
several cases in which various objects had been affixed 
with gum, and had soon become separated from the 
apex by a layer of fluid, as well as from some trials 
in which drops of thick gum-water alone had been 
applied, this fluid never causes bending. We have 
also seen in the numbered experiments that narrow 
splinters of quill and of very thin glass, affixed with 
shellac, caused only a slight degree of deflection, and 
this may perhaps have been 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 



CHAP. III. OF THE EAD1CLE 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 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 
she] lac was spread over a bit of card, and the tips of 
9 radicles were touched laterally with it ; only two of 
them became slightly deflected to the side opposite 
to that bearing the speck of dried shellac, and they 
afterwards straightened themselves. These specks 
were removed, and both together weighed less than 
y^jth of a grain; so that a weight of rather less 
than 2^th of a grain (0'82 mgs.) sufficed to excite 
movement in two out of the nine radicles. Here 
then we have apparently reached nearly the minimum 
weight which will act. 

A moderately thick bristle (which on measurement 
was found rather flattened, being 0'33 mm. in one 
diameter, and 0'20 mm. in the other) was cut into 
lengths of about T^th of an inch. These after being 
touched with thick gum-water, were placed on the tip 
of eleven radicles. Three of them were affected ; one 
being deflected in 8 h. 15 m. to an angle of about 90 
from the perpendicular ; a second to the same amount 
when looked at after 9 h. ; but after 24 h. from the 
time of first attachment the deflection had decreased 
to only 19 ; the third was only slightly deflected 
after 9 h., and the bit of bristle was then found not 
touching the apex ; it was replaced, and after 15 
additional hours the deflection amounted to 26 from 
the perpendicular. The remaining eight radicles 
were not at all acted on by the bits of bristle, so that 
we here appear to have nearly reached the minimum 



148 SENSITIVENESS OF THE APEX CHAP. III. 

of size of an object which will act on the radicle of 
the bean. But it is remarkable that when the bits of 
bristle did act, that they should have acted so quickly 
aiid 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 ^\jth of an inch), were fixed with 
shellac on opposite sides of the apices of 12 suspended 
radicles. The sanded card was between 0'15 and 
0-20 mm. (or between 0-0059 and 0-0079 of an inch), 
and the thin paper only 0'045 mm. (or 0' 00176 of an 
inch) in thickness. In 8 out of the 12 cases there 
could be no doubt that the radicle was deflected from 
the side to which the card-like paper was attached, and 
towards the opposite side, bearing the very thin paper. 
This occurred in some instances in 9 h., but in others 
not until 24 h. had elapsed. Moreover, some of the 
four failures can hardly be considered as really failures : 
thus, in one of them, in which the radicle remained 
quite straight, the square of thin paper was found, 
when both were removed from the apex, to have been 
so thickly coated with shellac that it was almost as 
stiff as the card : in the second case, the radicle was 
bent upwards into a semicircle, but the deflection 
was not directly from the side bearing the card, and 
this was explained by the two squares having become 
cemented laterally together, forming a sort of stiff 
gable, from which the radicle was deflected: in the 
third case, the square of card had been fixed by 
mistake in front, and though there was deflection 
from it, this might have been due to Sachs' curvature . 



OHAP.UL OF THE KADIGLE OF THE BEAN. 119 

iu the fourth case alone no reason could be assigned 
why the radicle had not been at all deflected. These 
experiments suffice to prove that the apex of the 
radicle possesses the extraordinary power of discri- 
minating between thin card and very thin paper, and 
is deflected from the side pressed by the more re- 
sisting or harder substance. 

Some trials were next made by irritating the tips 
without any object being left in contact with them. 
Nine radicles, suspended over water, .had their tips 
rubbed, each six times with a needle, with sufficient 
force to shake the whole bean ; the temperature was 
favourable, viz. about 63 F. In 7 out of these cases 
no effect whatever was produced ; in the eighth case 
tho radicle became slightly deflected from, and in thn 
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- 



150 SENSITIVENESS OF THE APEX CHAP IT. 

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. 
(57-61 F.). 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. Eight were examined after 19 h. : 
four after 22 h. 30 m. ; and three after 25 h. The 
final result was that out of the 18 radicles thus tried, 
13 were plainly bent from the cut surface after the 
above intervals of time; and one other became so 
after an additional interval of 13 h. 30 m. So that 
only 4 out of the 18 radicles were not acted on. To 
these 18 cases the 3 previously mentioned ones should 
be added. It may, therefore, be concluded that a thin 
slice removed by a razor from one side of the conical 
apex of the radicle causes irritation, like that from an 
attached object, and induces curvature from the injured 
surface. 

Lastly, dry caustic (nitrate of silver) was employed 
to irritate one side of the apex. If one side of the 
apex or of the whole terminal growing part of a 
radicle, is by any means killed or badly injured,' the 
other side continues to grow ; and this causes the pan 



CHAP. III. OF THE RADICLE OF THE BEAN. 151 

to bend over towards the injured side.* But i the 
following experiments we endeavoured, generally with 
success, to irritate the tips on one side, without ladly 
injuring them. This was effected by first drying tho 
tip as far as possible with blotting-paper, though it still 
remained somewhat damp, and then touching it onco 
with quite dry caustic. Seventeen radicles were thus 
treated, and were suspended in moist air over water at 
a temperature of 58 J F. They were examined after 
an interval of 21 h. or 24 h. The tips of two were 
found blackened equally all round, so that they could 
tell nothing and w-ere rejected, 15 being left. Of 
these, 10 were curved from the side which had been 
touched, where there was a minute brown or blackish 
mark. Five of these radicles, three of which were 
already slightly deflected, were allowed to enter the 
water in the jar, and were re-examined after an addi- 
tional interval of 27 h. (i.e. in 48 h. after the appli- 
cation of the caustic), and now four of them had 
become hooked, being bent from the discoloured side 
with their points directed to the zenith ; the fifth 
remained unaffected and straight. Thus 11 radicles 
out of the 15 were acted on. But the curvature of 
the four just described was so plain, that they alone 
would have sufficed to show that the radicles of the 
bean bend away from that side of the apex which has 
been slightly irritated by caustic. 

The power of an Irritant on the apex of the Radich 



* Ciesielski found this to be the pended over water, with a thick 

case (' Untersuchungen iiher die layer of grease, which is very 

Abwartskriimmung der Wurzel,' injurious or even fata] to grow- 

1871, p. 28) after burning with ing parts; 1'or after 48 hours 

heated platinum one side of a five of these radicles were curved 

rudicle. So did \vo when we towards tl.e greased side, two 

painted longitudinally half of the remaining straight, 
whole length of 7 radicles, sus- 

11 



152 SENSITIVENESS OF THE APEX CHAP. III. 

of tlie Bean, compared with that of Geotropism. Wo 
know that when a little square of card or other 
object is fixed to one side of the tip of a vertically 
dependent radicle, the growing part bends from it 
often into a semicircle, in opposition to geotropism, 
which force is conquered by the effect of the irri- 
tation from the attached object. Radicles were there- 
fore extended horizontally in damp air, kept at 
the proper low temperature for full sensitiveness, 
and squares of card were affixed with shellac on the 
lower sides of their lips, so that if the squares 
acted, the terminal growing part would curve upwards. 
Firstly, eight beans were so placed that their short, 
young, horizontally extended radicles would be simul- 
taneously acted on both by geotropism and by Sachs' 
curvature, if the latter came into play ; and they all 
eight became bowed downwards to the centre of the 
earth in 20 h., excepting one which was only slightly 
acted on. Two of them were a little bowed downwards 
in only 5 h. ! Therefore the cards, affixed to the lower 
sides of their tips, seemed to produce no effect ; and 
geotropism easily conquered the effects of the irritation 
thus caused. Secondly, 5 oldish radicles, li inch in 
length, and therefore less sensitive than the above- 
mentioned young ones, were similarly placed and 
similarly treated. From what has been seen on many 
other occasions, it may be safely inferred that if they 
had been suspended vertically they would have bent 
away from the cards ; and if they had been extended 
horizontally, without cards attached to them, they 
would have quickly bent vertically downwards through 
geotropism; but the result was that two of these 
radicles were still horizontal after 23 h. ; two were 
curved only slightly, and the fifth as much as 40 
beneath the horizon. Thirdly, 5 beans were fastened 



CHAP. III. OF THE EADICLE OF THE BEAN. 153 

with their flat surfaces parallel to the cork-lid, so that 
Sachs' curvature would uot 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 squai^s 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 dependent radicle, and the apex begins to 



154 SENSITIVENESS OF THE EADICLE. CHAP. Ill 

curve upwards, this movement will be opposed by geo- 
tropism acting only at a very oblique angle, and the 
irritation from the card will be strengthened by its 
previous action. We may therefore conclude that the 
initial power of an irritant on the apex of the radicle 
of the bean, is less than that of geotropism when 
acting at right angles, but greater than that of geo- 
tropism when acting obliquely on it. 

Sensitiveness of the tips of the Secondary Radicles of the 
Bean to contact. All the previous observations relate 
to the main or primary radicle. Some beans suspended 
to cork-lids, with their radicles dipping into water, had 
developed secondary or lateral radicles, which were 
afterwards kept- in very damp air, at the proper low 
temperature for full sensitiveness. They projected, 
as usual, almost horizontally, with only a slight 
downward curvature, and retained this position 
during several days. Sachs has shown* 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 I3ot. Inst., Wiirzburg,' Heft iv. 1874, p. 605-617. 



CHAP. TIL SENSITIVENESS OF THE RADICLE. 155 

curvature. The curvature amounted in some cases to 
about 45 above the horizon, in others to 90, and then 
the tip pointed to the zenith. In one instance a 
distinct upward curvature was observed in 8 h. 15 m., 
but usually not until 24 h. had elapsed. Although 
only 9 out of 39 radicles were affected, yet the curva- 
ture was so distinct in several of them, that there could 
be no doubt that the tip is sensitive to slight contact, 
and that the growing part bends away from the touch- 
ing object. It is possible that some secondary radicles 
are more sensitive than others ; for Sachs has proved * 
the interesting fact that each individual secondary 
radicle possesses its own peculiar constitution. 

Sensitiveness to contact of the Primary Eadicle, a little 
above the apex, in the Bean ( Vicia faba~) and Pea (Pisum 
sativu'in). 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 t has shown that pressure at the distance 
of a few millimeters above the apex causes the radicle 
to bend, like a tendril, towards the touching object. 
By fixing pins so that they pressed against the radicles 
of beans suspended vertically in damp air, we saw this 
kind of curvature ; but rubbing the part with a twig 
or needle for a few minutes produced no effect. Haber- 
landt remarks,} that these radicles in breaking through 
the seed-coats often rub and press against the ruptured 
edges, and consequently bend round them. As little 
squares of the card-like paper affixed with shellac to 
the tips were highly efficient in causing the radicles 
to bend away from them, similar pieces (of about Vo-th 



* ' Arbeiten Bot. Instit., Wiirz- } 'DieSchutzeinrichtungon dei 
burg,' Heft. iv. 1874, p. 620. Keimpflanzc,' 1877, p. 25. 

t lbi.1. Heft Hi. 1873, p. 437. 



156 SENSITIVENESS OF THE CHAP. IIL 

inch square, or rather less) were attached in the same 
manner to one side of the radicle at a distance of 3 or 
4 mm. above the apex. In our first trial on 15 radicles 
no effect was produced. In a second trial on the same 
number, three became abruptly curved (but only one 
strongly) towards the card within 24 h. From these 
cases we may infer that the pressure from a bit of card 
affixed with shellac to one side above the apex, is hardly 
a sufficient irritant ; but that it occasionally causes the 
radicle to bend like a tendril towards this side. 

We next tried the effect of rubbing several radicles 
at a distance of 4 mm. from the apex for a few seconds 
with lunar caustic (nitrate of silver) ; and although the 
radicles had been wiped dry and the stick of caustic 
was dry, yet the part rubbed was much injured and a 
slight permanent Depression was left. In such cases 
the opposite side continues to grow, and the radicle 
necessarily becomes bent towards the injured side. 
But when a point 4 mm. from the apex was momen- 
tarily touched with dry caustic, it was only faintly 
discoloured, and no permanent injury was caused. This 
was shown by several radicles thus treated straighten- 
ing themselves after one or two days ; yet at first they 
became curved towards the touched side, as if they had 
been there subjected to slight continued pressure. 
These cases deserve notice, because when one side of 
the apex was just touched with caustic, the radicle, as 
we have seen, curved itself in an opposite direction, that 
is, away from the touched side. 

The radicle of the common pea at a point a little 
above the apex is rather more sensitive to continued 
pressure than that of the beau, and bends towards the 
pressed side.* We experimented on a variety (Yorlc- 



* Saclis, 4 Aibeiten Bot. Institut, WurzburR,' Heft Hi. p. 438. 



CHAP. 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 
curved towards the cards. The greatest degree of 
curvature amounted to 62 from the perpendicular; 
but so large an angle was only once formed. On one 
occasion a slight curvature was perceptible after 5 h. 
45 m., and it was generally well-marked after 14 h. 
There can therefore be no doubt that with the pea, 
irritation from a bit of card attached to one side of the 
radicle above the apex suffices to induce curvature. 

Squares of card were attached to one side of the tips 
of 11 radicles within the same jars in which the above 
trials were made, and five of them became plainly, 
and one slightly, curved away from this side. Other 



158 SENSITIVENESS OF THE APEX CHAP. Ill 

analogous cases will be immediately described. The 
fact is here mentioned because it was a striking spec- 
tacle, showing the difference in the sensitiveness of 
the radicle in different parts, to behold in the same 
jar one set of radicles curved away from the squares OE 
their tips, and another set curved towards the squares 
attached a little higher up. Moreover, the kind of 
curvature in the two cases is different. The squares 
attached above the apex cause the radicle to bend 
abruptly, the part above and beneath remaining nearly 
straight ; so that here there is little or no transmitted 
effect. On the other hand, the squares attached to 
the apex affect the radicle for a length of from about 
4 to even 8 mm., inducing in most cases a sym- 
metrical curvature ; so that here some influence is 
transmitted from the apex for this distance along the 
radicle. 

Pisum sativum (var. Yorkshire Hero) : Sensitiveness of 
the apex of the Radicle. Little squares of the same card- 
like paper were affixed (April 24th) with shellac to 
one side of the apex of 10 vertically suspended radicles : 
the temperature of the water in the bottom of the jars 
was 60-61 F. Most of these radicles were acted on 
in 8 h. 30 m. ; and eight of them became in the course 
of 24 h. conspicuously, and the remaining two slightly, 
deflected from the perpendicular and from the side 
bearing the attached squares. Thus all were acted on ; 
but it will suffice to describe two conspicuous cases. 
Tn one the terminal portion of the radicle was bent at 
right angles (A, Fig. 66) after 24 h. ; and in the other 
(B) it had by this time become hooked, with the apex 
pointing to the zenith. The two bits of card here used 
were '07 inch in length and '04 inch in breadth. Two 
other radicles, which after 8 h. 30 m. were moderately 
deflected, became straight again after 24 h. Anothoi 



CHAP. IIL 



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. 




^~~ ^ 




A. 



B. 



Pisum sativum : deflection produced within 24 hours in the growth of 
vertically dependent radicles, by little squares of card affixed with 
shellac to one side of apex : A, bent at right angles ; li, hooked. 

interval of 5 h. 3.0 m. ; and we merely record in our 
notes " almost all bent slightly from the perpendicular, 
and away from the squares ; the deflection amounting 
in one or two instances to nearly a rectangle." These 
two sets of cases, especially the first one, prove that 
the apex of the radicle is sensitive to slight contact 
and that the upper part bends from the touching 
object. Nevertheless, on June 1st and 4th, 8 other 
radicles were tried in the same manner at a tempera- 
ture of 58-60 F., and after 24 h. only 1 was decidedly 
bent from the card, 4 slightly, 2 doubtfully, and 1 not 
in the least. The amount of curvature was unaccount- 
ably small ; but all the radicles which were at all bent, 
were bent away from the cards. 

We now tried the effects of widely different tempera- 
tures on the sensitiveness of these radicles with squares 



160 SENSITIVENESS OF THE APEX CIIAT. III. 

of card attached to their tips. Firstly, 13 peas, most 
of them having very short and young radicles, were 
placed in an ice-box, in which the temperature rose 
during three days from 44 to 47 F. They grew slowly, 
but 10 out of the 13 became in the course of the three 
days very slightly curved from the squares ; the other 
3 were not affected ; so that this temperature was too 
low for any high degree of sensitiveness or for much 
movement. Jars with 13 other radicles were next 
placed on a chimney-piece, where they were subjected 
to a temperature of between 68 and 72 F., and 
after 24 h., 4 were conspicuously curved from the 
cards, 2 slightly, and 7 not at all ; so that this tem- 
perature was rather too high. Lastly, 12 radicles 
were subjected to a temperature varying between 
72 and 85 F., and none o them were in the least 
affected by the squares. The above several trials, 
especially the first recorded one, indicate that the 
most favourable temperature for the sensitiveness of 
the radicle of the pea is about 60 F. 

The tips of 6 vertically dependent radicles were 
touched once with dry caustic, in the manner described 
under Vicia faba. After 24 h. four of them were bent 
from the side bearing a minute black mark ; and the 
curvature increased in one case after 38 h., and in 
another case after 48 h., until the terminal part pro- 
jected almost horizontally. The two remaining ra- 
dicles were not affected. 

With radicles of the bean, when extended horizontally 
in damp air, geotropism always conquered the effects 
of the irritation caused by squares of card attached to 
the lower sides of their tips. A similar experiment 
was tried on 13 radicles of the pea ; the squares being 
attached with shellac, and the temperature between 
58~60 F. The result was somewhat different ; for 



CHAP. 1IL OF THE RADICLE OF THE PEA. 161 

these radicles are either less strongly acted on by 
geotropisrn, or, what is more probable, are more sen- 
sitive to contact. After a time geotropism always 
prevailed, but its action was often delayed ; and iu 
three instances there was a most curious struggle 
between geotropism and the irritation caused by the 
cards. Four of the 13 radicles were a little curved 
downwards within 6 or 8 h., always reckoning from 
the time when the squares were first attached, and 
after 23 h. three of them pointed vertically down- 
wards, and the fourth at an angle of 45 beneath the 
horizon. These four radicles therefore did not seem 

Fig. 67. 





Pisum sntimim: a radicle extended horizontally in damp air with a little 
square of card affixed to the lower side of its tip, causing it to bend 
upwards in opposition to geotropism. The deflection of the radicle 
after 21 hours is shown at A, and of the same radicle after 45 hours at 
B, now forming a loop. 

to have been at all affected by the attached squares. 
Four others were not acted on by geotropism within 
the first 6 or 8 h., but after 23 h. were much bowed 
down. Two others remained almost horizontal for 
23 h., but afterwards were acted on. So that in these 
latter six cases the action of geotropism was much 
delayed. The eleventh radicle was slightly curved 
down after 8 h., but when looked at again after 23 h. 
the terminal portion was curved upwards; if it had 



lf>2 SENSITIVENESS OF THE APEX CHAP. Ml. 

been longer observed, the tip no doubt would have 
been found again curved down, and it would have 
formed a loop as in the following case. The twelfth 
radicle after 6 h. was slightly curved downwards ; but, 
when looked at again after 21 h., this curvature had 
disappeared and the apex pointed upwards ; after 30 h. 
the radicle formed a hook, as shown at A (Fig. 67) ; 
which hook after 45 h. was converted into a loop (B). 
The thirteenth radicle after 6 h. was slightly curved 
downwards, but within 21 h. had curved considerably 
up, and then down again at an angle of 45 beneath 
the horizon, afterwards becoming perpendicular. In 
these three last cases geotropism and the irritation 
caused by the attached squares alternately prevailed 
in a highly remarkable manner; geotropism being 
ultimately victorious. 

Similar experiments were not always quite so suc- 
cessful as in the above cases. Thus 6 radicles, horizon- 
tally extended with attached squares, were tried on 
June 8th at a proper temperature, and after 7 h. 30 m. 
none were in the least curved upwards and none were 
distinctly geotropic ; whereas of 6 radicles without any 
attached squares, which served as standards of com- 
parison or controls, 3 became slightly and 3 almost 
rectangularly geotropic within the 7 h. 30 m. ; but 
after 23 h. the two lots were equally geotropic. On 
July 10th another trial was made with 6 horizontally 
extended radicles, with squares attached in the same 
manner beneath their tips ; and after 7 h. 30 m., 4 were 
slight ly geotropic, 1 remained horizontal, and 1 was 
curved upwards in opposition to gravity or geotropism. 
This latter radicle after 48 h. formed a loop, like that 
at B (Fig. 67). 

An analogous trial was now made, but instead of 
attaching squares of card to the lower sides of the 



CHAP. III. OF THE RADICLE OF PHASEOLUS. 16S 

tips, these were touched with dry caustic. The details 
of the experiment will be given in the chapter on 
Geotropisin, 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 tc 
geotropism. This upward curvature was distinctly 
visible in 8 h. 45 m. after the lower sides of the tips 
had been cauterised. 

Little squares of card were affixed with shellac on 
two occasions to the tips of 22 young and short 
secondary radicles, which had been emitted from the 
primary radicle whilst growing in water, but were now 
suspended in damp air. Besides the difficulty of 
attaching the squares to such finely pointed objects 
as were these radicles, the temperature was too high, 
varying on the first occasion from 72 to 77 F., and 
on the second being almost steadily 78 F. ; and this 
probably lessened the sensitiveness of the tips. The 
result was that after an interval of 8 h. 30 m., 6 of the 
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 



181 SENSITIVENESS OF THE APEX CIIAP. Ill 

of various sizes of the same card -like paper, also with 
bits of thin glass and rough cinders, affixed with shellac 
to one side of the apex. 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 4G not being at 
all affected. It is therefore clear that the tips of the 
radicles of this Phaseolus are much less sensitive to 
contact than are those of the bean or pea. We 
thought that they might be sensitive to harder 
pressure, but after several trials we could not devise 
any method for pressing harder on one side of the 
apex than on the other, without at the same time 
offering mechanical resistance to its growth. We 
therefore tried other irritants. 

The tips of 13 radicles, dried with blotting-paper, 
were thrice touched or just rubbed on one side 
with dry nitrate of silver. They were rubbed thrice, 
because we supposed from the foregoing trials, that 
the tips were not highly sensitive. After 24 h. the 
tips were found greatly blackened ; 6 were blackened 
equally all round, so that no curvature to any one 
side could be expected ; 6 were much blackened on 
one side for a length of about T Vth of an inch, and 
this length became curved at right angles towards the 
blackened surface, the curvature afterwards increasing 
in several instances until little hooks were formed. 
1 1 was manifest that the blackened side was so much 
injured that it could not grow, whilst the opposite 
Bide continued to grow. One alone out of these 13 
radicles became curved from the blackened side, the 



CHAP. III. OF THE RADICLE OF PHASEOLUS. 16S5 

curvature extending for some little distance above 
the apex. 

After the experience thus gained, the tips of six 
almost dry radicles were once touched with the dry 
caustic on one side ; and after an interval of 10 in. 
were allowed to enter water, which was kept at a 
temperature of 65-67 F. The result was that after 
an interval of 8 h. a minute blackish speck could 
just be distinguished on one side of the apex of five 
of these radicles, all of which became curved towards 
the opposite side in two cases at about an angle 
of 45 in two other cases at nearly a rectangle and 
in the fifth case at above a rectangle, so that the apex 
was a little hooked ; in this latter case the black mark 
was rather larger than in the others. After 24 h. 
from the application of the caustic, the curvature of 
three of these radicles (including the hooked one) had 
diminished ; in the fourth it remained the same, and 
in the fifth it had increased, the tip being now hooked. 
It has been said that after 8 h. black specks could 
be seen on one side of the apex of five of the six 
radicles ; on the sixth the speck, which was extremely 
minute, was on the actual apex and therefore central ; 
and this radicle alone did not become curved. It was 
therefore again touched on one side with caustic, and 
after 15 h. 30 m. was found curved from the perpen- 
dicular and from the blackened side at an angle of 34, 
which increased in nine additional hours to 54. 

It is therefore certain that the apex of the radicle 
of this Phaseolus is extremely sensitive to caustic, 
more so than that of the bean, though the latter is 
far more sensitive to pressure. In the experiments 
just given, the curvature from the slightly cauterised 
side of the tip, extended along the radicle for a 
length of nearly 10 mm.; whereas in the first set 



166 SENSITIVENESS OF THE APEX CHAP. II! 

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 arid 
growing part of the radicle to bend. AVe have analo- 
gous cases with Drosera, for a strong solution of car- 
bonate of ammonia when absorbed by the glands, or 
too great heat suddenly applied to them, or crushing 
them, does not cause the basal part of the tentacles 
to bend, whilst a weak solution of the carbonate, or a 
moderate heat, or slight pressure always induces such 
bending. Similar results were observed with Dionsea 
and Pinguicula. 

The effect of cutting off with a razor a thin slice 
from one side of the conical apex of 14 young and 
short radicles was next tried. Six of them after being 
operated on were suspended in damp air ; the tips of 
the other eight, similarly suspended, were allowed to 
enter water at a. temperature of about 65 F. It was 
recorded in each case which side of the apex had 
been sliced off, and when they were afterwards 
examined the direction of the curvature was noted, 
"before the record was consulted. Of the six radicles 
in damp air, three had their tips curved after an 
interval of 10 h. 15 m. directly away from the sliced 
surface, whilst the other three were not affected and 
remained straight ; nevertheless, one of them after 
13 additional hours became slightly curved from the 
sliced surface. Of the eight radicles with their tips 
immersed in water, seven were plainly curved away 
irom the sliced surfaces after 10 h. 15 m. ; and with 



CHAP. 111. OF THE RADICLE OF TROP^EOLUM. 167 

respect to the eighth which remained quite straight, 
too thick a slice had been accidentally removed, so 
that it hardly formed .a real exception to the general 
result. When the seven radicles were looked at 
again, after an interval of 23 h. from the time of 
slicing, two had become distorted; four were deflected 
at an angle of about 70 from the perpendicular and 
from the cut surface ; and one was deflected at nearly 
90, so that it projected almost horizontally, but with 
the extreme tip now beginning to bend downwards 
through the action of geotropism. It is therefore 
manifest that a thin slice cut off one side of the conical 
apex, causes the upper growing part of the radicle of 
this Phaseolus to bend, through the transmitted effects 
of the irritation, away from the sliced surface. 

Tropasolum 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 waa 
in excess and had spread equally all round the apex. 
One radicle was deflected only slightly from the 
perpendicular and from the card ; whilst seventeen 
were plainly deflected. The angles in several of these 
latter cases varied between 40 and 65 from the 
perpendicular ; and in two of them it amounted after 
15 h. or 16 h. to about 90. In one instance a loop 
12 



1G8 SENSITIVENESS OF THE APEX CHAP. III. 

\vas nearly completed in 16 h. There can, therefore, 
be no doubt that the apex is higl.ly sensitive to slight 
contact, and that the upper part of the radicle bends 
away from the touching object. 

Gossypium lierbaceum: Sensitiveness of the apex of the 
Eadicle. Eadicles were experimented on in the same 
manner as before, but they proved ill-fitted for our 
purpose, as they soon became unhealthy when sus- 
pended in damp air. Of 38 radicles thus suspended, 
at temperatures varying from 66 to 69 F., with 
squares of card attached to their tips, 9 were plainly 
and 7 slightly or even doubtfully deflected from the 
squares and from the perpendicular; 22 not being- 
affected. We thought that perhaps the above tempera- 
ture was not high enough, so 19 radicles with attached 
squares, likewise suspended in damp air, were subjected 
to a temperature of from 74 to 79 F., but not one of 
them was acted on, and they soon became unhealthy. 
Lastly, 19 radicles were suspended in water at a tem- 
perature from 70 to 75 F., with bits of glass or 
squares of the card attached to their tips by means of 
Canada-balsam or asphalte, which adhered rather better 
than shellac beneath the water. The radicles did not 
keep healthy for long. The result was that 6 were 
plainly and 2 doubtfully deflected from the attached 
objects and the perpendicular ; 11 not being affected. 
The evidence consequently is hardly conclusive, 
though from the two sets of cases tried under a 
moderate temperature, it is probable that the radicles 
are sensitive to contact ; and would be more so under 
favourable conditions. 

Fifteen radicles which had germinated in friable peat 
were suspended vertically over water. Seven of them 
served as controls, and they remained quite straight 
during 24 h. The tips of the other eight radicles 



CHAP. III. OF THE EADICLE OF CUCURBITA. 169 

were just touched with dry caustic on one side. Aftei 
only 5 h. 10 m. five of them were slightly curved 
from the perpendicular and from the side bearing the 
little blackish marks. After 8 h. 40 m., 4 out of 
these 5 were deflected at angles between 15 and 65 
from the perpendicular. On the other hand, one 
which had been slightly curved after 5 h. 10 m., now 
became straight. After 24 h. the curvature in two 
cases had considerably increased; also in four other 
cases, but these latter radicles had now become so 
contorted, some being turned upwards, that it could no 
longer be ascertained whether they were still curved 
from the cauterised side. The control specimens ex- 
hibited no such irregular growth, and the two sets 
presented a striking contrast. Out of the 8 radicles 
which had been touched with caustic, two alone were 
not affected, and the marks left on their tips by the 
caustic were extremely minute. These marks in all 
cases were oval or elongated ; they were measured in 
three instances, and found to be of nearly the same 
size, viz. of a mm. in length. Bearing this fact in 
mind, it should be observed that the length of the 
curved part of the radicle, which had become deflected 
from the cauterised side in the course of 8 h. 40 m., 
was found to be in three cases 6, 7, and 9 mm. 

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 CHAP. Ill 

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 ^ mm. in length, or 
even less. In 4 h. 30 rri. after the cauterisation, 6 of 
them were plainly curved from the side with the 
brown mark, 4 slightly, and 1 not at all. The latter 
proved unhealthy, and never grew ; and the marks on 
2 of the 4 slightly curved radicles were excessively 
minute, one being distinguishable only with the aid 
of a lens. Of 10 control specimens tried in the same 
jars at the same time, not one was in the least curved. 
In 8 h. 40 m. after the cauterisation, 5 of the radicles 
out of the 10 (the one unhealthy one being omitted) 
were deflected at about 90, and 3 at about 45 from 
the perpendicular and from the side bearing the 
brown mark. After 24 h. all 10 radicles had in- 
creased immensely in length ; in 5 of them the curva- 
ture was nearly the same, in 2 it had increased, and 
in 3 it had decreased. The contrast presented by the 
10 controls, after both the 8 h. 40 m. and the 24 h. 
intervals, was very great ; for they had continued to 
grow vertically downwards, excepting two which, from 
some unknown cause, had become somewhat tortuous. 

In the chapter on Geotropism we shall see that 
10 radicles of this plant were extended horizontally on 
and beneath damp friable peat, under which conditions 



. III. OF THE RADICLE OF RAPHANUS. 17t 

they grow better and more naturally than in damp 
air; and their tips were slightly cauterised on the 
lower side, brown marks about i mm. in length 
being thus caused. Uncauterised specimens similarly 
placed became much bent downwards through geo- 
tropism in the course of 5 or 6 hours. After 8 h. 
only 3 of the cauterised ones were bowed downwards* 
and this in a slight degree ; 4 remained horizontal ; 
and 3 were curved upwards in opposition to geo- 
tropisrn 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. 

llaphanus sativus : Sensitiveness of the apex of the 
Radicle. We here encountered many difficulties in 
our trials, both with squares of card and with caustic j 
for when seeds were pinned to a cork-lid, many of the 
radicles, to which nothing had been done, grew irre- 
gularly, often curving upwards, as if attracted by the 
damp surface above ; and when they were immersed 
in water they likewise often grew irregularly. We 
did not therefore dare to trust our experiments with 
attached squares of card ; nevertheless some of them 
seemed to indicate that the tips were sensitive to 
contact. Our trials with caustic generally failed from 
the difficulty of not injuring too greatly the extremely 
fine tips. Out of 7 radicles thus tried, one became 
Lowed after 22 h. at an angle of 60, a second at 40 



172 SENSITIVENESS OF THE APEX OHAF. Ill 

und a third very slightly from the perpendicular and 
from the cauterised side. 

Msculus hippocastanum : Sensitiveness of the apex oj 
the Radicle. Bits of glass and squares of card were 
affixed with shellac or gum-water to the tips of 12 
radicles of the horse-chestnut ; and when these objects 
fell off, they were 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 
the radicles were slightly rubbed once with dry nitrate of silver; 
and after a few minutes were allowed to dip into water. They 
were subjected to a rather varying temperature, generally 
between 52 and 58 F. A few cases have not been thought 
worth recording, in which the whole tip was blackened, or in 
which the seedling soon became unhealthy. 

(1.) The radicle was slightly deflected from the cauterised 
side in one day (i.e. 24 h.) ; in three days it stood at 60 from 
the perpendicular ; in four days at 90 ; on the fifth day it was 
curved up about 40 above the horizon ; so that it had passed 
. through an angle of 130 in the five days, and this was the 
greatest amount of curvature observed. 

(2.) In two days radicle slightly deflected ; after seven days 



CHAP. IIL OF THE RADICLE OF ^SCULUS. 173 

deflected 69 from the perpendicular and from the cautcriseTi 
side ; after eight days the angle amounted to nearly 90. 

(3.) After one day slight deflection, but the cauterised mark 
was so faint that the same side was again touched with caustic. 
In four days from the first touch deflection amounted to 78, 
which in an additional day increased to 90. 

(4.) After two days slight deflection, which during the next 
three days certainly increased but never became great ; the 
radicle did not grow well and died on the eighth day. 

(5.) After two days very slight deflection; but this on the 
fourth day amounted to 56 from the perpendicular and from 
the cauterised side. 

(6.) After three days doubtfully, but after four days certainly 
deflected from the cauterised side. On the fifth day deflection 
amounted to 45 from the perpendicular, and this on the seventh 
day increased to about 90. 

(7.) After two days slightly deflected ; on the third day tho 
deflection amounted to 25 from the perpendicular, and thi.3 
did not afterwards increase. 

(8.) After one day deflection distinct ; on the third day i 1 
amounted to 44, and on the fourth day to 72 from the perper.- 
dicxilar 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 G3 from the perpendicular and the cauterised side. 

(13.) Whole tip blackened, but more on one side than the 
other ; on the fourth day slightly, and on the sixth day greatly 
deflected from the more blackened side ; the deflection on the 
EJnth day amounted to 90 from the perpendicular. 

(14.) Whole tip blackened in the same manner as in the last 
case : on the second day decided deflection from the more 
blackened side, which increased on the seventh day to nearlj 
90 ; on the following day the radicle appeared unhealthy. 

^15 ) Here we had the anomalous case of a radicle bending 



174 SENSITIVENESS OF THE APEX CHAP. Ill 

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 9U from the perpendicular. The cause 
appeared to lie in the tendril-like sensitiveness of the upper part 
of the radicle, against which the point of a large triangular flap 
of the seed-coats pressed with considerable force; and this 
irritation apparently conquered that from the cauterised apex. 

These several cases show beyond doubt that the 
irritation of one side of the apex, excites the upper 
part of the radicle to bend slowly towards the opposite 
side. This fact was well exhibited in one lot of five 
seeds pinned to the cork -lid of a jar ; for when after 
6 days the lid was turned upside down and viewed 
from directly above, the little black marks made by the 
caustic were now all distinctly visible on the upper 
sides of the tips of the laterally bowed radicles. 

A thin slice was shaved off with a razor from one 
side of the tips of 22 radicles, in the manner described 
under the common bean ; but this kind of irritation 
did not prove very effective. Only 7 out of the 22 
radicles became moderately deflected in from 3 to 5 
days from the sliced surface, and several of the others 
grew irregularly. The evidence, therefore, is far from 
conclusive. 

Quercus robur: Sensitiveness of the apex of the Radicle. 
The tips of the radicles of the common oak are fully 
as sensitive to slight contact as are those of any plant 
examined by us. They remained healthy in damp air 
for 10 days, but grew slowly. Squares of the card- 
like paper were fixed with shellac to the tips of 15 
radicles, and ten of these became conspicuously bowed 
from the perpendicular and from the squares ; two 
elightly, and three not at all. But two of the latter 
were not real exceptions, as they were at first very 
short, and haidly grew afterwards. Some of the more 



CHAP. III. OF THE RADICLE OF QUERGUS. 



175 



Fig. 08. 



remarkable cases are worth describing. The radicles 
were examined on each successive morning, at nearly 
the same hour, that is, after intervals of 24 h. 

No. 1. This radicle suffered from a series of accidents, and. 
acted in an anomalous manner, for the apex appeared at first 
insensible and afterwards sensitive to contact. The first square 
was attached on Oct. 19th ; on the 21st the 
radicle was not at all curved, and the square 
was accidentally knocked off; it was refixed 
on the 22nd, and the radicle became slightly 
carved from the square, but tno curvature 
disappeared on the 23rd, when the square 
was removed and refixed. No curvature en- 
sued, and the square was again accidentally 
knocked off, and refixed. On the morning of 
the 27th it was washed off by having reached 
the water in the bottom of the jar. The 
square was refixed, and on the 29th, that 
is, ten days alter the first square had been 
attached, and two days after the attachment 
of the last square the radicle had grown to 
the great length of 3'2 inches, and now Qwrcus robur . rai|iclfl 
the terminal growing part had become bent \ v ith square of card 
away from the square into a hook (see 
Fig. 68). 

No. 2. Square attached on the 19th ; on 
the 20th radicle slightly deflected from it 
and from the perpendicular; on the 21st 
deflected at nearly right angles ; it remained during the next 
two days in this position, but on the 25th the upward curva- 
ture was lessene I through the action of geotropism, and still 
more so on the 26th. 

No. 3. Square attached on the 19th; on the 21st a trace of 
curvature from the square, which amounted on the 22nd to 
aboiit 40, and on the 23rd to 53 from the perpendictilar. 

No. 4. Square' attached on the 21st,; on the 22nd trace of 
curvature from the square ; on the 23rd completely hooked 
\\ith the point turned up to the zenith. Three diiys afterwards 
(i.e. 26th) the curvature had wholly disappeared and the apex 
pointed perpendicularly downwards. 

No. 5. Square attached on the 21st ; on tho 22nd decided 




attached to one side 
of iipex, causing it 
to become hooked. 
Drawing one-half 
natural scale. 



176 SENSITIVENESS OF THE APKX CHAT. Ill 

though slight curvature from the square; on the 23rd the tip 
had curved up above the ho.izon, and on the 24th wak 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 24tli 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 tiie 
previous cases, with the exception of that of vtEsculus. 
As with the bean, the terminal growing part, after 
bending, sometimes straightened itself through the 
action of geotropisrn, 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 fdba) were attached with 
shellac on opposite sides (as accurately as could bo 
done) of the tips of 13 radicles, suspended in damp 
air, at a temperature of G5-66 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 papar, 
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 



(JHAP. TIL OF THE RADICLE OF ZEA. 17? 

case in which the deflection was 49, the two squares 
had accidentally come into contact on one side of the 
apex, and thus formed a lateral gable ;' and the deflec- 
tion was directed in part from this gable and in part 
from the thick paper. In three cases alone the radicles 
were not affected by the difference in thickness of the 
squares of paper attached to their tips, and conse- 
quently did not bend away from the side bearing the 
stiffer paper. 

Zea mays : Sensitiveness of the apex of the Radicle to 
contact. A large number of trials were made on this 
plant, as it was the only monocotyledon on which we 
experimented. An abstract of the results will suffice. 
In the first place, 22 germinating seeds were pinned to 
cork-lids without any object being attached to their 
radicles, some being exposed to a temperature of 65- 
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 08 
which certainly were not acted on. Some of the 
radicles which were experimented on. were young and 
short, most of them of moderate length, and two or 
three exceeded three inches in length. The curva- 
ture in the above cases occurred within 24 h., but it 
was often conspicuous within a much shorter period. 
For instance, the terminal growing part of one radicle 
was bent upwards into a rectangle in 8 h. 15 m., and 
of another in 9 h. On one occasion a hook was 
formed in 9 h. Six of the radicles in a jar containing 1 
nine seeds, which stood on a sand-bath, raised to 
a temperature varying from 76 to 82 F,, became 
hooked, and a seventh formed a complete loop, when 
first looked at after 15 hours. 

The accompanying figures of four germinating seeds 
(Fig. 69) show, firstly, a radicle (A) the apex of which 
has become so much bent away from the attached 
square as to form a hook. Secondly (B), a hook 
converted through the continued irritation of the 
card, aided perhaps by geotropisrn, 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 



CHAP. III. 



OF THE RADICLE OF ZITi. 



179 



to the continued irritation from the card, two com pi eta 
loops, that is, a helix of two spires ; w hich 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. 09 






.0 D. 

Zea mat/s: radicles excited to bend away from the little squares of card 
attached to one side of their tips. 

in making a second turn or spire, passed through the 
first loop, which was at first widely open, and in 
doing so knocked off the card ; it then grew perpen- 
dicularly downwards, and thus tied itself into a knot, 
which soon became tight ! 

Secondary Radicles of Zea. A short time after the 
first radicle has appeared, others protrude from the 



180 SENSITIVENESS OF THE APEX CHAP. ITT. 

seed, but not laterally from the primary one. Ten of 
these secondary radicles, which were directed obliquely 
downwards, were experimented on with very small 
squares of card attached with shellac to the lower 
sides of their tips. If therefore the squares acted, the 
radicles would bend upwards in opposition to gravity. 
The jar stood (protected from light) on a sand-bath, 
which varied between 76 and 82 F. After only 
5 h. one appeared to be a little deflected from the 
square, and after 20 h. formed a loop. Four others 
were considerably curved from the squares a f ter 20 h., 
and three of them became hooked, with their tips 
pointing to the zenith, one after 29 h. and the 
two others after 44 h. By this latter time a sixth 
radicle had become bent at a right angle from the side 
bearing the square. Thus altogether six out of the 
ten secondary radicles were acted on, four not being 
affected. There can, therefore, be no doubt that the 
tips of these secondary radicles are sensitive to slight 
contact, and that when thus excited they cause the 
upper part to bend from the touching object; but 
generally, as it appears, not in so short a time as in 
the case of the first-formed radicle. 

SENSITIVENESS OF THE TIP OF THE RADICLE TO 
MOIST AIE. 

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 Wiirzburg,' vol. i. 1872, p. 209. 



OflAi. III. OF THE RADICLE TO MOIST AIR. 181 

radicle to bend away from the source of irritation. 
In our experiments we followed Sachs' plan, and sieves 
with seeds germinating in damp sawdust were sus- 
pended so that the bottom was generally inclined at 
40 with the horizon. If the radicles had been acted 
on solely by geotropism, they would have grown out 
of the bottom of the sieve perpendicularly down- 
wards ; but as they were attracted by the adjoining 
damp surface they bent towards it and were deflected 
50 from the perpendicular. For the sake of ascertain- 
ing whether the tip or the whole growing part of the 
radicle was sensitive to the moist air, a length of from 
1 to 2 mm. was coated in a certain number of cases 
with a mixture of olive-oil and lamp-black. This 
mixture was made in order to give consistence to the 
oil, so that a thick layer could be applied, which 
would exclude, at least to a large extent, the moist air, 
and would be easily visible. A greater number of 
experiments than those which were actually tried 
would have been necessary, had not it been clearly 
established that the tip of the radicle is the part which 
is sensitive to various other irritants. 

Phaseolus multiflorus. Twenty-nine radicles, to which no- 
thing had been done, growing out of a sieve, were observed 
at the same time with those which had their tips greased, 
and for an equal length of time. Of the 29, 24 curved them- 
selves so as to come into close contact with the bottom of the 
sieve. The place of chief curvature was generally at a distance 
of 5 or 6 mm. from the apex. Eight radicles had their tips 
greased for a length of 2 mm., and two others for a length of 
la mm. ; they were kept at a temperature of 15-16 C. After 
intervals of from ]9 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 CIIAF. ItL 

considerably cnrved 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 ; t\ro of them 
having increased 2 and 3 mm. in length in 11 h. ; five others 
increased 5 to 8 mm. in 19 h. ; and two, which had been at first 
4 and G mm. in length, iLcreased in 24 h. to 15 and 20 mm. 

The tips of 10 radicles, which likewise grew well, were coated 
with the grease for a length of ojily 1 mm., and now the result 
was somt A\ hat different ; for of these 4 curved themselves to 
the sieve in from 21 h. to 24 h., whilst 6 did not do so. 
Five of the latter were observed for an additional day, and now 
all excepting one became curved to the sieve. 

The tips of 5 radicles were cauterised with nitrate of silver, 
and about 1 mm. in length was 'thus destroyed. They were 
observed for periods varying between 11 h. and 24 h., and were 
found to have grown well. One of them had curved until it 
came into contact with the sieve ; another was curving towards 
it; whilst the remaining three were still vertically dependent. 
Of 7 not cauterised radicles observed at the same time, all had 
come into contact with the sieve. 

The tips of 11 radicles were protected by moistened gold- 
beaters' skin, which adheres closely, lor a length varying from 
H to 2 mm. After 22 h. to 24 h , 6 of these radicles were 
clearly bent towards or had come into contact with the sieve ; 
2 were slightly curved in this direction, and 3 not at all. All 
had grown well. Of 14 control specimens observed at the same 
time, all excepting one had closely approached the sieve. It 
appears from these cases that a cap of goldbeaters' skin checks, 
though only to a slight degree, the bending of the radicles to 
an adjoining damp surface. Whether an extremely thin sheet 
of this substance when moistened allows moisture from the air 
to pass through it, we do not know. One case indicated that 
the caps were sometimes more efficient than appears from the 
above results; for a radicle, which after 23 h. had only 
slightly approached the sieve, had its cap (lj mm. in length) 
removed, and during the next 15j 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. 

View. faba. The tips of 13 radicles were coated with the 
grease for a length of 2 mm. ; and it should be remembered 
that with these radicles the seat of chief curvatxire is about 



CHAF. III. OF THE RADICLE TO MOIST AIR. 183 

4 or 5 mm. from the apex. Four of them were examined after 
22 h., three after 26 h., and six after 36 h., and none had 
been attracted towards the damp lower surface of the sieve. 
In another trial 7 radicles were similarly treated, and 5 of them 
still pointed perpendicularly downwards after 11 h,, whilst 

2 were a little curved towards the sieve; by an accident they 
were not subsequently observed. In both these trials the 
radicles grew well ; 7 of them, which were at first from 4 to 
11 mm. in length, were after 11 h. between 7 and 16 mm. ; 

3 which were at first from 6 to 8 mm. after 26 h. were 11'5 
to 18 mm. in length ; and lastly, 4 radicles which were at first 

5 to 8 mm. after 46 h. were 18 to 23 mm. in length. The 
control or ungreased radicles were not invariably attracted 
towards the bottom of the sieve. But on one occasion 12 out of 
13, which were observed for periods between 22 h. and 36 h., 
were thus attracted. On two other occasions taken together, 
38 out of 40 were similarly attracted. On another occasion 
only 7 out of 14 behaved in this manner, but after two more 
days the proportion of .the curved increased to 17 out of 23. 
On a last occasion only 11 out of 20 were thus attracted. If 
we add up these numbers, we find that 78 out of 96 of the 
control specimens curved themselves towards the bottom of the 
sieve. Of the specimens with greased tips, 2 alone out of the 
20 (but 7 of these were not observed for a sufficiently long 
time) thus curved themselves. We can, therefore, hardly doubt 
that the tip for a length of 2 mm. is the part which is sensitive 
to a moist atmosphere, and causes the upper part to bend 
towards its source. 

The tips of 15 radicles were cauterised with nitrate of silver, 
and they grew as well as those above described with greased 
tips. After an interval of 24 h., 9 of them were not at all 
curved towards the bottom of the sieve; 2 were curved towards 
it at angles of 20 and 12 from their former vertical position, 
and 4 had come into close contact with it. Thus the destruc- 
tion of the tip for a length of about 1 mm. prevented the curva- 
ture of the greater number of these radicles to the adjoining 
damp surface. Of 24 control specimens, 23 were bent to the 
Bieve, 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. 

Ava/a fmt'va. The tips of 13 radicles, which projected 
between 2 and 4 mm. from the bottom of the sieve, manj of 
13 



181 SENSITIVENESS OF THE APEX CHAP. TIL 

them not quite perpendicularly downwards, were coated with 
the black grease for a length of from 1 to 1| mm. The sieves 
were inclined at 30 with the horizon. The greater number of 
these radicles were examined after 22 h., and a few after 25 h., 
and within these intervals they had grown so quickly as to have 
nearly doubled their lengths. With the ungreased radicles the 
chief seat of curvature is at a distance of not less than between 
3'5 and 5'5 mm., and not more than between 7 and 10 mm. from 
the apex. Out of the 13 radicles with greased tips, 4 had not 
moved at all towards the sieve ; 6 were deflected towards it and 
from the perpendicular by angles varying between 10 and 35 ; 
and 3 had come into close contact witli 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 lj mm., they would not have l>een affected by the 
moist air and none would have become curved. 

Triticum vtilgare. Analogous trials were made on 8 radicles 
of the common wheat ; and greasing their tips produced much 
less effect than in the case of the oats. After 22 h., 5 of them 
had come into contact with the bottom of the sieve ; 2 had 
moved towards it 10 and 15, and one alone remained perpen- 
dicular. Not one of the very mmierous ungreased radicles 
failed to come into close contact with the sieve. These trials 
were made on Nov. 28th, when the temperature was only 4'8 C. 
at 10 A.M. We should hardly have thought this case worth 
notice, had it not been for the following circumstance. In the 
Ixjginniug of October, when the temperature was considerably 
higher, viz., 12 to 13 C., we found that only a few of the 
ungreased radicles became bent towards the sieve ; and this 
indicates that sensitiveness to moisture in the air is increased 
by a low temperature, as we have seen with the radicles of 
Vida faba relatively to objects attached to their tips. But m 
thu present instance it is possible that a difference in the dryness 



CHAP. III. OF THE RADICLE TO MOIST AIR. 185 

of the air may have caused the difference in the results at tho 
two periods. 

Finally, the facts just given with respect to Phaseolus 
multiflorus, Vicia faba, and Avena saliva show, as it 
seems to us, that a layer of grease spread for a length 
of 1 to 2 mm. over the tip of the radicle, or the 
destruction of the tip by caustic, greatly lessens or 
quite annuls in the upper and exposed part the power 
of bending towards a neighbouring source of moisture. 
We should bear in mind that the part which bends 
most, lies at some little distance above the greased or 
cauterised tip ; and that the rapid growth of this part, 
proves that it has not been injured by the tips having 
been thus treated. In those cases in which the radicles 
with greased tips became curved, it is possible that the 
layer of grease was not sufficiently thick wholly to ex- 
clude moisture, or that a sufficient length was not thus 
protected, or, in the case of the caustic, not destroyed. 
When radicles with greased tips are left to grow for 
several days in damp air, the grease is drawn out into 
the finest reticulated threads and dots, with narrow 
portions of the surface left clean. Such portions 
would, it is probable, be able to absorb moisture, and 
thus we can account for several of the radicles with 
greased tips having become curved towards the sieve 
after an interval of one or two days. On the whole, 
we may infer that sensitiveness to a difference in the 
amount of moisture in the air on the two sides of a 
radicle resides in the tip, which transmits some influ- 
ence to the upper part, causing it to bend towards the 
source of moisture. Consequently, the movement is 
the reverse of that caused by objects attached to one 
side of the tip, or by a thin slice being cut off, or by 
being slightly cauterised. In a future chapter it 
will be shown that sensitiveness to the attraction of 



186 THE EFFECT OF KILLING OR CHAP. III. 

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 EADICLES BECOMING VERTICALLY GEO- 
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. \Ye also modified the 
experiment, by pinching young radicles a little way 
above their tips, between the arms of a U-shaped 
piece of thick leaden wire. The part pinched was 
thus flattened, and was afterwards prevented from 
growing thicker. Five radicles had their ends cut 
off, and served as controls or standards. Eight were 
pinched ; of these 2 were pinched too severely and 
their ends died and dropped off ; 2 were not pinched 
enough and were not sensibly affected ; the remaining 
4: were pinched sufficiently to check the growth ol 
the terminal part, but did not appear otherwise injured. 
V\ T hen the U-shaped wires were removed, after an 

* ' Arbeitcn Bot. Institut., Wiirzburg,' Heft iv. 1874, p. 622. 



OHAF. 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 1^ incli 
in length) was somewhat distorted, and was not half 
as long as three adjoining secondary radicles, which 
had grown vertically, or almost vertically, downwards. 
Some of these secondary radicles adhered together or 
had become confluent. We learn from these four cases 
that it is not necessary, in order that a secondary 
radicle should assume the nature of a primary one, 
that the latter should be actually amputated ; it is 
sufficient that the flow of sap into it should be 
checked, and consequently should be directed into the 
adjoining secondary radicles; for this seems to be 
the most obvious result of the primary radicle being 
pinched between the arms of a U-shaped wire. 

This change in the nature of secondary radicles is 
dearly analogous, as Sachs has remarked, to that 
which occurs with the shoots of trees, when the leading 
one is destroyed and is afterwards replaced by one or 
more of the lateral shoots ; for these now grow upright 
instead of sub-horizontally. But in this latter case 
the lateral shoots are rendered apogeotropic, whereas 
with radicles the lateral ones are rendered geotropic. 
We are naturally led to suspect that the same cause 
acts with shoots as with roots, namely, an increased flow 
of sap into the lateral ones. We made some trials with 
Abies communis and 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 were pinched too severely, aud 



188 THE EFFECT OF KILLING OB CH*P. IH 

some not enough. Only one case succeeded, namely 
with the spruce-fir. The leading shoot was not killed, 
but its growth was checked ; at its base there were 
three lateral shoots in a whorl, two of which were 
pinched, one being thus killed ; the third was left 
untouched. These lateral shoots, when operated on 
(July 14th) stood at an angle of 8 above the horizon ; 
by Sept. 8th the unpinched one had risen 35 ; by 
Oct. 4th it had risen 46, and by Jan. 26th 48, and 
it had now become a little curved inwards. Part 
of this rise of 48 may be attributed to ordinary 
growth, for the pinched shoot rose 12 within the same 
period. It thus follows that the unpinched shoot 
stood, on Jan. 26th, 56 above the horizon, or 34 
from the vertical ; and it was thus obviously almost 
ready to replace the slowly growing, pinched, lead- 
ing shoot. Nevertheless, we feel some doubt about 
this experiment, for we have since observed with 
spruce-firs growing rather unhealthily, that the lateral 
shoots near the summit sometimes become highly 
inclined, whilst the leading shoot remains apparently 
sound. 

A widely different agency not rarely causes shoots 
which naturally would have grown out horizontally to 
grow up vertically. The lateral branches of the Silver 
Fir (A. peetinata) are often affected by a fungus, 
JEcidium elatinum, which causes the branch to enlarge 
into an oval knob formed of hard wood, in one of 
which we counted 24 rings of growth. According to 
De Bary,* when the mycelium penetrates a bud be- 
ginning to elongate, the shoot developed from it 
grows vertically upwards. Such upright shoots after- 



* See his valuable article in are called in German " Hexen 
Bot. Zeitung,' 1867, p. 257, on besen," or " witch-brooms." 
these monstrous growths, which 



CiiAr. III. INJURING THE PRIMARY RADICLE. 189 

wards 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 -ZEcidium, 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 steins of three 
species of Euphorbia and of Portulaca oleracea are 
" normally prostrate or procumbent ;" but when they 
are attacked by an ^Ecidiurn, they " assume an erect 
habit." Dr. Stahl informs us that lie knows of several 
analogous cases ; and these seem to be closely related 
to that of the Abies. The rhizomes of Sparganiiwi 
ramosum grow out horizontally in the soil to a con- 
siderable length, or are diageotropic ; but F. Elfving 
found that when they were cultivated in water 
their tips turned upwards, and they became apogeo- 
tropic. The same result followed when the stem of the 
plant was bent until it cracked or was merely much 
bowed.f 

No explanation has hitherto been attempted of such 
cases as the foregoing, namely, of secondary radicles 
growing vertically downwards, and of lateral shoots 
growing vertically upwards, after the amputation of 



* ' Proc. Acad. Xat. Sc. Thila- viously observed ('Flora,' 1878, 

dfclpliia,' June 16th, 1874, and p. 324) that the underground 

July 23rd, 1875. shoots of Triticum repens bead 

f See F. Elfving's interesting vertically up when the parts above 

paper in 'Arbeiten Bot. Institut., ground are removed, and wlien 

in Wiirzburg,' vol. ii. 1880, p. 489. the rhizomes are kept partly im 

Carl Kraus (Triesdorf) had pre- merscd in water. 



190 EFFECT OF KILLING PRIMARY RADICLE. CHAP. Ill 

tLe primary radicle or of the leading shoot. The 
following considerations give us, as we believe, the 
clue. Firstly, any cause which disturbs the con- 
stitution * is apt to induce reversion ; such as the 
crossing of two distinct races, or a change of con- 
ditions, as when domestic animals become feral. 
But the case which most concerns us, is the frequent 
appearance of peloric flowers on the summit of a stem, 
or in the centre of the inflorescence, parts which, it is 
believed, receive the most sap ; for when an irregular 
flower becomes perfectly regular or peloric, this may 
be attributed, at least partly, to reversion to a primi- 
tive and normal type. Even the position of a seed at 
the end of the capsule sometimes gives to the seedling 
developed from it a tendency to revert. Secondly, 
reversions often occur by means of buds, independently 
of reproduction by seed ; so that a bud may revert to 
the character of a former state many bud-generations 
ago. In the case of animals, reversions may occur in 
the individual with advancing age. Thirdly and 
lastly, radicles when they first protrude from the seed 
are always geotropic, and plumules or shoots almost 
always apogeotropic. If then any cause, such as an 
increased flow of sap or the presence of mycelium, 
disturbs the constitution of a lateral shoot or of a 
secondary radicle, it is apt to revert to its primordial 
state ; and it becomes either apogeotropic or geotropic, 
as the case may be, and consequently grows either 
vertically upwards or downwards. It is indeed pos- 



* The facts on which the fol- xiv. On ptloric flowers, clmp. 

lowing conclusions are founded xiii. p. M2 ; and see p. 337 on theii 

are given in 'The Variation of position on the plant. Witb 

Animals and Plants under Domes- respect to seeds, p. 340. On le- 

tication,' 2nd edit 1 875. On the version by means of buds, p. 438 

causes leading to reversion see chap, xi vol. i. 
chap. xii. vol. ii. and p. 59, chap. 



CHAP. Ill SUMMARY OF CHAPTER. 191 

sible, or even probable, that this tendency to reversion 
may have been increased, as it is manifestly of servicw 
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 Pliaseolus 
multiflorus the tip was hardly sensitive to small squares 
of attached card, but was sensitive to caustic and to 
slicing. The radicles of Tropseolum were highly sen- 
sitive to contact ; and so, as far as we could judge, 
were those of Gossypium herlaceum, and they were 
certainly sensitive to caustic. The tips of the radicles 
of Cucwbita ovifera were likewise highly sensitive to 
caustic, though only moderately so to contact. Ra- 
phanus sativus offered a somewhat doubtful case. 
With ^iEsculus the tips were quite indifferent to 
bodies attached to them, though sensitive to caustic. 
Those of Quereus robur 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 -ZEsculus, 
the tips were not at all sensitive to small bodies 
attached to them ; but it does not follow from this 



L92 SUMMARY OF CHAPTER. CHAP. 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 radiclo 
for a length of from 1 mm. to 1 5 mm. When this 
part is irritated by contact with any object, by caustic, 
or by a thin slice being cut off, the upper adjoining 
part of the radicle, for a length of from 6 or 7 to 
even 12 mm., is excited to bend away from the side 
which has been irritated. Some influence must there- 
fore be transmitted from the tip along the radicle for 
this length. The curvature thus caused is generally 
symmetrical. The part which bends most apparently 
coincides with that of the most rapid growth. The 
tip and the basal part grow very slowly and they 
bend very little. 

Considering the widely separated position in the 
vegetable series of the several above-named genera, 
we may conclude that the tips of the radicles of all, or 
almost all, plants are similarly sensitive, and transmit 
an influence causing the upper part to bend. With 
respect to the tips of the secondary radicles, those of 
Vicia faba, Pisum sativum, and Zea mays were alone 
observed, and they were found similarly sensitive. 

In order that these movements should be properly 
displayed, it appears necessary that the radicles 
should grow at their normal rate. If subjected to a 
high temperature and made to grow rapidly, the 
tips seem either to lose their sensitiveness, or the 
upper part to lose the power of bending. So it 
appears to be if they grow very slowly from not being 
vigorous, or from being kept at too low a temperature , 
also when they are forced to germinate in the middle 
of the winter. 



CHAI III. SUMMARY OF CHAPTER. 393 

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 .^Eseulus. The curva- 
ture often amounts to a rectangle, that is, the ter- 
minal part bends upwards until the tip, which is but 
little carved, projects almost horizontally. Occa- 
sionally the tip, from the continued irritation of the 
attached object, continues to bend up until it forms a 
hook with the point directed towards the zenith, or 
a loop, or even a spire. After a time the radicle 
apparently becomes accustomed to the irritation, as 
occurs in the case of tendrils, for it again grows down- 
wards, although the bit of card or other object may 
remain attached to the tip. 

It is evident that a small object attached to the free 
point of a vertically suspended radicle can offer no 
mechanical resistance to its growth as a whole, for the 
object is carried downwards as the radicle elongates, 
or upwards as the radicle curves upwards. Nor can 
the growth of the tip itself be mechanically checked 
by an object attached to it by gum-water, which 
remains all the time perfectly soft. The weight of 
the object, though quite insignificant, is opposed 
to the upward curvature. We may therefore conclude 
that it is the irritation due to contact which excites 
the movement. The contact, however, must be pro- 
longed, for the tips of 15 radicles were rubbed for a 
yhort time, and this did not cause them to bend. Here 
then we have a case of specialised sensibility, like 
that of the glands of Drosera ; for these are ex- 
quisitely sensitive to the slightest pressure if prolonged, 
bat 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 



19 1 SUMMARY OF CHAPTER. CHAP. 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 adjoining upper part 
does not bend. The stimulus is too powerful or the 
shock too great for the proper influence to be trans- 
mitted from the tip. We have strictly analogous cases 
with Drosera, Dionaea and Pinguicula, with which 
plants a too powerful stimulus does not excite the 
tentacles to become incurved, or the lobes to close, or 
the margin to be folded inwards. 

With respect to the degree of sensitiveness of the 
apex to contact under favourable conditions, we have 
seen that with Vicia 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 j^yth 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 bristla 
and by a very much less weight than 3 Jjjth of a grain. 



CHAP. 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 
lias been excited. The tips of the radicles of the pea 
seem to be more sensitive to contact than those of the 
bean, for when they were extended horizontally with 
squares of card adhering to their lower sides, a most 
curious struggle occasionally arose, sometimes one 
and sometimes the other force prevailing, but ulti- 
mately geotropism was always victorious ; neverthe- 
less, in two instances the terminal part became so 
much curved upwards that loops were subsequently 
formed. With the pea, therefore, the irritation from 
an attached object, and from geotropism when acting 
at right angles to the radicle, are nearly balanced 
forces. Closely similar results were observed with the 
horizontally extended radicles of Cuourbita ovifera, 



196 SUMMARY OF CHAPTER. CHAI>. I, 

when their tips were slightly cauterised on the lowei 
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, 
geotropisin 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 geotropisin in such a 
manner that they tend to bend only obliquely down- 
wards. If they had been acted on like the primary 
radicle, all the radicles would have penetrated the 
ground in a close bundle. We have seen that if 
the end of the primary radicle is cut off or in- 
jured, the adjoining secondary radicles become geo- 
tropic and grow vertically downwards. This power 
must often be of great service to the plant, when the 
primary radicle has been destroyed by the larva) 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 geotropisin ; 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 surroundiDg 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 



1 Ar'x-iten Bot. Institut., Wiirzburg,' Heft iv. 1S74, pp. G05-G31. 



CHAP. 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- 
gi}n, both of which forces act powerfully. It is, 
however, indispensable that the seeds should be at 
first held down in some manner. When they lie 
on the bare surface they are held down by the attach- 
ment of the root-hairs to any adjoining objects ; and 
this apparently is effected by the conversion of 
their outer surfaces into a cement. But many seeds 
get covered up by various accidents, or they fall into 
crevices or holes. With some seeds their own weight 
suffices. 

The circumnutating movement of the terminal grow- 
ing part both of the primary and secondary radicles 
is so feeble that it can aid them very little in pene- 
trating the ground, excepting when the superficial 
layer is very soft and damp. But it must aid them 
materially when they happen to break obliquely into 
cracks, or into burrows made by earth-worms or larvae. 
This movement, moreover, combined with the sen- 
sitiveness of the tip to contact, can hardly fail to be 
of the highest importance ; for as the tip is always 
endeavouring to bend to all sides it will press on all 
sides, and will thus be able to discriminate between 
the harder and softer adjoining surfaces, in the same 
manner as it discriminated between the attached 
squares of card-like and thin paper. Consequently it 
will tend to bend from the harder soil, and will thus 
follow the lines of least resistance. So it will be if it 
meets with a stone or the root of another plant in the 
soil, as must incessantly occur. If the tip were not 
sensitive, and if it did not excite the upper part of the 
root to bend away, whenever it encountered at right 
angles some obstacle in the ground, it would be liable 



198 SUMMARY OF CHAPTER. CHAP. Ill 

to be doubled up into a contorted mass. But we have 
seen with radicles growing down inclined plates of 
glass, that as soon as the tip merely touched a slip of 
wood cemented across the plate, the whole terminal 
growing part curved away, so that the tip soon stood 
at right angles to its former direction ; and thus it 
would be with an obstacle encountered in the ground, 
as far as the pressure of the surrounding soil would 
permit. We can also understand why thick and strong 
radicles, like those of .ZEsculus, 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 



1 Arbeitcn But. lust. Wurzburg,' Heft iii. p. 456. 



CHAP. 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 geotropisrn 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 
Bide to side, or circumnutating, he will feel any stone 
14 



200 SUMMARY OF CHAPTER. CHAP. 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. 



GIECUMNUTATIOH 201 



CHAPTEE IV. 

TUB ClRCtJMNUTATING MOVEMENTS OF THE SEVERA1 PARTS OF 
MATURE PLANTS. 

Circumnutation of stems : concluding remarks on Circumnutatiou of 
stolons: aid thus afforded in winding amongst the stems of sur- 
rounding plants Circumnutation of flower-stems Circumnutution 
of Dicotyledonous leaves Singular oscillatory movement of leaveo 
of Diontea Leaves of Cannabis sink at night Leaves of Gymno- 
eperms Of Monocotvledons Cryptogams Concluding remarks 
on the circumnututioii 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 thai 
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 t( 
score of genera, belonging to widely distinct families 
and inhabitants of various countries. Several plants 



202 CIECUMNUTATION OF STEMS. CHAP. IV 

were selected which, from being woody, or for other 
reasons, seemed the least likely to circumnutate. The 
observations and the diagrams were made in the 
manner described in the Introduction. Plants in pots 
were subjected to a proper temperature, and whilst 
being observed, were kept either in darkness or were 
feebly illuminated from above. They are arranged 
in the order adopted by Hooker in Le Maout and 
Decaisne's ' System of Botany.' The number of the 
family to which each genus belongs is appended, as 
this serves to show the place of each in the series. 

(1.) fberis nm^ellata (Cruciferse, Fam. 14). The movement of 
the stem of a young plant, 4 inches in height, consisting of 
four internodes (the hypocotyl included) besides a large bud 

Fig. 70. 



-is. 



Iberis nmhellata : circumiiutation 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 maguirie I 
between 4 and 5 times. 

on the summit, -was traced, as here shown, during 24 h. 
(Fig. 70). As far as we could judge the uppermost inch alone 
of the stem circumnutated, and this in a simple manner. The 
movement was slow, and the rate very unequal at different 
times. In part of its course an irregular ellipse, or rather 
triangle, was completed in 6 h. 30 m. 

(2.) Brassica oleracea (Cruciferse). 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 




CHAP. IV. CIKCUMXUTATION OF STEMS. 203 

found to be in constant movement. It crossed five di visions of 
the micrometer, that is, ygoth of an inch, in 6 ni. 20 s. There 
could hardly be a doubt that it was the stem which chiefly 
moved, for the tip did not get quickly out of focus ; and this 
would have occurred had the movement been confined to the 
leaf, which moves up or down in nearly the same vertical plane. 

(3.) Linum usitatissimum (Linea3, Fam. 39). The stems of this 
plant, shortly before the flowering period, are stated by Fritz 
Muller (' Jenaische Zeitschrift,' B. v. p. 137) to revolve, or 
circumnutate. 

(4.) Pelargonium zonale (Geraniaccae, Fam. 47). A young 
plant, 7 2 inches in height, was observed in the usual manner ; 
but, in order to see the bead at the end of the glass filament 



Fig. 71. 




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 llth. 

and at the same time the mark beneath, it was necessary to cut 
off three leaves on one side. We do not know whether it was 
owing to this cause, or to the plant having previously become 
bent to one side through heliotropism, but from the morning of 
the 7th of March to 10.30 P.M. on the 8th, the stem moved 
a considerable distance in a zigzag line in the same general 
direction. During the night of the 8th it moved to some 
distance at right angles to its former course, and next morning 
(9th) stood for a time almost still. At noon on the 9th a new 
tracing was begun (see Fig. 71), which was continued till 8 A.M. 
on the llth. Between noon on the 9th and 5 P.M. on the 10th 
(i.e. in the course of 29 h.), the stem described a circle. This 
plant therefore circumnutates, but at a very slow rate, and to a 
small extent. 

(5.) Tropceolum mafus (?) (dwarfed var. called Tom Thumb) ; 
(Geraniaceac, Fam. 47). The species of this genus climb by the 



204 



CIECUMNUTATION OF STEMS. CHAP. TV 



aid of their sensitive petioles, but some of them also twine 
round supports; but even these latter species do not begin to 
circumnutato in a conspicuous manner whilst young. The 

Fig. 72. 




Tropccolum nvtjiis (?) : circumnutation of stem of youug 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 treated 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- 

Fi *' 73 ; temodes, 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.) 'Jrifoliitm resvpi- 
natum (Leguminosre, 
Fam. 75). When we 
treat of the sleep of 
plants, we shall see that 
the stems in several 

, Leguminous genera, for 

Tn folium rcsnpmatum oircumnutation of . , , 

.tern, traced on vertical glass from 9.30 ^stance, those of Hedy- 

A.M. to 4.30 P.M. Nov. 3rd. Tracing not, sarum, Mimosa, Meli- 

greatly magnified, reduced to half of lotus, &C., which are not 

fromTbovt 6 ' * climbers, circumnutate 

in a con spicuoiTS manner. 

We will here give only a single instaace (Fig. 73), showing 
the circumnntation of the stem of a large plant of a clover, 
Trifolium resupinatum. In the course of 7 h. the stem changed 




CHAT. IV 



CIRCUMNUTATIOX OF STEMS. 



205 



its course greatly eight times and completed three irregular 
circles or ellipses. It therefore circumnutated rapidly. Some 
of the lines run at right angles to one another. 



Fig. 74. 




yboid) : 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.) liubus idceus (hybrid) (Bosacese, Fam. 76). As we hap- 



Fig. 75. 



pened to have a young plant, 11 inches 
in height and growing vigorously, 
which had been raised from a cross 
between the raspberry (Kubus idceus) 
and a North American Eubus, it was 
observed in the usual manner. During 
the morning of March 14th the stem 
almost completed a circle, and then 
moved far to the right. At 4 P.M. it 
reversed its course, and now a fresh 
tracing was begun, which was con- 
tinued during 40i- h., and is given in 
Fig. 74. We here have well-marked 
circumnutation. 

(8.) Deut.zia yracilis (Saxifragese, 
Fam. 77). A shoot on a bush about 
18 inches in height was observed. The 
bead changed its course greatly eleven 
times in the course of 10 h. 30 m. 
(Fig. 75), and there could be no 
doubt about the circumnutation of the 
item. 

(9.) Fuchsia (greenhouse van, with 
large flowers, probably a hybrid) (Ona- 
grnviefe, Fam. 100). A young plant, 
15 inches in height, was observed during nearly 48 h. The 




ifi graoOa: circum nu- 
tation of stem, kept in 
darkness, traced on hori- 
zontal glass, from 8.30 
A.M. to 7 P.M. March 20th. 
Movement of bead origin- 
ally magnified about 20 
times, here reduced to 
half scale. 



206 CIRCUMNUTATION OF STEMS. CHAP. IV 

accompanying figure (Fig. 76) gives the necessary particulars, 
and shows that the stem circumnutated, though rather 
slowly. 




Fitchsia (garden var.): circumuutation of stem, kejit 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 sp<ciocis.iimus (garden var., sometimes called 
Phyllocactus multiflorus) (Cactefe, 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 li in diameter, was chosen for observa- 
tion, as less likely to circumnutate than a flabelliform branch. 
The movement of the bead at the end of the glass filament, 
affixed to the summit of the branch, was traced (A, Fig. 77) 
from 9.23 A.M. to 4.30 P.M. on Nov. 23rd, during which time it 
changed its course greatly six times. On the 24th another 
tracing was made (see B), and the bead on this day changed its 
course oftener, making in 8 h. what may be considered as four 
ellipses, with their longer axes differently directed. The position 
of the stem and its commencing course on the following 
morning arc likewise shown. There can be no doubt that this 
branch, though appearing quite rigid, circumnutated; but the 



CHAP. IV. 



CIBCUMNUTATION OF STEMS. 



207 



exlreme amount of movement during the time was very small, 
probably rather less than the ^th of an inch. 

Fig. 77. 





Oerius speciocissimus : circumnutation of stem, illuminated from above, 
traced on a horizontal glass, in A from 9 A.M. to 4.30 P.M. on Nov. 
23rd ; and in B from 8.30 A.M. on the 24th to 8 A.M. on the 25th. 
Movement of the bead in B magnified about 38 times. 

(11.) Hfl^ra 'itlix (Araliacese, Fam. 114). The stem is known 
to be apheliotropic, and several seedlings growing in a pot in 
the greenhouse became bent in the middle of the summer at 
right angles f i om 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 
Blow rate, but there could be no doubt about their circumnuta- 
tion. The plants were kept exactly in the same place before the 
window, and after an interval of 15 days the stems were 
again observed during 2 days and their movements traced, aiid 



208 



CIRCUMNUTATION OF STEMS. 



CHAP. 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. 73. 




Gazania ringens: circunmutation of stem traced from 9 A.M. March 21st 
to 6 P.M. on 22nd; j>l;int 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 (Ericinese, Fam. 128). A bush 21 inches 
in height was selected for observation, and the circumuutation 
of its leading shoot was traced during 26 h. 40 m , as shown 
in the following figure (Fig. 79). 

(14.) Plumbago C<ipen*is (Plumbaginese, Fam. 134). A small 
lateral branch which projected from a tall freely growing biish, 
at an angle of 35 above the horizon, was selected for obser- 
vation. For the first 11 h. it moved to a considerable distance 
in a nearly straight line to one side, owing probably to its 
having been previously deflected by the light whilst standing in 
the greenhouse. At 7.20 P.M. on March 7th a fresh tracing was 
begun and continued for the next 43 h. 40 m. (see Fig. 80). 
During the first 2 h. it followed nearly the Fame direction as 
before, and then changed it a little; during the night it 
moved at nearly right angles to its previous course. Next 



CHAP. IV 



CIECUMNUTATION OF STEMS 



209 



day (8th) it zigzagged greatly, and on the 9th moved irregu- 
larly round and round a small circular space. By 3 P.M. on 
the 9th the figure had become so complicated that no more dots 
coiild be mado ; but the shoot continued during the evening of 
the 9th, the whole of the 10th, and the morning of the llth to 



Fig. 79. 




Azalea In dica : circumnutation 
of stem, illuminated from 
above, traced on horizontal 
glass, from 9.30 A.M. March 
9th to 12.10 P.M. on the 10th. 
But on the morning of the 
10th only four dots were 
made between 8.30 A.M. 
and 12.10 P.M., both hours 
included, so that the circum- 
nntation is not fairly repre- 
sented in this part of the 
diagram. Movement of the 
bead here magnified about 
30 times. 



Fig. 80. 




Plumbago Capensis : c 
tation of tip of 
branch, traced on h 



rizontal 



glass, from 7.20 .M. on 
March 7th to 3 I'.S on the 
9th, Movement f bead 
magnified 13 times. Plant 
feebly illuminated from 
above. 



circumnutate over the same small space, which was only about 
the -Troth of an inch ('97 mm.) in diameter. Although this 
branch circumnutated to a very small extent, yet it changed its 
course frequently. The movements ought to have been more 
magnified. 

(15.) Aloysia citriodora fVerbenacese, Fam. 173). The follow- 
ing figure (Fig. 81) gives the movements of a shoot during 



210 CIBCUMNUTATION OF STEMS. CHAP. IV. 

31 h. 40 m., and shows that it circumnutated. The bush was 
15 inches in height. 

Fig. 81. 




Aloysia citriodora : circumnutation of stem, traced from 8.20 A.M. on March 
22nd to 4 P.M. on 23rd. Plant kept in darkness. Movement raagmtm 
about 40 times. 

(16.) Verbena melindres (?)(a scarlet-flowered herbaceous var.) 
(Verbenacesj). 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 
11 inches. A glass filament, with a bead at the end, was fixed 




i-ej : circumuutadon 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 circum- 
stances the lateral movements were chiefly shown; but as the 
hues from side to side are not on the same level, the shoot 



CHAP. IV. CIRCUMNUTATION OF STEMS. 211 

must have moved in a plane at right angles to that of the lateral 
movement, that is, it must have circumnutated. On the next day 
(6th) the shoot moved in the course of 16 h. four times to the right, 
and four times to the left ; and this apparently represents the 
formation of four ellipses, so that each was completed in 4 h. 

(17.) Ceratophyllum demersum (Ceratophyllefe, Fam. 220). An 
interesting account of the movements of the stem of this water- 
plant has been published by M. E. Eodier.* The movements are 
confined to the young internodes, becoming less and less lower 
down the stem ; and they are extraordinary from their amplitude. 
The stems sometimes moved through an angle of above 200 in 
6 h., and in one instance through 220 in 3 h. They generally 
bent from right to left in the morning, and in an opposite direc- 
tion in the afternoon ; but the movement was sometimes tempo- 
rarily reversed or quite arrested. It was not affected by light. 
It does not appear that M. Eodier made any diagram on a hori- 
zontal plane representing the actual course pursued by the 
apex, but he speaks of the " branches executing round their 
axes of growth a -movement of torsion." From the particulars 
above given, and remembering in the case of twining plants and 
of tendrils, how difficult it is not to mistake their bending to all 
points of the compass for true torsion, we are led to believe that 
the stems of this Ceratophyllum circumnutate, probably in the 
shape of narrow ellipses, each completed in about 26 h. The 
following statement, however, seems to indicate something 
different from ordinary circumnutation, but we cannot fully 
understand it. M. Kodier says : " II est alors facile de voir que 
le mouvement de flexion se produit d'abord dans les merithalles 
superieurs, qu'il se propago ensuite, en s'amoindrissant du faint 
en.bas; tandis qu'au contraire le mouvement de redressement 
commence par la partie inferieure pour se terminer h la partie 
superieure qui, quelquefois, peu de temps avant de se relever 
tout a fait, forme avec Faxe uu angle tres aigu." 

(18 ) Conifirce. Dr. Maxwell Masters states (' Journal Linn 
Soc.,' Dec. 2nd, 1879) that the leading shoots of many Conifera 
during the season of their active growth exhibit very remark- 
able movements of revolving nutation, that is, they circumnu- 
tate. We may feel sure that the lateral shoots whilst growing 
would exhibit the same movement if carefully observed. 



* 'Comptes Rendus,' April 30th. 1877. Also a second notio 
published separately in Bourdeaux, Nov. 12th, 1877. 



21-2 
C1UO 



C1BCUMXUTATIOX OF STEMS. CHAP. IV 

auralum (Fam. Liliaceae). The circumnutatioc 
Fig. 83. 




Liiittm atf'-tu'n: circumnutationofastem in darkness, traced on a horizontal 
glass, from 8 A.M. on March 14th to 8.35 A.M. on 16th. But it should 
be noted that our observations were interrupted between 6 P.M. on the 
14th and 12.15 P.M. on 15th, and the movements during this interval 
of 18 h. 15 m. are represented bra 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. 83). 

Fig. 48. 




Cypena alternifolius : circumnutstion of stem, illuminated from above, 
trard 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. Cyperacese.) A glass 



CHAP. IV. CIECUMXUTATIOX OF STEMS. 213 

filament, with a bead at the end, was fixed across the summit 
of a young stem 10 inches in height, close beneath the crown of 
elongated leaves. On March 8th, between 12.20 and 7.20 P.M., 
the stem described an ellipse, open at one end. On the follow- 
ing day a new tracing was begun (Fig. 84), which plainly shows 
that the stem completed three irregular figures in the course of 
35 h. 15 m. 

Concluding Remarks on tlie 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 circummitate, not 
forgetting the number of plants distributed in the 
most distinct families which climb by a similar move- 
ment, will probably admit that the growing stems 
of all plants, if carefully observed, would be found 
to circumnutate to a greater or less extent. When 
we treat of the sleep and other movements of plants, 
many other cases of circumnutating stems will be 
incidentally given. In looking at the diagrams, we 
should remember that the stems were always growing, 
so that in each case the circumnutating apex as it 
rose will have described a spire of some kind. The 
dots were made on the glasses generally at intervals 
of an hour, or hour and a half, and were then joined 
by straight lines. If they had been made at intervals 
of 2 or 3 minutes, the lines would have been more 
curvilinear, as in the case of the tracks left on the 
loked glass-plates by the tips of the circumnutating 
licles of seedling plants. The diagrams generally 
ipproach 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 
line day or on succeeding days. The stems there- 



214 CIRCUMNUTATION OF STOLONS. CHAP. IV 

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 11J h., and those of the 
Trifolium three triangular or quadrilateral figures 
in 7 h. 

CIRCUMNUTATION OF STOLONS OR KUNNERS. 

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.): Rosacece. 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 12 h. it moved twice up 
and twice down in somewhat zigzag lines, and no doubt tra- 
velled in the same manner during the night. On the following 



CHAP. IV. CIRCUMNUTATION OF STOLONS. 



215 



morning after an interval of 20 h. the apex stood a 'little higher 
than it did at first, and this shows that the stolon had not beeo 

Fig. 85. 




Frarja'-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 retixed 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 tho course of 14 h. 30 m. 
five times up and four times down, besides some lateral move- 
ment. On the 20th the course was even more complicated, .and 
can hardly be followed in the figure ; but the filament moved in 
16 h. at least five times up and five times down, with very little 



* Dr. A. B. Frank stntos ('Die acted on by gfofropism, hut only. 
Naturliclie \vagi-rechte Itiehtung nfter a cons.idt-ruhlu interval o{ 
von Pflanzenth.-ilen,' 1870, p. 20) time, 
that the stolons of this plant are 
15 



216 



CIRCUMNUTATION OF STOLONS. CHAP. IV 



lateral deflection. The first and last dots made on this second 
day, viz., at 7 A.M. and 11 P.M., were close together, showing 
that the stolon had not fallen or risen. Nevertheless, by com- 
paring its position on 
the morning of the 19th 
and 21st, it is obvious 
that the stolon had sunk ; 
and this may be attri- 
buted to slow bending 
down either from its own 
weight or from geotro- 
pism. 

During a part of the 20th 
an orthogonal tracing was 
made by applying a cube 
of wood to the vertical 
glass and bringing the 
apex of the stolon at suc- 
cessive periods into a line 
with one edge; a dot 
being made each time on 
the glass. This tracing 
therefore represented very 
nearly the actual amount 
of movement of the apex ; 
and in the course of 9 h. 
the distance of the ex- 
treme dots from one an- 
other was '45 inch. By 
the same method it was 
ascertained that the apex 
moved between 7 A.M. on 
the 20th and 8 A.M. on the 

Wrngarin : circumnutation of the same stolon 21st a distance of '82 inch. 
as in the last figure, observed in the same A youn g er alK j shorter 
m:\nner, and traced from 8 A.M. May 19th " , 

to 8 A M 2lst. stolon was supported so 

that it projected at about 

45 above the horizon, and its movement was traced by the 
saino orthogonal method. On the first clay the apex soon 
rose above the field of vision. By the next morning it had 
sunk, and the course pursued was now traced during 14 h. 
30 m. (Fig. 87). The amount of movement was almost the same, 




CHAP IV. CIRCUMNUTATIOJf OF STOLONS. 217 

from side to side as up and down ; and differed in this respect 
remarkably from the movement in the previous cases. During 
the latter part of the day, viz., between 3 and 10.30 P.M., the 




Fraga'-in: circumnutation 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 T55 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 
wo could not perceive any effect from the above feeble degree of 
ilhimination. We may add that on another occasion, late in the 
summer, some stolons, placed upright before a south-west window 



1 Arbeiten Bot. Inst., Wiirzburg,' 1872, p. 431. 



2] 8 CIRCUMNUTATION OF STOLONS. CHAP IV 

3u a cloudy day, became distinctly curved towards the light, and 
were therefore heliotropic. Close in front of the tips of the 
prostrate stolons, a crowd of very thin sticks and the dried 
haulms of grasses were driven into the sand, to represent the 
crowded stems of surrounding plants in a state of nature. Thia 
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 sti 1 ! 
straight. But to this subject we shall recur under Saxifraga. 

Saxiffoga sarmentona (Saxifrageae). A plant in a suspended 
pot had omitted long branched stolons, which depended like 



Fig. 88. 




Sax'fraf/a sarmentosa: circumnutation of an inclined stolon, traced in 
darkness on a horizontal glass, from 7.45 A.M. April 18th to 9 A.M. oft 
9th. Movement of end of stolon magnified 2-2 times. 

threads on all sides. Two were tied up so as to stand vertically, 
and their upper ends became gradually bent downwards, but so 
slowly in the course of several days, that the bending was pro- 
bably due to their weight and not to geotropism. A glass fila- 
ment with little triangles of paper was fixed to the end of one of 
these stolons, which was 17J inches in length, and had already 
become much bent down, but still projected at a considerable 
angle above the horizon. It moved only slightly three times 
from sido to side and then upwards ; on the following day 



CHAP. IV. CIRCUMNUTATION OF STOLONS. 219 

the movement was even less. As this stolon was so long we 
thought that its growth was nearly completed, so we tried 
another which was thicker and shorter, viz., 10 i 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 15 h. The extreme actual amount of 
movement of the tip during the '25 h. was '75 inch. 

Several stolons were laid on a flat surface of damp sand, in the 
same manner as with those of the strawberry. The friction of 
the sand did not interfere with their circumnutation ; nor could 
we detect any evidence of their being sensitive to contact. In 
order to see how in a state of nature they would act, when 
encountering a stone or other obstacle on the ground, short 
pieces of smoked glass, an inch in height, were stuck upright 
into the sand in front of two thin lateral branches. Their tips 
scratched the smoked surface in various directions ; one made 
three upward and two downward lines, besides a nearly hori- 
zontal one; the other curled quite away from the glass; but 
ultimately both surmounted the glass and pursued their original 
course. The apex of a third thick stolon swept up the glass in a 
curved line, recoiled and again came into contact with it ; it then 
moved to the right, and after ascending, descended vertically ; 
ultimately it passed round one end of the glass instead of over it. 

Many long pins were next driven rather close together into 
the sand, so as to form a crowd in front of the same two thin 
lateral branches; but these easily wound their way through 
the crowd. A thick stolon was much delayed in its passage ; 
at one place it was forced to turn at right angles to its former 
course; at another place it could not pass through the pins, 
and the hinder part became bowed; it then curved upwards 
and passed through an opening between the upper part of some 
pins which happened to diverge ; it then descended and finally 
emerged" through the crowd. This stolon was rendered perma- 
nently sinuous to a slight degree, and was thicker where sinuous 
than elsewhere, apparently from its longitudinal growth having 
been checked. 

Cotyledon umbilicus (Crassulaceae). A plant growing in a pan 



220 



CIRCUMNUTATION OF STOLONS. 



CHAT. IV 



of damp moss had emitted 2 stolons, 22 and 20 inches in length, 
One of these was supported, so that a length of 4^ inches pro 
iected in a straight and horizontal line, and the movement 
of the apex was traced. The first dot was made at 9.10 A.M. 

Fig. 89. 




iJfa.rn.S7 1 ? 1 



59O'j> J m.B6 e * 



Uo'3&p.m.35<*' 



Cotyledon umbilicus: circumnutation of stolon, traced from 11.15 A.M 
Aug. 25th to 11 A.M. 27th. Plant illuminated from above. Th 
terminal internode was *25 inch in length, the penultimate 2 '25, auo 
the third 30 inches in length. Apex of stolon stood at a distance of 
5'75 inches from the vertical glass ; but it was not possible to ascertain 
how much the tracing was magnified, as it was not known how great 
a length of the internode circumnutated. 

the terminal portion soon began to bend downwards and con- 
tinued to do so until noon. Therefore a straight line, very 
nearly as long as the whole figure here given (Fig. 89), was first 
traced on the glass ; but the upper part of this line has not been 
copied in the diagram. The curvature occurred in the middle 



CHAP. IV. 



CIRCUMNUTATION OF STOLONS. 



221 



of the penultimate internode ; and its chief seat was at the 
distance of IT inch from the apex; it appeared due to the 
weight of the terminal portion, acting on the more flexible 
part of the internode, and not to gcotropism. The apex after 
thus sinking down from 9.10 A.M. to noon, moved a little to the 
left; it then rose up and circumnutated in a nearly vertical 
plane until 10.35 P.M. On the following day (26th) it was ob- 

Fig. 90. 




ITa.m i^. 

Cotyledon umbilicus: circutnnutation and downward movement of another 
itolon, traced on vertical glass, from 9.11 A.M. Aug. 25th to 11 A.M. 27th. 
Apex close to glass, so that figure but little magnified, and here reduced 
to two-thirds of original size. 

served from 6.40 A.M. to 5.20 P.M., and within this time it moved 
twice up and twice down. On the morning of the 27th the apex 
stood as high as it did at 11.30 A.M. on the 25th. Nor did it 
sink down during the 28th, but continued to circumnutate abotit 
the same place. 
Another stolon, which resembled the last in almost every 



222 CIRCUMNUTAT1ON OF STOLONS. CHAP. IV. 

respect, was observed during the same two days, but only two 
inches of the terminal portion was allowed to project freely and 
horizontally. On the 25th it continued from 9.10 A.M. to 1.30 P.M. 
to bend straight downwards, apparently owing to its weight 
(Fig. 90); but after this hour until 10.35 P.M. 'it zigzagged. 
This fact deserves notice, for we here probably see the combined 
effects of the bending down from weight and of circumnutation. 
The stolon, however, did not circuinuutate when it first began 
to bend down, as may be observed in the present diagram, and 
as was still more evident in the last case, when a longer portion 
of the stolon was left unsupported. On the following day 
(26th) the stolon moved twice up and twice down, but still con- 
tinued to fall ; in the evening and during the night it travelled 
from some unknown cause in an oblique direction. 

We see from these three cases that stolons or 
runners circumnutate in a very complex manner. The 
lines generally extend in a vertical plane, and this 
may probably be attributed to the effect of the weight 
of the unsupported end of the stolon ; but there is 
always some, and occasionally a considerable, amount 
of lateral movement. The circumnutation is so great 
in amplitude that it may almost be compared with 
that of climbing plants. That the stolons are thus 
aided in passing over obstacles and in winding between 
the stems of the surrounding plants, the observations 
above given render almost certain. If they had not 
circumnutated, their tips would have been liable to 
have been doubled up, as often as they met with 
obstacles in their path ; but as it is, they easily avoid 
them. This must be a considerable advantage to the 
plant in spreading from its parent-stock ; but we are 
far from supposing that the power has been gained 
by the stolons for this purpose, for circumnutation 
seems to be of universal occurrence with all growing 
parts; but it is not improbable that the amplitude 
of the movement may have been specially increased 
for this purpose. 



CHAI IV. CIECUMNUTATION OF FLOWEE -STEMS. 223 



ClRCUMNUTATION OF FLOWEE-STEMS. 

We did not think it necessary to make any special 
observations on the circumnutation of flower-stems, 
these being axial in their nature, like stems or stolons ; 
but some were incidentally made whilst attending 
to other subjects, and these we will here briefly give. 
A few observations have also been made by other 
botanists. These taken together suffice to render it 
probable that all peduncles and sub-peduncles cir- 
cunmutate whilst growing. 

Oxalis carnosa.Tlie peduncle which springs from the thick 
and woody stem of this plant bears three or foiu 1 sub-peduncles. 

Fig. 91. 




Oxalis carnosa : flower-stem, feebly illuminated from above, its circumnuta 
tion traced from 9 A.M. April "l 3th to 9 A.M. 15th. Summit of flower 
8 inches beneath the horizontal glass. Movement probably magnified 
about 6 times. 

A- filament with little triangles of paper was fixed within the 
calyx of a flower which stood upright. Its movements were 
observed for 48 h. ; during the first half of this time the flower 
was fully expanded, and during the second half withered. The 
figure here given (Fig. 91) represents 8 or 9 ellipses. Although 
the main peduncle circumnutated, and described one large and 



224 CIECUMNUTAT10N OF FLOWER-STEMS CHAI>. IV 

two smaller ellipses in the course of 2i h., yet the chief seat of 
movement lies in the sub-peduncles, which ultimately bend 
vertically downwards, as will be described in a future chapter. 
The peduncles of Oxalis aceiosella likewise bend downwards, and 
afterwards, when the pods are nearly mature, upwards ; and this 
is effected by a circumnutating movement. 

It may be seen in the above figure that the flower-stem of 
O.carnosa circumnutated during two days about the same spot. 
On the other hand, the flower-stem of U. xet/sitiua undergoes a 
strongly marked, daily, periodical change of position, when kept 
at a proper temperature. In the middle of the day it stands 
vertically up, or at a high angle ; in the afternoon it sinks, and 
in the evening projects horizontally, or almost horizontally, 
rising again during the night. This movement continues from 
the period when the flowers are in bud to when, as we believe, 
the pods are mature : and it ought perhaps to have been included 
amongst the so-called sleep-movements of plants. A tracing 
was not made, but the angles were measured at successive periods 
during one whole day; and these showed that the movement 
was not continuous, but that the peduncle oscillated up and 
down. We may therefore conclude that it circumnutated. At 
the base of the peduncle there is a mass of small cells, forming 
a well- developed pulvinus, which is exteriorly coloured purple 
and hairy. In no other genus, as far as we know, is the peduncle 
furnished with a pulvinus. The peduncle of O. Orhgesii behaved 
differently from that of O. sensitiua, for it stood at a less angle 
above the horizon in the middle of the day, than in the morning 
or evening. By 10.20 P.M. it had risen greatly. During the 
middle of the day it oscillated much up and down. 

TrifoUum subterraneum. A filament was fixed vertically to 
the uppermost part of the peduncle of a young and upright 
flower-head (the stem of the plant having been secured to a 
stick); and its movements were traced during 36 h. Within 
this time it described (see Fig. 92) a figure which represents four 
ellipses; but during the latter part of the time the peduncle 
began to bend downwards, and after 10.30P.M. on the 24th it 
curved so rapidly down, that by 6.45 A.M. on the 25th it stood 
only 19 above the horizon. It went on circumnutating in nearly 
the same position for two days. Even after the flower-heads 
have buried themselves in the ground th-sy continue, as will 
hereafter be shown, to circumnutate. It will also be seen in the 
next chapter that the sub-peduncles of the separate flowers of 



CHAP. IV. CIRCUMNUTATION OF FLOWER-STEMS. 225 

Trifolium repens circumnutate in a complicated course during 
several days. I may add that the gynophore of Arachis hypogcea, 



Fig. 92. 




Trifolium subterraneum : main flower-peduncle, illuminated from above, 
circumnutation traced on horizontal glass, from 8.40 A.M. July 23rd 
to 10.30 P.M. 24th. 

which looks exactly like a peduncle, circumnutates whilst growing 
vertically downwards, in order to bury the young pod in the 
ground. 

The movements of the flowers of Cyclamen Persicum were not 
observed; but the peduncle, whilst the pod is forming, increases 
much in length, and bows itself down by a circumnutating 
movement. A young peduncle of Maurandia semperflorens, 
li inch in length, was carefully observed during a whole day, 
and it made 41 narrow, vertical, irregular and short ellipses, 
each at an average rate of about 2 h. 'J5 m. An adjoining 
peduncle described during the same time similar, though fewer, 
ellipses.* According to Sachs f the flower-stems, whilst growing, 

* ' The Movements and Habits 1875, p. 68. 
of Climbing Plants,' 2nd pdit., f ' Text-Book of Botany,' 1875 



226 CIRCUMNUTATION OF LEAVES. CHAP. IV. 

of many plants, for instance, those of Brassica napus, revolve or 
circutn nutate; those of Alliam porrum bend from side to side, 
aud, if this movement had been traced on a horizontal glass, 
no doubt ellipses would have been formed. Fritz Miiller has 
described * the spontaneous revolving movements of the flower- 
stems of an Alisma, which he compares with those of a climbing 
plant. 

We made no observations on the movements of the different 
parts of flowers. Morren, however, has observed f in the 
stamens of Sparmannia and Cereus a " fren Jsseaient spontane," 
which, it may be suspected, is a circumnutating movement. 
The circumnutation of the gynostemium of Stylidium, as de- 
scribed by Gad,J is highly remarkable, and apparently aids in 
the fertilisation of the flowers. The gynostemium, whilst spon- 
taneously moving, comes into contact with the viscid labellum, 
to which it adheres, until freed by the increasing tension of the 
parts or by being touched. 

We have now seen that the flower-stems of plants 
belonging to such widely different families as the 
Cruciferae, Oxalidse, Leguminoste, Primulacere, Scro- 
phularineae, Alismaceae, and Liliacefe, circumnutate ; 
and that there are indications of this movement in 
many other families. With these facts before us, 
bearing also in mind that the tendrils of not a few 
plants consist of modified peduncles, we may admit 
without much doubt that all growing flower-stems 
circumnutate. 

CIRCUMNUTATIOX OP LEAVES : DICOTYLEDONS. 

Several distinguished botanists, Hofmeister, Sachs, 
Pfeffer, De Vries, Batalin, Millardet, &c., have ob- 



p. 766. Lmnrens and Trevintnus plies drcumnutation. 

(according to Pfeffer, 'Die Pe- * 'Jenaische Zeitsch.,' B. T. 

riodischen Bewegungen,' &c., p. p. 133. 

162) stato that the flower-stalks f ' N". Mem. de 1'Acad. R. da 

of many plants occupy different Bruxelles,' torn. xiv. 1841. p. 5$. 

positions by night and day, and J ' Sitzungbericht des hot. Ve- 

we shall see in the chapter on reiis der P. Brandenburg,' xxi 

the Sleep of Plants that this im- p. 84. 



DICOTYLEDONS. 227 

served, and some of them with the greatest care, the 
periodical movements of leaves ; bat their attention 
has been chiefly, though not exclusively, directed to 
those which move largely and are commonly said to 
sleep at night. From considerations hereafter to be 
given, plants of this nature are here excluded, and 
will be treated of separately. As we wished to ascer- 
tain whether all young and growing leaves circumnu- 
tated, we thought that it would be sufficient if we 
observed between 30 and 40 genera, widely distributed 
throughout the vegetable series, selecting some un- 
usual forms and others on woody plants. All the 
plants were healthy and grew in pots. They were 
illuminated from above, but the light perhaps was not 
always sufficiently bright, as many of them were ob- 
served under a skylight of ground-glass. Except in a 
few specified cases, a fine glass filament with two minute 
triangles of paper was fixed to the leaves, and their 
movements were traced on a 
vertical glass (when not stated 
to the contrary) in the manner 
already described. I may repeat 
that the broken lines represent 
the nocturnal course. The stem 
was always secured to a stick, 
close to the base of the leaf 
under observation. The ar- 
rangement Of the Species, with s^cenfapurpurea: cireum- 

the number of the Family ap- nutation of yonng pitcher, 

- ... i traced from 8 A.M. July 3rd 

pended, is the same as in the to 10.15 A.M. 4th. Temp, 
case of stems. I? - 1 8 c - ^ ex of 'pitcher 

20 inches from glass, so 
movement greatly mag- 

(1.) Sarracenia purpurea (Sarra- n ified. 
cenose, Fam. 11). A young leaf, or 

pitcher, 8 inches in height, with the bladder swollen, hut with 
the hood not as vet open, had a filament fixed transversely 




228 



CIRCUMNUTATION OF LEAVES. 



CHAP. IV. 



Fig. 94 



across its apex ; it was observed for 43 h., and during the whole 
of this time it circumnutated in a nearly similar manner, but 
to a very small extent. The tracing given (Fig. 93) relates 
only to the movements during the first 20 h. 

(2.) fjluucium luteum (Papave- 
raceso, Fam. 12). A young plant, 
bearing only 8 leaves, had a fila- 
ment attached to the youngest leaf 
but one, which was 3 inches in 
length, including the petiole. The 
circumnutating movement was 
traced during 47 h. On both days 
the leaf descended from before 7 A.M. 
until about 11 A.M., and then 
ascended slightly during the rest 
of the day and the early part of 
the night. During the latter part 
of the night it fell greatly. It did 
not ascend so much during the 
second as during the first day, and 
it descended considerably lower on 
the second night than on the first. 
This difference was probably due 
to the illumination from above 
having been insufficient during the 
two days of observation. Its course 
during the two days is shown in 
Fig. 94. 

(3.) Crambe maritima (Cruciferae, 
Fam. 14). A leaf 9 inches in length 
on a plant not growing vigorously 

/was first observed. Its apex was 
in constant movement, but. this 
could hardly be traced, from being 
so small in extent. The apex, how- 
ever, certainly changed its course at 
least 6 times in the course of 14 h. 
A more vigorous young plant, tear- 
ing only 4 leaves, was then selected, 
and a filament was affixed to the 
midrib of the third leaf from the base, which, with the petiole, was 
5 inches in length. The leaf stood up almost vertically, but the t ip 




Glaucium lulettm: circumnuta- 
tion of young leaf, traced 
from 9.30 A.M. June 14th 
to 8.30 A.M. 16th. Tracing 
not much magnified, as apex 

from the glass. 



CHAP. IV. 



DICOTYLEDONS. 



229 



rig. 95. 



was deflected, so that the filament projected almost horizontally, 
and its movements were traced during 48 h. on a vertical glass, 
as shown in the accompanying figure (Fig. 95). We here plainly 
see that the leaf was con- 
tinually circumnutating ; 
but the proper periodicity 
of its movements was dis- 
turbed by its being only 
dimly illuminated from 
above through a double 
skylight. We infer that 
this was the case, because 
two leaves on plants grow- 
ing out of doors, had their 
angles above the horizon 
measured in the middle 
of the day and at 9 to 
about 10 P.M. on succes- 
sive nights, and they 
were found at this latter 
hour to have risen by an 
average angle of 9 above 
their rnid-day position : 
on the following morning 
they fell to their former 
position. Now it may be 
observed in the diagram 
that the leaf rose during 
the second night, so that 
it stood at 6.40 A.M. higher 
than at 10/20 P.M. on the Crambe maritima: circumnutation of leaf, 
preceding night; and this disturbed by being insufficiently illumi- 
, i ot+^K,,*^ * *!, nated from above, traced from 7.50 A.M. 
June 23rd to 8 A.M. 25th. Apex of leaf 
15J inches from the vertical glass, so that 
the tracing was much magnified, but is 
here reduced to one-fourth of original scale. 

(4.) Bratsica ohracta (Crucifene). Hofmeister and Batalin * 
state that the leaves of the cabbage rise at night, and fall by 
d;iy. We covered a young plant, bearing 8 leaves, under a large 
bell-glass, placing it in the same position with respect to the 



7: Sift 




may be attributed to the 
leaf adjusting itself to the 
dim light, coming exclu- 
sively from above. 



Flora,' 1873, p. 437 



230 CIRCUMNUTATION OF LEAVES. CHAP. IV 

light in which it had long remained, and a filament was fixed 
at the distance of '4 of an inch from the apex of a young leaf 
nearly 4 inches in length. Its movements were then traced 
during three days, but the tracing is not worth giving. The 
leaf fell during the whole morning, and rose in the evening and 
during the early part of the night. The ascending and descend- 
ing lines did not coincide, so that an irregular ellipse was formed 
each 24 h. The basal part of the midrib did not move, as was 
ascertained by measuring at successive periods the angle which 
it formed with the horizon, so that the movement was confined 
to the terminal portion of the leaf, which moved through an 
angle of 11 in the course of 24 h., and the distance travelled by 
the.apex, up and down, was between '8 and '9 of an inch. 

In order to ascertain the effect of darkness, a filament was 
fixed to a leaf 05 inches in length, borne by a plant which after 
forming a head had produced a stem. The leaf was inclined 
44 above the horizon, and its movements were traced on a 
vertical glass every hour by the aid of a taper. During the 
first day the leaf . rose from 8 A.M. to 10.40 P.M. in a slightly 
zigzag course, the actual distance travelled by the apex being 
67 of an inch. During the night the leaf fell, whereas it ought 
to have risen ; and by 7 A.M. on the following morning it had 
fallen '23 of an inch, and it continued falling until 9.40 A.M. It 
then rose until 10.50 P.M., but the rise was interrupted by one 
considerable oscillation, that is, by a fall and re-ascent. During 
the second night it again fell, but only to a very short distance, 
and on the following morning re-ascended to a very short 
distance. Thus the normal course of the leaf was greatly 
disturbed, or rather completely inverted, by the absence of 
light ; and the movements were likewise greatly diminished in 
amplitude. 

We may add that, according to Mr. A. Stephen Wilson * the 
young leaves of the Swedish turnip, which is a hybrid between 
B. oleracea and rapa, draw together in the evening so much 
"that the horizontal breadth diminishes about 30 per cent, of 
the daylight breadth." Therefore the leaves must me con- 
siderably at night. 

(5.) Dianthus caryoplyllus (Caryophyllcse, Fain. 26). The 



* ' Trans. Bot. fine. Edinburgh,' see Darwin. 'Animals and Plants 
vol. xiii. p. i!2. With respect to under Domestication,' 2nd edit 
the origin of the Swedish turnip, vol. i. p. 3i. 



CHAP IV. DICOTYLEDONS. 231 

terminal shoot of a young plant, growing very vigorously, was 
selected for observation. The young leaves at first stand up 
vertically and close together, but they soon bend outwards and 
downwards, so as to become horizontal, and often at the same 
time a little to one side. A filament was fixed to the tip of a 
young leaf whilst still highly inclined, and the first dot was 
made on the vertical glass at 8.30 A.M. June 13th, but it curved 
downwards so quickly that by 6.40 A.M. on the following 
morning it stood only a little above the horizon. In Fig. 96 



Fig. 96 





1D C '35 'p .misfy 



Dianthus caryophyllus : circumnutation of young leaf, traced from 10.15 
P.M. June 13th to 10..35 P.M. 10th. Apex of leaf stood, at the close of 
our observations, 8f inches from the vertical glass, so tracing not 
greatly magnified. The leaf was 5 inches long. Temp. 15-17$ C. 

the long, slightly zigzag line representing this rapid downward 
r-ourse, which was somewhat inclined to the left, is not given- 
but the figure shows the highly tortuous and zigzag course, 
icgether with some loops, pursued during the next 2i days. 
As ths leaf continued to move all the time to the left, it is 
evident that the zigzag line represents many circumnutations. 

(6.) Camellia Japonica (Camelliacese, Fain. 32). A youngish 
leaf, which together with its petiole was 21 inches in length and 
which arose from a side branch on a tall bush, had a filament 
attached to its apex. This leaf sloped downwards at an angle 
of 40 beneath the horizon. As it was thick and rigid, and its 
10 



232 



C1RCUMNUTATIOX OF LEAVES. 



CIIAP. 



petiole 



Fig. 97. 



very short, much movement could not be expected. 
Nevertheless, the apex changed its course 
completely seven times in the course of 
Hi 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. 
cjr (as shown in Fig. U7), and was nearly of 
j ea f the same nature, but rather less complex. 
6.40 The movement seems to be periodical, for 
on both days the leaf circumnutated in tho 
forenoon, fell in the afternoon (on the first 



from 

A.M. June 14th to 
6.50 A.M. loth. 
Apex of leaf 12 
inches from the ver- day until between o and 4 P.M., and on the 
tical glass, so figure 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 



considerably mag- 
nified. Temp. 16- 
16J C. 




6.30'n.m. 

10.S5'p.m.l 

Pelargonium zonale : circumnutation and downward movement of yottiijr 
leaf, traced from 9.30 A.M. June 14th to 6.30 P.M. 16th. Apex of leaf 
9| inches from the vertical glass, so figure moderately magnified. 
Temp. 15-16J C. 

at night; and as they often do not then occupy a vertical 
position, especially if they have not been well illuminated during 



CHAP. IV. DICOTYLEDONS. 233 

the day, it is doubtful whether some of these cases ought not 
to have been included in the present chapter. 

(7.) Pelargonium zonale (Geraniaceas, Fam. 47). A young 
leaf, lr inch in breadth, with its petiole 1 inch long, borne on 
a young plant, was observed in the usual manner during 61 h. ; 
and its course is shown in the preceding figure (Fig. 98). 
During the first day and night the leaf moved downwards, but 
circumnutated between 10 A.M. and 4.30 P.M. On the second 
day it sank and rose again, but between 10 A.M. and 6 P.M. it 
circumnutated on an extremely small scale. On the third day 
the circumnutation was more plainly marked. 

(8.) Cissus discolor (Ampelidese, Fam. 67). A leaf, not nearly 
full-grown, the third from the apex of v \g. 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 (Leguminosse, 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 51i h. The filament fell 
all morning (July 2nd) tffl 3 P.M., and 
then rose greatly till 10.3o P.M. ; but tation of leaf, traced 
the rise this day was so great, com- from 10.35 A.M. May 

pared with that which subsequently ^ 8th *? , 6 ** . 29 * h - 
, , . 1,1, . Apez of leaf 82 inches 

occurred, that it was probably due in from the ver tical 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 



CIKCUMNUTATION OF LEAVES. 



CHAP. IV 



As tlic evening rise and the early morning full were unusually 
large, the angle of the petiole above the horizon was measured 
it the two periods, and the. leaf was found to have risen 1'.) 

Fig. 100. 




l*fet'a faba: circumnutation of leaf, traced from 7.15 P.M. July 2nd tc 
10.15 A.M. 4th. Apex of the two terminal leaflets 1\ inches from the 
vertical glass. Figure here reduced to two-thirds of original scale. 
Temp. 17-18 C. 

between 12.20 P.M. and 10.45 P.M., and to have fallen 23 30 1 

between the latter hour and 10.20 A.M. on the following morning. 

The main petiole was now secured to a stick close to the base 



CHAP. IV. 



DICOTYLEDONS. 



235 



of 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 theif 

Fig. 101. 



J.0*40'a.m.4, 




\ 



Vicia faba: circumnutation of one of the two terminal leaflets, the mala 
petiole having been secured, traced from 10.40 A.M. July 4th to 10.30 A.!*. 
6th. Apex of leaflet 6| inches from the vertical glass. Tracing IKI 
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. 



230 



CIRCUMNUTATION- OF LEAVES. CHAP. 



Fig 102. 




Fig. 103. 



(10.) Acacia rctinoides (Leguminosae). The movement of a 

young phyllode, 2 inches in length, and inclined at a consider- 
able angle above the horizon, was traced 
during 45 h. 30 in. ; but in the figure here 
given (Fig. 102), its circumnutation is shown 
during only 21 h. 30 m. During part of 
this time (viz., 14 h. 30 m.) the phyllode 
described a figure re- 
presenting 5 or 6 
small ellipses. The 
actual amount of 
movement in a ver- 
tical direction was '3 
inch. The phyllode 
rose considerably be- 
tween 1.30 P.M. and 
4 P.M., but there was 
no evidence on either 
day of a regular pe- 
riodic movement. 

(11.) Lvpintis s r c- 
ciosus (Leguminosre). 
Plants were raised 

from seed purchased under this name. 

This is one of the species in this large 

genus, the leaves of which do not sleep 

at night. The petioles rise direct from 

the ground, and are from 5 to 7 inches 

in length. A filament was fixed to the 

midrib of one of the longer leaflets, and 

the movement of the whole leaf was traced, 

as shown in Fig. 103. In the course of 

6 h. 30 m. the filament went four times up 

and three times down. A new tracing 

was then begun (not hero given), and 

during 12^ h. the leaf moved eight times 

up and seven times down; so that it 

described 7j ellipses in this time, and 

this is an extraordinary rate of movement. 

The summit of the petiole was then secured 

to a stick, and the separate leaflets were found to be continually 

circumnutating. 



Acacia retino'des : cir- 
cumnutation of a 
young phyllode, 
traced from 10.45 
A.M. July 18th to 
8.15 A.M. 19th. 
Apex of phyllode 9 
inches from the 
vertical glass; tern p. 
16i-17J C. 




: cir- 
' leaf 
traced on vertical 
glass, from 10.15A.M. 
to 5.45 P.M.; i.e., 
during 6 h. 30 m. 




CHAP. IV. DICOTYLEDONS. 237 

(12.) Eclieveria stolonifera (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- ^' ^^' 

nutation could be detected. A fila- 
ment was fixed to a young upwardly 
inclined leaf, -75 inch in length and 
"28 in breadth, which stood on the 
outside of a terminal rosette of leaves, 
produced by a plant growing very 
vigorously. Its movement was traced 
during 3 days, as here shown (Fig. 
104). The course was chiefly in an 
upward direction, and this may be 
attributed to the elongation of the 
leaf through growth ; but we see that 
the lines are strongly zigzag, and that *rf~^r\ 
occasionally there was distinct cir- / 

cumnutation, though on a very small Echeverit stolonifera : circutn- 
sca]e nutation of leaf, traced 

(13.> Bryophyllum (vel Calanchce) taMEUS A M. 28th. Apex 

calycinum (Crassulacese). Duval- of leaf 12* inches from the 

JOUVO ('Bull. SOC. Bot. de France,' glass, so that the movement 

p i lAi-v. IQCO\ -i j.t T was much magnified; temp. 

Feb. 14th, 1868) measured the dis- 23-24 l0 C. 

tance between the tips of the upper 

pair of leaves on this plant, with the result shown in the following 

Table. It should be noted that the measurements on Dec. 2nd 

were made on a different pair of leaves : 

8 A.M. 2 P.M. 7 P.M. 

Nov. 16 . .15 mm. . . .25 mm. ...(?) 

19 ... 48 ... 60 ... 48 mm. 
Dec. 2 ... 22 ... 43 ... 28 

"We see from this Table that the leaves stood considerably 
further apart at 2 P.M. than at either 8 A.M. or 7 P.M. ; and this 
shows that they rise a little in the evening and fall or open 
in the forenoon. 

(14.) Drosera rotundifolia (Droseracese, Fam. 85). The move- 
ments of a young leaf, having a long petiole but with iis tentacles 
(01 gland-bearing hairs) as yet unfolded, were traced during 
47 h. 15 m. The figure (Fig. 105) shows that it circumnutated 
largely, chiefly in a vertical direction, making two ellipses each 



238 



CIRCUMNUTATION OF LEAVES. CHAP. IV. 



Fig. 105. 



45'pm. 



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 observ- 
ing three other leaves during 
several successive days and 
nights, we found this to be an 
error; and the case is given 
merely as a caution. On the 
third morning the above le.af 
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 whicli 
time it continued to sink a 
little in a slightly zigzag line. 
On the following morning, at 
Drosera rotundifolia : circumnutation 7 A.M., a drop of a solution 
of young leaf, with filament fixed o f car bonate of ammonia (2 
to back of blade, traced from 9.1o , , ,. . \ 

A.M. June 7th to 8.30 A.M. June &' to 1 7 " f ter > WftS 
9th. Figure here reduced to one- placed on the disc, and this 
.half original scale. 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 3 P.M. It then circumnutated about 
the same spot on a very small scale for 21 h. ; and during the 
next 21 h. it sank in a zigzag line to nearly the same level 
which it had held when the ammonia was first administered. 
By this time the tentacles had re-expanded, and the glands had 
recovered their proper colour. We thus learn that an old leal 




CHAP. IV. DICOTYLEDONS. 239 

circtimnutates on a small scale, at least whilst absorbing car- 
bonate of ammonia ; for it is probable that this absorption may 
stimulate growth and thus re-excite circumnutation. Whether 
the rising of the glass filament which was attached to the back 
of the leaf, resulted from its margin becoming slightly inflected 
(as generally occurs), or from the rising of the petiole, was not 
ascertained. 

In order to learn whether the tentacles or gland-bearing hairs 
circumuutate, 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 - 5 -^o 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 
considenibly 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 5-^ of an inch in 
20 m., or ^ of inch in 1 h. 40 m. ; but as it likewise moved 
from side to side to an extent of above ^ f mcu > tne move- 
ment was probably one of modified circumuutation. A tentacle 
on an old leaf was next observed in the same manner. In 
15 m. after being placed under the microscope it had moved 
about -5^00 of an inch. During the next 7 ' h. it was looked at 
repeatedly, and during this whole time it moved only another 
YoVo 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 rasv 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.) Dionwa iniuscipv.la (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 tbe petiole, and ultimately in 
nearly a straight line with it. A young leaf, which with I hit 



240 



CIRCUMNUTAT10N OF LEAVES. 



CIIAP. IV 



Fig. 106. 



petiole was only T2 inch in length, had a filament fixed exter- 
nally along the midrib of the still closed lobes, which projected 
at right angles to the petiole. In the evening this leaf com- 
pleted an ellipse in the course of 2 h. On 
the following day (Sept. 25th) its move- 
ments were traced during 22 h. ; and we 
see in 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 bo 
no doubt that this young leaf circumnu- 
tated. 

A rather old, horizontally extended 
leaf, with a filament attached along the 
under side of the midrib, was next 
observed during 7 h. It hardly moved, 
but when one of its sensitive hairs 
was touched, the blades closed, though 
not very quickly. A new dot was now 
made on the glass, but in the course of 
14 h. 2J m. there was hardly any change 
in the position of the filament. We may 
therefore infer that an old and only 
moderately sensitive leaf does not circum- 
nutate plainly ; but we shall soon see 
that it by no means follows that such 
a leaf is absolutely motionless. We may 
further infer that the stimulus from a 
zontal plass in dark- touch does not re-excite plain circumnu- 
ness, from noon Sept. f^ion. 

Apex of leaf 13J Another full-grown leaf had a filament 
inches from the glass, attached externally along one side of the 
so tracing consider- midrib and parallel to it, so that the fila- 

ably magnified. , , j ... 

ment would move if the lobes closed. It 

should be first stated that, although a touch on one of the sensi- 
tive hairs of a vigorous leaf causes it to close quickly, often 
almost instantly, yet when a bit of damp meat or some solution 
of carlionate 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 circumnutatc, but it ought to have been observed for a 




Docea mvscfpuia : cir- 
cumnutation of a 
young and expanding 
leaf, traced on a hori- 



CHAP. IV. DICOTYLEDONS 241 

longer period ; although, as we have seen, a young leaf com- 
pleted a fairly large ellipse in 2 h. A drop of an infusion of 
raw meat was then placed ou the leaf, and within 2 h. the glass 
filament rose a little ; and this implies that the lobes had begun 
to close, and perhaps the petiole to rise. It continued to rise 
with extreme slowness for the next 8 h. 30 m. The position of 
the pot was then (7.15 P.M., Sept. 24th) slightly changed and 
an additional drop of the infusion given, and a new tracing 
was begun (Fig. 107). By 10.50 P.M. the filament had risen 
only a little more, and it fell during the night. On the follow- 
ing morning the lobes were closing more quickly, and by 5 P.M. 
it was evident to the eye that they had closed considerably ; by 
8.48 P.M. this was still plainer, and by 10.45 P.M. the marginal 
spikes were interlocked. The leaf fell a little during the night, 
and next morning (25th) at 7 A.M. the lobes were completely 
shut. The course pursued, as may be seen in the figure, was 



Fig. 107. 




Dionasa 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 8 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, 
vary 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 



242 C1RCUMNUTATIOX OF LEAVES. CHAP. IV. 

seem to be continually opening and shutting to a very small 
extent. A nearly full-grown leaf (afterwards proved to be 
highly sensitive to contact) stood almost horizontally, so 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 ^ of an iuch. The apex of 
the paper-triangle was now seen to be in constant slight move- 
ment ; for in 4 h. it crossed nine divisions, or g-^ of an inch, 
and after ten additional hours it moved back and had crossed 
5^ in an opposite direction. The plant was kept in rather 
too cool a place, and on the following day it moved rather less, 
namely, -^ in 3 h., and -^ in an opposite direction during the 
next G h. The two lobes, therefore, seem to be constantly 
closing or opening, though to a very small distance; for we must 
remember that the little triangle of paper affixed to the marginal 
spike increased its length, and thus exaggerated somewhat the 
movement. Similar observations, with the important difference 
that the petiole was left free and the plant kept under a high 
temperature, were made on a leaf, which was healthy, but so old 
that it did not close when its sensitive hairs were repeatedly 
touched, though judging from other cases it would have slowly 
closed if it had been stimulated by animal matter. The apex of 
the triangle was in almost, though not quite, constant movement, 
sometimes in one direction and sometimes in an opposite one ; 
and it thrice crossed five divisions of the micrometer (i.e. y^ of 
an inch) in 30 m. This movement on so small a scale is hardly 
comparable with ordinary circumnutation ; but it may perhaps 
be compared with the zigzag lines and little loops, by which the 
larger ellipses made by other plants are often interrupted. 

In the first chapter of this volume, the remarkable oscillatory 
movements of the circumnutating hypocotyl of the cabbage 
have been described. The leaves of Dionaca present the same 
phenomenon, which is a wonderful one, as viewed under a lovr 
power (2-inch object-glass), with an eye-piece micrometer of 
which each division (^ of an inch) appeared as a rather wide 
space. The young unexpanded leaf, of which the circumnutating 
movements were traced (Fig. 106), had a glass filament fixed 
perpendicularly to it; and the movement of the apex was 
observed in the hot-house (temp. 81 to 86 F.), with light 
admitted only from above, and with any lateral currents of air 



tfUAr. 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 yoVo or icfoo? an( ^ * u 
one instance of TO %Q 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 yoW inch ; and there were 
longer motionless periods. As it appeared possible that the 
movements might be due to currents of air, a wax taper was 
held close to the leaf during one of the motionless periods, but 
no oscillations were thus caused. After 10 m., however, vigorous 
oscillations commenced, perhaps owing to the plant having been 
warmed and thus stimulated. The candle was then removed and 
before long the oscillations ceased ; nevertheless, when looked at 
again after an interval of 1 h. 30 m., it was again oscillating. 
The plant was taken back into the hot-house, and on the 
following morning was seen to be oscillating, though not very 
vigorously. Another old but healthy leaf, which was not in the 
least sensitive to a touch, was likewise observed during two 
days in the hot-house, and the attached filament made many 
little jerks forwards of about y %o or only y^ 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 y^ of 
an inch stil] continued. On the following day a little infusion 



244 



CIKCUHNUTATION OF LEAVES. CHAP IV 



of raw meat was placed on the leaf, which caused the lobes tc 
close together very slowly in the course of two days ; and the 
oscillations continued during this whole time and for the next 
two days. After nine additional days the leaf began to open 
and the margins were a little everted, and now the apex of the 
glass filament remained for long periods motionless, and then 
moved backwards and forwards for a distance of about i<} 00 of 
an inch slowly, without any jerks. Nevertheless, after warming 
the leaf with a taper held close to it, the jerking movement 
recommenced. 

This same leaf had been observed 2.] months previously, and 
was then found to be oscillating or jerking. We may therefore 
infer that this kind of movement goes on night and day for a 
very long period ; and it is common to young unexpanded leaves 
and to leaves so old as to have lost their sensitiveness to a 
touch, but which were still capable of absorbing nitrogenous 
matter. The phenomenon when well displayed, as in the young 
leaf just described, is a very interesting one. It often brought 
before our minds the idea of effort, or of a small animal 
struggling to escape from some constraint. 
(16.) Eucalyptus rtslnifera (MyrtacesB, Fam. 94). A young leaf, 
two inches in length together with 
the petiole, produced by a lateral 
shoot from a cut-down tree, was 
observed in the usual manner. 
The blade had not as yet as- 
sumed its vertical position. On 
June 7th only a few observations 
were made, and the tracing merely 
showed that the leaf had moved 
three times upwards and three 
downwards. On the following 
day it was observed more fre- 
quently; and two tracings were 
made (see A and B, Fig. 108), as 
a single one would have been too 
complicated. The apex changed 
its course 13 times in the course 
of 16 h., chiefly up and down, but 
The actual amount of movement 



Fig. 108. 




Kucalnptus reiinifera : circumuu- 
tation of a leaf, traced, A, from 
G.40 A.M. to 1 P.M. June 8th ; 
B, from 1 P.M. 8th to 8.30 A.M. 
9th. Apes of leaf U inches 
from the horizontal glass, so 
figures considerably magnified. 



with some lateral movement. 
in any one direction was small. 
(17 ) Dallia (garden var.) (Composite, Fam. 122). A fine young 



THAP. IV. 



DICOTYLEDONS. 



245 



leftf 5f inches in length, produced by a young plant 2 feet high, 
growing vigorously in a large pot, was directed at an angle of 
about 45 beneath the horizon. On June 18th the leaf descended 
from 10 A.M. till 11 35 A.M. (see Fig. 109) ; it then ascended 
greatly till 6 P.M., this ascent being probably due to the light 

Fig. 109. 




Dtihh-t: circumnutation of leaf, traced from 10AM. June 18th to 8.10 A.M. 
20th, but with a break of 1 h. 40 m. on the morning of the 19th, as, 
owing to the glass filament pointing too much to one side, the pot had 
to be slightly moved ; therefore the relative position of the two tracings 
is somewhat arbitrary. The figure here given is reduced to one-fifth of 
the original scale. Apex of leaf 9 inches from the glass in the line 
of its inclination, and 4| in a horizontal line. 

coming only from above. It zigzagged between 6 r.M. and 
10.35 P.M., and ascended a little during the night. It should bo 
remarked that the vertical distances in the lower part of the 
diagram are much exaggerated, as the leaf was at first deflected 
beneath the horizon, and after it had sunk downwards, the 
filament pointed in a very oblique line towards the glass. Next 



246 CIRCUMNUTATION OF LEAVES. CHAP. IV. 

day the leaf descended from 8.20 A.M. till 7.15 P.M., then zigzagged 
and ascended greatly during the night. On the morning of 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.) Muttsia clematis (Composite). The leaves terminate in 
tendrils and circumnutate like those of other tendril-bearers ; 
but this plant is here mentioned, on accoimt 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.) Cyclamen Persicum (Primulacese, Fam. 135). A young 
leaf, 1'8 of an inch in length, petiole included, produced by an 
old root-stock, was observed during three days in the usual 
manner (Fig. 110). On the first day the leaf fell more than after- 
wards, apparently from adjusting itself to the light from above. 
On all three days it fell from the early morning to about 7 P.M., 
and from that hour rose during the night, the course being 
slightly zigzag. The movement therefore is strictly periodic. 
It should be noted that the leaf would have sunk each evening 
a little lower down than it did, had not the glass filament rested 
between 5 and 6 P.M. on the rim of the pot. The amount of 
movement was considerable ; for if we assume that the whole 
leaf to the base of the petiole Became bent, the tracing would 
be magnified rather less than five times, and this would give 
to the apex a rise and fall of half an inch, with some lateral 
movement. This amount, however, would not attract attention 
without the aid of a tracing or measurement of some kind. 



'Tho Movements and Habits of Climbing Plants,' 1875, p. 118. 



CHAP. IV. 



DICOTYLEDONS. 



247 



(20.) Allamanda Schottii (Apocynese, Fam. 144). The young 
loaves of this shrub are elongated, with the blade bowed so much 

Fig. 110. 




m 4'? 1 

Cyclamen Persicum : circumnutation of leaf, traced from 6.45 A.M. Jane 2afl 
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 
petioleof a young leaf. 41 inches in length, stood at 2.50 P.M on 
17 



248 



CIKCUMNUTATION OF LEAVES. 



CHAP. 17 



Doc. 5th at an angle of 13 beneath the horizon, but by 9.30 P.M, 

the blade had straightened itself 

Fl S- 111% so much, which implies the 

\raising of the apex, that the 
chord now stood at 37 above the 
horizon, and had therefore risen 
- 50. On the next day similar 

angular measurements of the 
same leaf were made; and at 
noon the chord stood 36 be- 
neath the horizon, and 9.30 P.M. 
3J above it, so had risen 39J. 
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 
4. in the curvature of the young 

y<\ leaves; so that previous expo- 

/ *^^ sure to a strong light is appa- 

^~-~w7 rently requisite for the periodi- 

vj cal change of curvature in the 

blade, and for the slight rising 
Frfwma violacea : downward move- ' 

ment and circumnutation of a of the petiole, 
rery young leaf, traced from 10 (21.) Wigandia (HydroleaceJB, 



ery yo 
.M. Ju 



informs us that the leaves of this 



6th. N.B. At 6.40 A.M. on the 

5th it was necessary to move the 

pot a little, and a new tracing plant rise in the evening; but as 

was begun at the point where \ye do not know whether or not 

two dots are not joined in the ^ rising ig g^ tMg gpeciea 

diagram. Apex of leaf 7 inches v v i x 

from the vertical glass. Temp. OUght perhaps to be classed 

generally 17J C. amongst sleeping plants. 



OHAP. IV. DICOTYLEDONS. 249 

(22.) Petunia violacta (Solanese, Fam. 157). A very young 
leaf, only f inch in length, highly inclined upwards, was observed 
for four days. During the whole of this time it bent outwards 
and downwards, so as to become more and more nearly hori- 
zontal. The strongly marked zigzag line in the figure on p. 248 
(Fig. Ill), shows that this was effected by modified circum- 
uutation ; and during the latter part of the time there was much 
ordinary circumnutation on a small scale. The movement in 
the diagram is magnified between 10 and 11 times. It exhibits 
a clear trace of periodicity, as the leaf rose a little each evening ; 
but this upward tendency appeared to be almost conquered by 
the leaf striving to become more and p . 112 

more horizontal as it grew older. The 
angles which two older leaves formed 
together, were measured in the even- 
ing and about noon on 3 successive 
days, and each night the angle de- 
creased a little, though irregularly. 

(23.) Acanthus mollis (Acanthacese, 
Fam. 168). The younger of two 
leaves, 2r inches in length, petiole 
included, produced by a seedling 
plant, was observed during 47 h. 
Early on each of the three morn- 
ings, the apex of the leaf fell ; and 
it continued to fall till 3 P.M., on 
the two afternoons when observed. 
After 3 P.M. it rose considerably, and 
continued to rise on the second night 
until the early morning. But on 
the first night it felt instead of rising, 
and we have little doubt that this Acanthus wjotas.-circumnuta- 
was owing to the leaf being very *on of young leaf, traced 
young and becoming through epi- $tiJ^&*-%* 
nastic growth more and more hori- of i ea f n inches from the 
zontal ; for it may be seen in the vertical glass, so movement 
diagram (Fig. 112), that the leaf stood considerably magnified. 
on a higher level on the first than on ^^^ ^ 
kne second day. The leaves of an Temp. 15-16| C. 
allied species (A. spinosus) certainly 

rose every night ; and the rise between noon and 10.15 P.M., 
when measured on one occasion, was 10. This rise was chiefly 




250 CIRCUMNUTATIOX OF LEAVES. CHAP. IT. 

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 (Cannabinese, Fam. 195). We have 
here the rare case of leaves moving downwards in the evening, 
but not to a sufficient degree to be called sleep.* In the early 
morning, or in the latter part of the night, they move upwards. 
For instance, all the young leaves near the summits of several 
stems stood almost horizontally at 8 A.M. May 29th, and at 
10.30p.sr. were considerably declined. On a subsequent day two 
leaves stood at 2 P.M. at 21 and 12 beneath the horizon, and at 
10 P.M. at 38 beneath it. Two other leaves on a younger plant 
were horizontal at 2 P.M., and at 10 P.M. had sunk' to 36 beneath 
the horizon. With respect to this downward movement of the 
leaves, Kraus believes that it is due to their epinastic growth. 
He adds, that the leaves are relaxed during the day, and tense 
at night, both in sunny and rainy weather. 

(25.) Pinus pinnster (Coniferae, Fam. 223). The leaves on the 
Biimmits of the terminal shoots stand at first in a bundle almost 
upright, but they soon diverge and ultimately become almost 
horizontal. The movements of a young leaf, nearly one inch in 
length, on the summit of a seedling plant only 3 inches high, 
were traced from the early morning of June 2nd to the evening 
of the 7th. During these five days the leaf diverged, and its apex 
descended at first in an almost straight line ; but during the two 
latter days it zigzagged so much that it was evidently circumnu- 
tating. The same little plant, when grown to a height of 5 inches, 
was again observed during four days. A filament was fixed 
transversely to the apex of a leaf, one inch in length, and which 
had already diverged considerably from its originally upright 
position. It continued to diverge (see A, Fig. 113), and to 
descend from 11.45 A.M. July 31st to 6.40 A.M. Aug. 1st. On 
August 1st it circumnutated about the same small space, and 
again descended at night. Next morning the pot was moved 
nearly one inch to the right, and a new tracing was begun (B), 
From this time, viz., 7 A.M. August 2nd to 8.20 A.M. on the 4th, 



* Wo were led to observe this Flcra, 1879, p. 66. We regret that 

pliint by Dr. Carl Kraus' paper, we cannot fully understand p;irtu 

' Beitriijre zur Kentuias der Bewe- of this paper, 
eii Wuchscnder Laubbliitter,' 



CHAP. IV. 



DICOTYLEDONS. 



251 



the leaf manifestly circumnutated. It does not appear from the 
diagram that the leaves move periodically, for the descending 
course during the first two nights, was clearly due to epinastio 



Fig. 113. 




Pinus pinaster: circumnutation of young leaf, traced from 11.45 A.M. 
July 31st to 8.20 A.M. Aug. 4th. At 7 A.M. Aug. 2nd the pot was 
moved an inch to one side, so that the tracing consists of two figures. 
Apex of leaf 14J 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 



CIBCUMNUTATION OF LEAVES. CHAP. IV. 



quite fully grown, produced by a lateral shoot, on a young tree 
3 feet in height, were observed during 29 h. (July 31st), in the 
game manner as the leaves of the previous species. Both tLeso 
leaves certainly circumnutated, making 
Kg. 114. within the above period two, or two and 

a half, small, irregular ellipses. 

(26.) Cycas pectinata (Cycadese, Fain 
224). A young leaf, Hi inches in 
length, of which the leaflets had only 
recently become uncurled, was observed 
during 47 h. 30 m. The main petiole 
was secured to a stick at the base of the 
two terminal leaflets. To one of the 
latter, 31 inches in length, a filament 
was fixed ; the leaflet was much bowed 
downward, but as the terminal part was 
upturned, the filament projected almost 
horizontally. The leaflet moved (see 
Fig. 114) largely and periodically, for it 
fell until about 7 P.M. and rose during 
the night, falling again next morning 
after 6.40 A.M. The descending lines 
are in a marked manner zigzag, and so 
probably would have been the ascending 
lines, if they had been traced throughout 
the night. 



Qycas pectinata : circum- 
nutation of one of the 
terminal leaflets, traced 
from 8.30 A.M. June 
22nd to 8 A.M. June 
24th. Apex of leaflet 
7f inches from the ver- 



ClKCUMNUTATION OP LEAVES: 
MONOCOTYLEDONS. 



(27.) Canna Warscewiczii (Cannacese, 
tical glass, so tracing Fam. 2). The movements of a young 

not greatly magnified, leaf g inclies in length ftnd 3, 
and here reduced to , ,,, , , , . 

one-third of original breadth, produced by a vigorous young 

cale; temp. 19-21C. plant, were observed during 45 h. 

50 iu., 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 ia 
periodical, as the leaf descended from the early morning untiJ 
about 5 P.M., and ascended during the rest of the evening and 



OHAP. IV. 



MONOCOTYLEDONS. 



253 



part of the night. On the evening of the llth it circumnu toted 
on a small scale for some time about the same spot. 



Fig. 115. 




A. ** 

Canna Warsccwiczii circumnutation of leaf, traced (A) from 11.30 A.M 
June 10th to 6.40 A.M. llth ; and (B) from 6.40 A.M. llth to 8.40 A.M. 
Apex of leaf 9 inches from the vertical glass. 



12th. 



(28.) Iris pseudo-acorus (Irideae, Fam. 10). The movements 
of a young leaf, rising 13 inches above the water in which the 
plant grew, were traced as shown in the 
figure (Fig. 116), during 27 h. 30 m. 
It manifestly circumnutated, though 
only to a small extent. On the second 
morning, between 6.40 A.M. and 2 P.M. 
(at which latter hour the figure here 
given ends), the apex changed its coiTrse 
five times. During the next 8 h. 40 m. it 
zigzagged much, and descended as far 
as the lowest dot in the figure, making 
in its course two very small ellipses ; 
but if these lines had been added to 
the diagram it would have been too 
complex. 

(29.) Crinum Capense (Amaryllidese, 
Fam. 11). The leaves of this plant 
are remarkable for their great length 
and narrowness: one was measured 
and found to be 53 inches long and 
only 1'4 broad at the base. Whilst quite young they stand up 
almost vertically to the height of about a foot; afterwards 



Fig. 116. 

T 

Iris pseudo-acorus : circum- 
nutation of leaf, traced 
from 10.30 A.M. May 28th 
to 2 P.M. 29th. Tracing 
continued to 11 P.M., but 
not here copied. Apex 
of leaf 12 inches beneath 
the horizontal glass, so 
figure considerably mag- 
nified. Temp. 15-16 C. 



254 CIBCUMNUTATION OP LEAVES. CHAP. 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 
62 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 bladk dot on one side; and by bringing the dependent 
apex of the leaf into a line with this dot, the accompanying 
figure (Fig. 117) was traced on the other side of the bell, during 
2 days. During the first day (22nd) the tip travelled laterally 
far to the left, perhaps in consequence of the plant having boon 



Fig. 117. 




Crinum captnse : 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 3 P.M. May 22nd to 10.15 A.M. 25th. 
The leaf was growing rapidly, so that the apex ascended greatly 
during this period ; as it zigzagged much it was clearly circum- 
nutating, and it apparently tended to form one ellipse each 
day. The lines traced during the night were much more vertical 
than those traced during the day ; and this indicates that the 
tracing would have exhibited a nocturnal rise and a diurnal 
fall, if -the leaf had not grown so quickly. The movement of 
this same leaf after an interval of six days (May 31st), by which 
time the tip had curved outwards into a horizontal position, 



CHIP. IV. MONOCOTYLEDONS. 255 

and had thus made the first step towards becoming dependent, 
was traced orthogonically by the aid of a ciibe of wood (in the 
manner before explained) ; and it was thus ascertained that the 
actual distance travelled by the apex, and due to circumnutation, 
was 3 inches in the course of 20$ h. During the next 24 h. it 
travelled 2j inches. The circumnutating movement, therefore, 
of this young leaf was strongly marked. 

(30.) Pancratium littorale (Amaryllideae). The movements, 
much magnified, of a leaf, 9 inches in length and inclined at 
about 45 above the horizon, were traced during two days. On 
the first day it changed its course completely, upwards and 
downwards and laterally, 9 times in 12 h. ; and the figure traced 
apparently represented five ellipses. On the second day it was 
observed seldomer, and was therefore not seen to change its 
course so often, viz., only 6 times, but in the same complex 
manner as before. The movements were small in extent, but 
there could be no doubt about the circumnutation of the leaf, 

(31.) Imatophyllum vel Clivia (sp. ?) (Amaryllidese). A long 
glass filament was fixed to a leaf, and the angle formed by it 
with the horizon was measured occasionally during three suc- 
cessive days. It fell each morning until between 3 and 4 P.M., 
and rose at night. The smallest angle at any time above the 
horizon was 48, and the largest 50; so that it rose only 2 
at night; but as this was observed each day, and as similar 
observations were nightly made on another leaf on a distinct 
plant, there can be no doubt that the leaves move periodically, 
though to a very small extent. The position of the apex when 
it stood highest was '8 of an inch above its lowest point. 

(32.) Pistia stratiotes (Aroideae, Fam. 30). Hofmeister 
remarks that the leaves of this floating water-plant are more 
highly inclined at night than by day.* We therefore fastened 
a fine glass filament to the midrib of a moderately young 
leaf, and on Sept. 19th measured the angle which it formed 
with the horizon 14 times between 9 A.M. and 11.50 P.M. The 
temperature of the hot-house varied during the two days of 
observation between 18i and 23 C. At 9 A.M. the filament 
stood at 32 above the horizon ; at 3.34 P.M. at 10 and at 
LI .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 CIBCUMNUTATION OF LEAVES. CHAP. IV 

6 and 6 P.M. On the next day the leaf stood at only 10 abovo 
the horizon at 8.25A.M., and it remained at about 15 3 till past 
3 P.M.; at 5.40 P.M. it was 23, and at 9.30 P.M. 58; so that 
the rise was more sudden this evening than on the previous 
one, and the difference in the angle amounted to 48. The 
movement is obviously periodical, and as the leaf stood on the 
first night at 55, and on the second night at 58 above the 
horizon, it appeared very steeply inclined. This case, as wo 
shall see in a future chapter, ought perhaps to have been 
included under the head of sleeping plants. 
(33.) Pontederia (sp. ?) (from the highlands of St. Catharina, 

Fig. 118. 




Pontederia (sp. ?) : circumnutation of leaf, traced from 4.50 P.M. July 2nd 
to 10.15 A.M. 4th. Apex of leaf 1GJ inches from the vertical glass, so 
tracing greatly magnified. Temp, about 17 C., and therefore rather 
too low 

Brazil) (Pontederiaceaa, Fam. 46). A filament was fixed across 
the apex of a moderately young leaf, 7s inches in height, and 
its movements were traced during 42 h. (see Fig. 118). On 
the first evening, when the tracing was begun, and during the 
night, the leaf descended considerably. On the next morning 
it ascended in a strongly marked zigzag line, and descended 
again in the evening and during the night. The movement, 
therefore, seems to be periodic, but some doubt is thrown on 
this conclusion, because another leaf, 8 inches in height, 
appearing older and standing more highly inclined, behaved 
differently. During the first 12 h. it circumnutated over a 



CHAP. IV. CIRCUMNUTATION OF CRYPTOGAMS. 257 

small space, but during the night, and the whole following day 
it ascended in the same general direction; the ascent being 
effected by repeated up and down well-pronounced oscillations. 



CRYPTOGAMS. 

(34.) Nephrodium molle (Filices, Fam. 1). A filament was 
fixed near the apex of a young frond of this Fern, 17 inches 
in height, which was not as yet fully uncurled ; and its move- 
ments were traced during 24 h. We see in Fig. 119 that it 

Fig. 119. 




Nepbrodium molle: circumnutation of rachis, traced from 9.15 A.M. Slay 
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. Loomia and Prof. Asa 
Gray have described (' Botanical 
Gazette,' 1880, pp. 27, 43), an 
extremely curious case of move- 
ment in the fronds, but only in 
the fruiting fronds, of Asplenium 
trtchomanes. They move almost 
as rapidly as the little leaflets 



of Desmodium gyran*, alternately 
backwards and forwards through 
from 20 <o 40 degrees, in a plane at 
right angles to that of the frond. 
The apex of the frond describes " 
long and very narrow ellipse," so 
that it circumnutates. But tho 
movement differs from ordinary 



258 CIRCUMNUTATION OP CRYPTOGAMS. CHAP. 17, 



Fig. 120. 



In the chapter on the Sleep of Plants the conspicuous circum- 
nutation of Marsilea quadri/oliata (Marsileaceee, Fam. 4) will be 
described. 

It has also been shown in Chap. I. that a very young Selar 
ginella (Lycopodiaceae, Fam. 6), only '4 inch in height, plainly 
circumnutated ; we may therefore conclude that older plants, 
whilst growing, would do the same. 

(35.) Lunularia vulgaris (Hepaticse, Fam. 11, Muscales). 
The earth in an old flower-pot was 
coated with this plant, bearing 
gemmae. A highly inclined frond, 
which projected - 3 inch above the 
soil and was '4 inch in breadth, was 
selected for observation. A glass 
hair of extreme tenuity, -75 inch 
in length, with its end whitened, 
was cemented with shellac to the 
frond at right angles to its breadth ; 
and a white stick with a minute 
black spot was driven into the soil 
close behind the end of the hair. 
The white end could be accurately 
brought into a line with the black 
spot, and dots could thus be suc- 
cessively made on the vertical 
glass-plate in front. Any move- 
ment of the frond would of course 
be exhibited and increased by the 
long glass hair; and the black spot 
was placed so close behind the end 
of the hair, relatively to the dis- 
tance of the glass-plate in front, 
that the movement of the end was 
magnified about 40 times. Never- 
theless, we are convinced that our 
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, 




Lunidaria vulgar is: circumnutn- 
tion of a frond, traced from 
9 A.M. Oct 25th to 8 A.M. 27th. 



circumnutation as it occurs only 
when the plant is exposed to the 
light; even artificial light "is 



sufficient to excite motion for a 
few minutes." 



CHAP. 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 
know. As the sinking was slight on the fifth day, a new tracing 
was begun on the sixth day (Oct. 25th), and was continued 
for 47 b. ; it is here given (Fig. 120). Another tracing was made 
on the next day (27th) and the frond was found to be still cir- 
cuinnutating, for during 14 h. 30 m. it changed its course com- 
pletely (besides minor changes) 10 times. It was casually 
observed for two more days, and was seen to be continually 
moving. 

The lowest members of the vegetable series, the Thallogens, 
apparently circumnutate. If an Oscillaria be watched under 
the microscope, it may be peen to describe circles about every 
40 seconds. After it has bent to one side, the tip first beginw 
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 2j h. the filament moved 4 
times to the left and 3 times to the right, and he refers to a 
movement at right angles to the above. The tip moved at the 
rate of about O'l mm. in five minutes. He compares the move- 
ment with the nutation of the higher plants.f We shall hereafter 
see that heliotropic movements result from modified circum- 
nutation, and as unicellular Moulds bend to the light we may 
infer that they also circumnutate. 

CONCLUDING BEMARKS ON THE CIRCUMNUTATION 
OF LEAVES. 

The circumnutating movements of young leaves in 
3.'J genera, belonging to 25 families, widely distributed 



* ' Ucber die Bewegungen der 1880, vol. iii. p. 320) that the 

Fuden der Spirogyra princeps: movements of Spirtilina, a mem- 

.lahreshefte des Vereins fur vater- ber of the OscillatorieiO, are closely 

liindisulie Naturkunde in Wiirt- analogous "to the well-known 

temberjr,' 1874, p. 211. rotation of growing shoots and 

f Znkalalso remarks (as quoted tendrils." 
in 'Journal 11. Mieroscop. Soo.,' 



260 CIRCUMNUTATION OF LEAVES. CHAP. 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 nave seen 
reason to conclude is the case with cotyledons. The 
seat of movement generally lies in the petiole, but 
sometimes both in the petiole and blade, or in the 
blade alone. The extent of the movement differed much 
in different plants ; but the distance passed over was 
never great, except with Pistia, which ought perhaps 
to have been included amongst sleeping plants. The 
angular movement of the leaves was only occasionally 
measured ; it commonly varied from only 2 (and pro- 
bably even less in some instances) to about 10 ; but 
it amounted to 23 in the common bean. The move- 
ment is chiefly in a vertical plane, but as the ascending 
and descending lines never coincided, there was always 
some lateral movement, and thus irregular ellipses 
were formed. The movement, therefore, deserves to 
be called one of circumnutation ; for all circumnuta- 
ting organs tend to describe ellipses, that is, growth 
on one side is succeeded by growth on nearly but not 
quite the opposite side. The ellipses, or the zigzag 
lines representing drawn-out ellipses, are generally 
very narrow ; yet with the Camellia, their minor axes 
were half as long, and with the Eucalyptus more than 
half as long as their major axes. In the case of Cissus, 
parts of the figure more nearly represented circles than 
ellipses. The amount of lateral movement is therefore 
sometimes considerable. Moreover, the longer axes 
of the successively formed ellipses (as with the Bean, 
Cissus, and Sea-kale), and in several instances the 
zigzag lines representing ellipses, were extended in 
very different directions during the same day or on 



UHAP. IV. CIKCUMNUTATION 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. A single large irregular 
ellipse may be described on one day, and two smaller 
ones by the same plant on the next day. With Drosera 
two, and with Lupinus, Eucalyptus and Pancratium, 
several were formed each day. 

The oscillatory and jerking movements of the leaves 
of Dionsea, which resemble those of the hypocotyl of 
the cabbage, are highly remarkable, as seen under the 
microscope. They continue night and day for some 
months, and are displayed by young unexpanded leaves, 
and by old ones which have lost their sensibility to a 
touch, but which, after absorbing animal matter, close 
their lobes. We shall hereafter meet with the same 
kind of movement in the joints of certain Graminese, 
and it is probably common to many plants while cir- 
cumnutating. It is, therefore, a strange fact that no 
such movement could be detected in the tentacles of 
Drosera rotundifolia, though a member of the same 
family with Diona?a ; yet the tentacle which was ob- 
served was so sensitive, that it began to curl inwards 
in 23 seconds after being touched by a bit of raw meat. 

One of the most interesting facts with respect to 
the circurnnutation of leaves is the periodicity of their 
movements ; for they often, or even generally, rise a 
little in the evening and early part of the night, and 
sink again on the following morning. Exactly the 
same phenomenon was observed in the case of coly- 
ledons. The leaves in 16 genera out of the 33 which 
were observed behaved in this manner, as did probably 
2 others. Nor must it be supposed that in the remain- 
ing 15 genera there was no periodicity in their move- 
ments ; for 6 of them were observed during too short 
a period for any judgment to be formed on this head, 



262 CIRCUMNUTATION OF LEAVES. CHAP. IV 

and 3 were so young that their epinastic growth 
which serves to bring them clown into a horizontal 
position, overpowered every other kind of movement. 
In only one genus, Cannabis, did the leaves sink in 
the evening, and Kraus attributes this movement to 
the prepotency of their epinastic growth. That the 
periodicity is determined by the daily alternations 
of light and darkness there can hardly be a doubt, as 
will hereafter be shown. Insectivorous plants are 
very little affected, as far as their movements are con- 
cerned, by light ; and hence probably it is that their 
leaves, at least in the cases of Sarracenia, Drosera, and 
Dionaea, do not move periodically. The upward move- 
ment in the evening is at first slow, and with different 
plants begins at very different hours ;-r -with Grlaucium 
as early as 11 A.M., commonly between 3 and 5 P.M., 
but sometimes as late as 7 P.M. It should be observed 
that none of the leaves described in this chapter 
(except, as we believe, those of Lupinus speciosus) 
possess a pulvinus ; for the periodical movements of 
leaves thus provided have generally been amplified 
into so-called sleep-movements, with which we are not 
here concerned. The fact of leaves and cotyledons 
frequently, or even generally, rising a little in the 
evening and sinking in the morning, is of interest as 
giving the foundation from which the specialised sleep- 
movements of many leaves and cotyledons, not pro- 
vided with a pulvinus, have been developed. The 
above periodicity should be kept in mind, by any one 
considering the problem of the horizontal position of 
leaves and cotyledons during the day, whilst illumi- 
nated from above. 



CHAP V MODIFIED CIRCUMNUTATION. 263 



CHAPTEK V. 

MODIFIED CIRCUMNTJTATION : CLIMBING PLAIT, s ; EPINASTIC AND 
HYPONASTIC MOVEMENTS. 

Circumnutat.ion modified through innate causes or through the action 
of external conditions Innate causes Climbing plants ; similarity 
of their movements with those of ordinary plants ; increased ampli- 
tude ; occasional points of difference Epinastic growth of young 
leaves Hypouastic growth of the hypo^otyls and epicotyls of seed- 
lingsHooked tips of climbiug and other plants due to modified 
circumnutation Ampelopsis tricuspidata Sinithia Pfundii 
Straightening of the tip due to hyponasty Epinastic growth and 
circumuutation of the flower-peduncles of Trifolium repena and 
Oxalis carnosa. 

THE radicles, hypocotyls and epicotyls of seedling 
plants, even before they emerge from the ground, and 
afterwards the cotyledons, are all continually circum- 
nutating. So it is with the stems, stolons, flower- 
peduncles, and leaves of older plants. We may, there- 
fore, infer with a considerable degree of safety that all 
the growing parts of all plants circumnutate. Although 
this movement, in its ordinary or unmodified state, 
appears in some cases to be of service to plants, 
either directly or indirectly for instance, the circum- 
nutation of the radicle in penetrating the ground, or 
that of the arched hypocotyl and epicotyl in breaking 
through the surface yet circumnutation is so general, 
or rather so universal a phenomenon, that we cannot 
suppose it to have been gained for any special pur- 
pose. We must believe that it follows in some un- 
known way from the manner in which vegetable tissues 
grow. 

18 



204 MODIFIED CIRCUMNUTATION. CHAP. V. 

We shall now consider the many cases in which 
eircumnutation has been modified for various special 
purposes ; that is, a movement already in progress is 
temporarily increased in some one direction, and tem- 
porarily diminished or quite arrested in other direc- 
tions. These cases may be divided in two sub-classf s ; 
in one of which the modification depends on innate or 
constitutional causes, and is independent of external 
conditions, excepting in so far that the proper ones for 
growth must be present. In the second sub-class the 
modification depends to a large extent on external 
agencies, such as the daily alternations of light and 
darkness, or light alone, temperature, or the attraction 
of gravity. The first small sub-class will be considered 
in the present chapter, and the second sub-class in the 
remainder of this volume. 



THE CIRCUMNUTATION OF CLIMBING PLANTS. 

The simplest case of modified eircumnutation is that 
offered by climbing plants, with the exception of 
those which climb by the aid of motionless hooks or 
of rootlets : for the modification consists chiefly in the 
greatly increased amplitude of the movement. This 
would follow either from greatly increased growth over 
a small length, or more probably from moderately in- 
creased growth spread over a considerable length of the 
moving organ, preceded by turgescence, and acting suc- 
cessively on all sides. The eircumnutation of climbers 
is more regular than that of ordinary plants ; but in 
almost every other respect there is a close similarity 
between their movements, namely, in their tendency 
to describe ellipses directed successively to all points 
of the compass in their courses being often inter- 
rupted oy zigzag lines, triangles, loops, or smal] 



CHAP. V CLIMBING PLANTS 265 

ellipses in the rate of movement, and in different 
species revolving once or several times within the samo 
length of time. In the same internode, the move- 
ments cease first in the lower part and then slowly 
upwards. In both sets of cases the movement may bo 
modified in a closely analogous manner by geotropism 
and by heliotropism ; though few climbing plants are 
aeliotropic. Other points of similarity might be 
pointed out. 

That the movements of climbing plants consist of 
ordinary circumnutation, modified by being increased 
in amplitude, is well exhibited whilst the plants are 
very young ; for at this early age they move like other 
seedlings, but as they grow older their movements 
gradually increase without undergoing any other 
change. That this power is innate, and is not excited 
by any external agencies, beyond those necessary for 
growth and vigour, is obvious. No one doubts that 
this power has been gained for the sake of enabling 
climbing plants to ascend to a height, and thus to 
reach the light. This is effected by two very different 
methods ; first, by twining spirally round a support 
but to do so their stems must be long and flexible ; 
and, secondly, in the case of leaf-climbers and tendril- 
bearers, by bringing these organs into contact with a 
support, which is then seized by the aid of their 
sensitiveness. It may be here remarked that these 
latter movements have no relation, as far as we can 
judge, with circumnutation. In other cases the tips 
of tendrils, after having been brought into contact with 
a support, become developed into little discs which 
adhere firmly to it. 

We have said that the circumnutatiou of climbing 
plants differs from that of ordinary plants chiefly by 
its greater amplitude. But most leaves circumnutate 



266 MODIFIED CIECUMNUTATION. CHAP. V. 

in an almost vertical plane, and therefore describe very 
narrow ellipses, whereas the many kinds of tendrils 
which consist of metamorphosed leaves, make much 
broader ellipses or nearly circular figures ; and thus 
they have a far better chance of catching hold of a 
support on any side. The movements of climbing 
plants have also been modified in some few other 
special ways. Thus the circumnutating stems of Sol- 
nanum dulcamara can twine round a support only 
when this is as thin and flexible as a string or thread. 
The twining stems of several British plants cannot 
twine round a support when it is more than a few 
inches in thickness ; whilst in tropical forests some 
can embrace thick trunks ;* and this great difference 
in power depends on some unknown difference in 
their manner of circurnnutation. The most remarkable 
special modification of this movement which we have 
observed is in the tendrils of Ecliinocystis lobata ; these 
are usually inclined at about 45 above the horizon, 
but they stiffen and straighten themselves so as to 
stand upright in a part of their circular course, namely, 
when they approach and have to pass orer the summit 
of the shoot from which they arise. If they had not 
possessed and exercised this curious power, they would 
infallibly have struck against the summit of the shoot 
and been arrested in their course. As soon as one of 
these tendrils with its three branches begins to stiffen 
itself and rise up vertically, the Devolving motion 
becomes more rapid; and as soon as it has passed 
over the point of difficulty, its ir.otion coinciding 
with that from its own weight, causes it to fall into its 
previously inclined position so quickly, that the apex 
can be seen travelling like the hand of a gigantic clock. 



* 'The Movements and Habits of Climbing Plants,' p. 38. 



V. EPINASTY AND HYPONASTY. 267 

A large number of ordinary leaves and leaflets and 
a few flower-peduncles are provided with pulvini ; but 
this is not the case with a single tendril at present 
known. The cause of this difference probably lies in 
the fact, that the chief service of a pulvinus is to 
prolong the movement of the part thus provided after 
growth has ceased ; and as tendrils or other climbing- 
organs are of use only whilst the .plant is increasing 
in height or growing, a pulvinus which served to 
prolong their movements would be useless. 

It was shown in the last chapter that the stolons or 
runners of certain plants circumnutate largely, and 
that this movement apparently aids them in finding a 
passage between the crowded stems of adjoining plants. 
If it could be proved that their movements had been 
modified and increased for this special purpose, they 
ought to have been included in the present chapter ; 
but as the amplitude of their revolutions is not so 
conspicuously different from that of ordinary plants, 
as in the case of climbers, we have no evidence on 
this head. We encounter the same doubt in the case 
of some plants which bury their pods in the ground. 
This burying process is certainly favoured by the 
circumnutation of the flower-peduncle ; but we do not 
know whether it has been increased for this special 
purpose. 

EPINASTY HYPONASTY. 

The term epinasty is used by De Vries * to express 
greater longitudinal growth along the upper than 



* 'Arbciten des Bot. Inst., two terms as first used by Schim- 

in Wiirzburg,' Heftii. 1872, p. 223. per, and they have been adopted 

DC- Yries hae slightly modified in this sense by Sachs, 
(p. 252; the meaning of the above 



208 MODIFIED C1BCUMNUTATION. CHAP. \ 

along the lower side of a part, which is thus caused to 
bend downwards; and hyponasty is used for the reversed 
process, by which the part is made to bend upwards. 
These actions come into play so frequently that the 
use of the above two terms is highly convenient. The 
movements thus induced result from a modified form 
of circumnutation ; for, as we shall immediately see, 
an organ under the influence of epinasty does not 
generally move in a straight line downwards, or under 
that of hyponasty upwards, but oscillates up and down 
with some lateral movement : it moves, however, in a 
preponderant manner in one direction. This shows 
that there is some growth on all sides of the part, but 
more on the upper side in the case of epinasty, and 
more on the lower side in that of hyponasty, than on 
the other sides. At the same time there may be in 
addition, as De Vries insists, increased growth on one 
side due to geotropism, and on another side due to 
heliotropism ; and thus the effects of epinasty or of 
hyponasty may be either increased or lessened. 

He who likes, may speak of ordinary circumnutation 
as being combined with epinasty, hyponasty, the effects 
of gravitation, light, &c. ; but it seems to us, front 
reasons hereafter to be given, to be more correct to 
say that circumnutation is modified by these several 
agencies. We will therefore speak of circumnutation, 
which is always in progress, as modified by epinasty, 
hyponasty, geotropism, or other agencies, whether 
internal or external. 

One of the commonest and simplest cases of epinasty is that 
offered by leaves, which at an early age are crowded together 
round the buds, and diverge as they grow older. Sachs first 
remarked that this was due to increased growth along the nppev 
side of the petiole and blade ; and De Vries has now shown in 
more detail that the movement is thus caused, aided slightly by 



CHAP. V. EPINASTY AND HYPONASTY. 269 

the weight of the leaf, and resisted as he believes by apogeo- 
tropism, at least after the leaf has somewhat diverged. In out 
observations on the circumnutation of leaves, some were selected 
which were rather too young, so that they continued to diverge 
or sink downwards whilst their movements were being traced. 
This may be seen in the diagrams (Figs. 98 and 112, pp. 232 
and 249) representing the circumnutation of the young leaves ot 
Acanthus mallis and Pelargonium zonale. Similar cases were ob- 
served with Drosera. The movements of a young leaf, only I inch 
in length, of Petunia violacea were traced during four days, and 
offers a better instance (Fig. Ill, p. 248), as it diverged during 
the whole of this time in a curiously zigzag line with some of the 
angles sharply acute, and during the latter days plainly circum- 
nutated. Some young leaves of about the same age on a plant 
of this Petunia, which had been laid horizontally, and on another 
plant which was left upright, both being kept in complete dark- 
ness, diverged in the same manner for 48 h., and apparently 
were not affected by apogeotropism ; though their stems were in 
a state of high tension, for when freed from the sticks to which 
they had been tied, they instantly curled upwards 

The leaves, whilst very young, on the leading shoots of the 
Carnation (Diantlms caryopliyllus} are highly inclined or vertical ; 
and if the plant is growing vigorously they diverge so quickly 
that they become almost horizontal in a day. But they move 
downwards in a rather oblique line and continue for some time 
afterwards to move in the same direction, in connection, we pre- 
sume, with their spiral arrangement on the stem. The course 
pursued by a young leaf whilst thus obliquely descending was 
traced, and the line was distinctly yet not strongly zigzag ; the 
larger angles formed by the successive lines amounting only to 
135, 154, and 163. The subsequent lateral movement (shown 
in Fig. 96, p. 231) was strongly zigzag with occasional circum- 
nutations. The divergence and sinking of the young leaves 
of this plant seem to be very little affected by -geotropism or 
heliotropism ; for a plant, the leaves of which were growing 
rather slowly (as ascertained by measurement) was laid hori- 
zontally, and the opposite young leaves diverged from one 
another symmetrically in the usual manner, without any up- 
turning in the direction of gravitation or towards the light-. 

The needle-like leaves of Finn* pinaster form a bundle whilst 
young ; afterwards they slowly diverge, so that those on the up- 
right shoots become horizontal. The movements of one such 



270 



MODIFIED CIRCUMNUTATION. 



CHAP. V. 



Fig. 121. 



young leaf was traced during 4 J days, and the tracing here given 
(Fig. 121) shows that it descended at first in a nearly straight 
line, but afterwards zigzagged, 
making one or two little loops. 
The diverging and descend- 
ing movements of a rather 
older leaf were also traced 
(see former Fig. 113, p. 251) : 
it descended during the first 
day and night in a some- 
what zigzag line ; it then cir- 
cumnutated round a small 
space and again descended. 
By this time the leaf had 
nearly assumed its final posi- 
tion, and now plainly circum- 
nutated. As in the case of the 
Carnation, the leaves, whilst 
very young, do not seem to be 
much affected by geotropism 
or heliotropism, for those on a 
young plant laid horizontally, 
and those on another plant 
left upright, both kept in the 
dark, continued to diverge in 
the usual manner without 
bending to either side. 

"With Cobcea scandens, the 
young leaves, as they succes- 
sively diverge from the lead- 
ing shoot which is bent to 
one side, rise up so as to pro- 
ject vertically, and they retain 
this position for some time 
whilst the tendril is revolving, 
The diverging and ascending 
<* movements of the petiole of 

. , one such a leaf, were traced on 

xi-xu r*yster : epmastic downward . 

movecusnt of a young leaf, pro- a vertical glass under a sky- 

dnced by a young plant in a pot, light ; and the course pursued 

traced on a vertical glass under a was j n mos t par ts nearly 

rynght, from (3.45 A.M. June 2nd _.._ , . , , ,, . 




ight 
to 10.40 P.M. Gth. 



straight, but there were twc 



CHAP. V. EPINASTY AND HYPONASTY. 271 

well-marked zigzags (one of them forming an angle of 112), 
and this indicates circumnutation. 

The still closed lobes of a young leaf of Diontea projected at 
right angles to the petiole, and were in the act of slowly rising. 
A glass filament was attached to the under side of the midrib, 
und its movements were traced on a vertical glass. It circum- 
nutated once in the evening, and on the next day rose, as already 
described (see Fig. 106, p. 240), by a number of acutely zigzag 
lines, closely approaching in character to ellipses. This move- 
ment no doubt was due to epinasty, aided by apogeotropisnij 
for the closed lobes of a very young leaf on a plant which had 
been placed horizontally, moved into nearly the same line with 
the petiole, as if the plant had stood upright ; but at the same 
time the lobes curved laterally upwards, and thus occupied an 
unnatural position, obliquely to the plane of the foliaceous petiole. 

As the hypocotyls and epicotyls of some plants protrude from 
the seed-coats in an arched form, it is doubtful whether the 
arching of these parts, which is invariably present when they 
break through the ground, ought always to be attributed to 
epinasty ; but when they are at first straight and afterwards 
become arched, as often happens, the arching is certainly due to 
epinasty. As long as the arch is surrounded by compact earth 
it must retain its form; but as soon as it rises above the 
surface, or even before this period if artificially freed from the 
surrounding pressure, it begins to straighten itself, and this no 
doubt is mainly due to hyponasty. The movement of the 
upper and lower half of the arch, and of the crown, was occa- 
sionally traced ; and the course was more or less zigzag, showing 
modified circunmutation. 

With not a few plants, especially climbers, the summit of the 
shoot is hooked, so that the apex points vertically downwards. 
In seven genera of twining plants * the hooking, or as it has been 
called by Sachs, the nutation of the tip, is mainly due to an 
exaggerated form of circumnutation. That is, the growth is so 
great along one side that it bends the shoot completely over to 
the opposite side, thus forming a hook; the longitudinal line or 
zone of growth then travels a little laterally round the shoot, 
and the hook points in a slightly different direction, and so 
onwards until the hook is completely reversed. Ultimately it 



' The Movements and Habits of Climbing Plants,' 2nd edit u. 13. 



'/72 MODIFIED CIRCUMNUTATION. CHAP. V. 

comes back to the point whence it started. This was ascertained 
by painting narrow lines with Indian ink along the convex 
surface of several hooks, and the line was found slowly to be- 
come at first lateral, then to appear along the concave surface, 
and ultimately back again on the convex surface. In the case of 
Lonictra brachypoda the hooked terminal part of the revolving 
shoot straightens itself periodically, but is never reversed ; thftt 
is, the periodically increased growth of the concave side of the 
hook is sufficient only to straighten it, and not to bend it over 
to the opposite side. The hooking of the tip is of service to 
twining plants by aiding them to catch hold of a support, and 
afterwards by enabling this part to embrace the support much 
more closely than it could otherwise have done at first, thus 
preventing it, as we often observed, from being blown away by a 
strong wind. Whether the advantage thus gained by twining 
plants accounts for their summits being so frequently hooked, 
we do not know, as this structure is not very rare with plants 
which do not climb, and with some climbers (for instance, Vitis, 
Ampelopsis, Cissus, &c.) to whom ib does not afford any assist- 
ance in climbing. 

With respect to those cases in which the tip remains always 
bent or hooked towards the same side, as in the genera just 
named, the most obvious explanation is that the bending is due 
to continued growth in excess along the convex side. Wiesner, 
however, maintains * that in all cases the hooking of the tip is 
the result of its plasticity and weight, a conclusion which from 
what we have already seen with several climbing plants is 
certainly erroneous. Nevertheless, we fully admit that the 
weight of the part, as well as geotropism, &c., sometimes come 
into play. 

Ampelopsis tricu?pidata. This plant climbs by the aid of 
adhesive tendrils, and the hooked tips of the shoots do not 
appear to be of any service to it. The hooking depends chiefly, 
as far as we could ascertain, on the tip being affected by epinasty 
and geotropism ; the lower and older parts continually straight- 
ening themselves through hyponasty and apogeotropism. We 
believe that the weight of the apex is an unimportant element, 
because on horizontal or inclined shoots the hook is often 
extended horizontally or even faces upwards. Moreover shoots 
frequently form loops instead of hooks; and in this case the 



' Sitzb. der k. Akad. der Wisseiiseh.,' Vienna, Jan. 1880, p. 1G. 



CHAP. V. 



EPINASTY AND HYPONASTY. 



273 



extreme part, instead of hang- Fig. 12'2. 

ing vertically down as would 
follow if weight was the efficient 
cause, extends horizontally or 
even points upwards. A shoot, 
which terminated in a rather 
open hook, was fastened in 
a highly inclined downward 
position, so that the concave 
side faced upwards, and the 
result was that the apex at first *05O'a#ni4 '' 
curved upwards. This ap- 
parently was due to epinasty \ 
and not to apogeotropism, for / 
the apex, soon after passing / 
the perpendicular, curved so / 
rapidly downwards that we ' 
could not doubt that the move- / 
ment was at least aided by / 
geotropism. In the course of / 
a few hours the hook was thus 
converted into a loop with the 
apex of the shoot pointing 
straight downwards. The 
longer axis of the loop was at ta>.j(> 
first horizontal, but after- 
wards became vertical. During 
this same time the basal part 
of the hook (and subsequently 
of the loop) curved itself slowly 
upwards ; and this must have 
been wholly duo to apogeo- 
tropism in opposition to hypo- 
nasty. The loop was then y 
fastened upside down, so that 
its basal half would be simul- 
taneously acted on by hypo- 8 
nasty (if present) and by apo- Ampelopsis tricuspidata .- hynonastu 



geotropism ; and now it curved 
itself so greatly upwards in 
the course of only 4h. that 
there could hardly be a doubt 
that both forces were acting 



movement of hooked tip of leading 
shoot, traced from 8.10 A.M. July 
13th to 8A.M. 15th. Apex of shoot 
5 inches from the vertical glass. 
Plant illuminated through a sky- 
light. Temp. 17$ C -19C. Diagram 
reduced to one-third of original scala 



274 



MODIFIED CIRCUMNUTATION. 



CUAT 1 . V. 




Smithia Pfundii : hypouastic movement 
of thecurved summitof astern, whilst 
straightening itself, traced from 9 
A.M. July 10th to 3 P.M. 13th. Apex 
9| inches from the vertical glass. 
Diagram reduced to one-fifth of 
original scale. Plant illuminated 
through okylight ; temp. 17j-19C. 



together. At the same time 
the loop became open and 
was thus reconverted into a 
hook, and this apparently 
was effected by the geotropio 
movement of the apex in 
opposition to epinasty. In 
the case of Ampelopsis hede- 
racea, weight plays, as far as 
we could judge, a more im- 
portant part in the hooking 
of the tip. 

In order to ascertain 
whether the shoots of A. tri- 
cuspid'tta in straightening 
themselves under the com- 
bined action of liyponasty and 
apogeotropism moved in a 
simple straight course, or 
whether they circumnutated, 
glass filaments were fixed to 
the crowns of four hooked 
tips standing in their natural 
position ; and the movements 
of the filaments were traced 
on a vertical glass. All four 
tracings resembled each other 
in a general manner ; but we 
will give only one (see Fig. 
122, p. 273). Tha filament 
rose at first, which shows 
that the hook was straighten- 
ing itself ; it then zigzagged, 
moving a little to the left 
between 9.25 A.M. and 9 P.M. 
From this latter hour on the 
13th to 10.50 A.M. on the fol- 
lowing morning (14th) the 
hook continued to straighten 
itself, and then zigzagged a 
short distance to the right. 
But from 1 P.M. to 10.40 P.M. 
on the 14th the movement 



,^AI>. V. EPINASTY AND HYPONASTY. 275 

was reversed and the shoot became more hooked. During 
the night, after 10.40 P.M. to 8.15 A.M. on the 15th, the hook 
again opened or straightened itself. By this time the glass 
filament had become so highly inclined that its movements could 
no longer be traced with accuracy ; and by 1.30 P.M. on this same 
day, the crown of the former arch or hook had become perfectly 
straight and vertical. There can therefore be no doubt that the 
straightening of the hooked shoot of this plant is effected by 
the circumnutation of the arched portion that is, by growth 
alternating between the upper and lower surface, but prepon- 
derant on the lower surface, with some little lateral movement. 

We were enabled to trace the movement of another straight- 
ening shoot for a longer period (owing to its slower growth and 
to 4ts having been placed further from the vertical glass), namely, 
from the early morning on July 13th to late in the evening of the 
16th. During the whole daytime of the 14th, the hook straight- 
ened itself very little, but zigzagged and plainly circumnutated 
about nearly the same spot. By the 16th it had become nearly 
straight, and the tracing was no longer accurate, yet it was 
manifest that there was still a considerable) amount of movement 
both up and down and laterally; for the crown whilst con- 
tinuing to straighten itself occasionally became for a short time 
more curved, causing the filament to descend twice during the 
day. 

Kmitltia Ffundii. The stiff terminal shoots of this Legu- 
minous water-plant from Africa project so as to make a rectaugle 
with the stem below ; but this occurs only when the plants are 
growing vigorously, for when kept in a cool place, the summits 
of the stems become straight, as they likewise did at the close 
of the growing season. The direction of the rectangularly bent 
part is independent of the chief source of light. But from 
observing the effects of placing plants in the dark, in which 
case several shoots became in two or three days upright or nearly 
upright, and when brought back into the light again became 
rectangularly curved, we believe that the bending is in part 
due to apheliotropism, apparently somewhat opposed by apogeo- 
tropism. On the other hand, from observing the effects of tying 
a shoot downwards, so that the rectangle faced upwards, we are 
led to believe that the curvature is partly due to epinasty. As 
the rectangularly bent portion of an upright stem grows older, 
the lower part straightens itself; and this is effected through 
hyponasty. He who has read Sachs' recent Essay on the vertical 



276 MODIFIED CIECUMNUTATION. CIIAP. V 

and inclined positions of the parts of plants* will see how diffi- 
cult a subject this is, and will feel no surprise at our expressing 
ourselves doubtfully in this and other such cases. 

A plant, 20 inches in height, was secured to a stick close 
beneath the curved summit, which formed rather less than a 
rectangle with the stem below. The shoot pointed away from the 
observer ; and a glass filament pointing towards the vertical glass 
on which the tracing was made, was fix~ed to the convex surface of 
the curved portion. Therefore the descending lines in the figure 
represent the straightening of the curved portion as it grew 
older. The tracing (Fig. 123, p. 274) was begun at 9 A.M. on 
July 10th; the filament at first moved but little in a zigzag line, 
but at 2 P.M. it began rising and continued to do so till 9 P.M. ; 
and this proves that the terminal portion was being more bent 
downwards. After 9 P.M. on the 10th an opposite movement 
commenced, and the curved portion began to straighten itself, 
and this continued till 11.10 A.M. on the 12th, but was interrupted 
by some small oscillations and zigzags, showing movement in 
different directions. After 11.10 A.M. on the 12th this part of 
the stem, still considerably curved, circumnutated in a con- 
spicuous manner until nearly 8 P.M. on the 13th; but during all 
this time a downward movdhient of the filament prevailed, 
caused by the continued straightening of the stem. By the 
afternoon of the 13th, the summit, which had originally been 
deflected more than a right angle from the perpendicular, had 
grown so nearly straight that the tracing could no longer be 
continued on the vertical glass. There can therefore be no 
doubt that the straightening of the abruptly curved portion of 
the growing stem of this plant, which appears to be wholly due 
to hyponasty, is the result of modified circumnutation. We 
will only add that a filament was fixed in a different manner 
across the curved summit of another plant, and the same general 
kind of movement was observed. 

Trifolium repens. In many, but not in all the species of Tri- 
folium, as the separate little flowers wither, the sul>-peduncleg 
bend downwards, so as to depend parallel to the upper part of 
the main peduncle. In Tr. subhrranmm the main peduncle 
curves downwards for the sake of burying its capsules, and in 
this species the sub-peduncles of the separate flowers bend 



* ' Ueber Orthotrope und Pla- ten dee Bot. Inst., in Wurzburg,' 
giotrope Pflanzentheile ;' 'Arbei- Heft ii. 1879, p. 226. 



CHAP. V, 



SPIN AST Y AND HYPONASTY. 



277 



Fig. 124. 





Tfifolhtm repens: circumnn- 
tating and epinastic move- 
ments of the sub-peduncle 
of a sinsle flower, traced 
OH a vertical ginss under 
a skylight, in A from 11.30 
A.M. Aug. 27th to 7 A.M. 
30th ; in B from 7 A.M. 
Ang. 30th to a little after 
6 P.M. Sept. 8th. 




278 MODIFIED CIKCUMNUTATION. CHAT. V. 

upwards, so as to occupy the same position relatively to the 
upper part ot the main peduncle as in Tr. repens. This fact 
alone would render it probable that the movements of the sub- 
peduncles in Tr. repens were independent of geotropism. Never 
theless, to make sure, some flower-heads -were tied to little stirka 
upside down aud others in a horizontal position ; their sub- 
peduncles, however, all quickly curved upwards through tlio 
action of heliotropism. We therefore protected some flower- 
heads, similarly secured to sticks, from the light, and although 
some of them rotted, many of their sub-peduncles turned very 
slowly from their reversed or from their horizontal positions, 
so as to stand in the normal manner parallel to the upper part 
of the main peduncle. These facts show that the movement is 
independent of geotropism or apheliotropism ; it must there- 
be attributed to epinasty, which however is enecked, at least as 
long as the flowers are young, by heliotropism. Most of the 
above flowers were never fertilised owing to the exclusion of 
bees ; they consequently withered very slowly, and the movements 
of the sub-peduncles were in like manner much retarded. 

To ascertain the nature of the movement of the sub-peduncle, 
whilst bending downwards, a filament was fixed across the 
summit of the calyx of a not fully expanded and almost upright 
flower, nearly in the centre of the head. The main peduncle 
was secured to a stick close beneath the head. In order to see 
the marks on the glass filament, a few flowers had to be cut 
away on the lower side of the head. The flower under obser- 
vation at first diverged a little from its upright position, so as 
to occupy the open space caused by the removal of the adjoining 
flowers. This required two days, after which time a new tracing 
was begun (Fig. 124). In A we see the complex circumnutating 
course pursued from 11.30 A.M. Aug. 26th to 7 A.M. on the 
30th. The pot was then moved a very little to tho right, and 
the tracing (B) was continued without interruption from 7 A.M. 
Aug. 30th to after 6 P.M. Sept. 8th. It should be observed that 
on most of these days, only a single dot was made each morning 
at the same hour. Whenever the flower was observed carefully, 
as on Aug. 30th and Sept. 5th and 6th, it was found to be cir- 
cumnutating over a small space. At last, on Sept. 7th, it 
began to bend downwards, and continued to do so until after 
6 P.M. on the 8th, and indeed until the morning of the 9th, when 
its movements could no longer be traced on the vertical glass. 
It was carefully observed during the whole of the 8th, and by 



CSAP. V. EPINASTY AND HYPONASTY. 279 

10.30 P.M. it had descended to a point lower down by two-thirds 
of the length of the figure as here given; but from want of space 
the tracing has been copie I in B, only to a little aftor 6 P.M. On 
the morning of the 9th the flower was withered, and the sub- 
peduncle now stood at an angle of 57 beneath the horizon. If 
the flower had been fertilised it would have withered rnuuh 
sooner, and have moved much more quickly. We thus see that 
the sub-peduncle oscillated up and down, or circumnutattd, 
during its whole downward epinastic course. 

The sub-peduncles of the fertilised and withered flowers 
of Oxalis carnoxa likewise bend downwards through epinasty, 
as will be shown in a future chapter; and theii downward 
course is strongly zigzag, indicating circumnutation. 

The number of instances in which various organs 
move through epinasty or hyponasty, often in com- 
bination with other forces, for the most diversified 
purposes, seems to be inexhaustibly great; and from 
the several cases which have been here given, we may 
safely infer that such movements are due to modified 
circumnutation. 



280 MODIFIED CIRCUMNUTATIOX. OHAP. VI. 



CHAPTER VI. 

MODIFIED CIBCTTMNUTATION : SLEEP OR NYCTITROPIC MOVEMENTS, 
THEIR USE: SLEEP OP COTYLEDONS. 

Preliminary sketch of the sleep or nyctitropic movements of leaves 
Piesence of pulvini The lessening of radiation the final cause of 
nycrtitropie movements Manner of trying experiments on leaves of 
Oxalis. Arachis, Cassia, Melilotus, Lotus and Murnilea, and on the 
cot\ ledons of Miuuua Concluding remarks on radiation from leaves 
Small differences in the conditions make a great difference in the 
result Description of the nyctitropic position and movements of 
the cotyl.dons of various plants List of species Concluding 
reiiiaiks Independence of the mctifropic movements of the leaves 
and cotyledons of the s;ime species Reasons for believing that the 
movements have been acquired for a special purpose. 

THE so-called sleep of leaves is so conspicuous a 
phenomenon that it was observed as early as the 
time of Pliny ;* and since Linnaeus published his 
famous Essay, ' Sornnus Plantarurn,' it has been the 
subject of several memoirs. Many flowers close at 
night, and these are likewise said to .sleep ; but we 
are not here concerned with their movements, for 
although effected by the same mechanism as in the 
case of young leaves, namely, unequal growth on the 
opposite sides (as first proved by Pfeffer), yet they differ 
essentially in being excited chiefly by changes of 
temperature instead of light; and in being effected, as 
far as we can judge, for a different purpose. Hardly 
any one supposes that there is any real analogy 



* Pfeffer has given a clear and riodis-clien Bewegungen der Blat 
interesting sketch of the history toryane,' 1S75, p. 1G3 
of this subject in his 'Die Pe- 



CHAP. VI. SLEEP MOVEMENTS. 281 

between the sleep of animals and that of plants,* 
whether of leaves or flowers. It seems, therefore, 
advisable to give a distinct name to the so-called 
sleep-movements of plants. These have also generally 
been confounded, under the term " periodic," with the 
slight daily rise and fall of leaves, as described in the 
fourth chapter ; and this makes it all the more desir- 
able to give some distinct name to sleep-movements. 
Nyctitropism and nyctitropic, i.e. night-turning, may 
be applied both to leaves and flowers, and will be 
occasionally used by us ; but it would be best to con- 
fine the term to leaves. The leaves of some few plants 
move either upwards or downwards when the sun shines 
intensely on them, and this movement has sometimes 
been called diurnal sleep ; but we believe it to be of 
an essentially different nature from the nocturnal 
movement, and it will be briefly considered in a 
future chapter. 

The sleep or nyctitropisin of leaves is a large 
subject, and we think that the most convenient plan 
will be first to give a brief account of the position 
which leaves assume at night, and of the advantages 
apparently thus gained. Afterwards the more re- 
markable cases will be described in detail, with 
respect to cotyledons in the present chapter, and to 
leaves in the next chapter. Finally, it will be shown 
that these movements result from circumnutatiou, 
much modified and regulated by the alternations of 
day and night, or light and darkness ; but that they 
are also to a certain extent inherited. 

Leaves, when they go to sleep, move either upwards 
or downwards, or in the case of the leaflets of com- 



* Ch. Royer must, however, bo Nat.' (5th series), But. voL iz 

exempted ; see * Annalcs des So. 18G8, p. 378. 



282 MODIFIED CIRCUMNUTATION. CHAP. VI 

pound leaves, forwards, that is, towards the apex of the 
leaf, or backwards, that is, towards its base ; or, again, 
they may rotate on their own axes without moving 
either upwards or downwards. But in almost every 
case the plane of the blade is so placed as to stand 
nearly or quite vertically at night. Therefore the apex, 
or the base, or either lateral edge, may be directed 
towards the zenith. Moreover, the upper surface of 
each leaf, and more especially of each leaflet, is often 
brought into close contact with that of the opposite 
one ; and this is sometimes effected by singularly 
complicated movements. This fact suggests that the 
upper surface requires more protection than the lower 
one. For instance, the terminal leaflet in Trifolium, 
after turning up at night so as to stand vertically, 
often continues to bend over until the upper surface is 
directed downwards whilst the lower surface is fully 
exposed to the sky ; and an arched roof is thus 
formed over the two lateral leaflets, which have their 
upper surfaces pressed closely together. Here we have 
the unusual case of one of the leaflets not standing 
vertically, or almost vertically, at night. 

Considering that leaves in assuming their nycti- 
tropic positions often move through an angle of 
90 ; that the movement is rapid in the evening ; 
that in some cases, as we shall see in the next 
chapter, it is extraordinarily complicated; that with 
certain seedlings, old enough to bear true leaves, 
the cotyledons move vertically upwards at night, 
whilst at the same time the leaflets move ver- 
tically downwards ; and that in the same genus 
the leaves or cotyledons of some species move 
upwards, whilst those of other species move down- 
wards ; from these and other such facts, it is hardly 
possible to doubt that plants must derive some 



CHAP. VI. SLEEP MOVEMENTS. 283 

great advantage 'from such remarkable powers of 
movement. 

The nyctitropic movements of leaves and cotyledons 
are effected in two ways,* firstly, by means of pulvini 
which become, as Pfeffer has shown, alternately more 
turgescent on opposite sides ; and secondly, by in- 
creased growth along one side of the petiole or 
midrib, and then on the opposite side, as was first 
proved by Batalin.j But as it has been shown by 
De Vries | that in these latter cases increased growth 
is preceded by the increased turgescence of the cells, 
the difference between the above two means of move- 
ment is much diminished, and consists chiefly in the 
turgescence of the cells of a fully developed pulvinus, 
not being followed by growth. When the move- 
ments of leaves or cotyledons, furnished with a pul- 
vinus and destitute of one, are compared, they are seen 
to be closely similar, and are apparently effected for 
the same purpose. Therefore, with our object in view, 
it does not appear advisable to separate the above two 
sets of cases into two distinct classes. There is, how- 
ever, one important distinction between them, namely, 
that movements effected by growth on the alternate 
sides, are confined to young growing leaves, whilst those 
effected by means of a pulvinus last for a long time. 
We have already seen well-marked instances of this 
latter fact with cotyledons, and so it is with leaves, as 
has been observed by Pfeffer and by ourselves. The 
long endurance of the nyctitropic movements when 
effected by the aid of pulvini indicates, in addition to 
the evidence already advanced, the functional imDort- 



* This distinction was first Dassen in 1837. 

pointed out (according to Pfeffer, t ' Flora,' 1873, p. 433. 

'Die Periodischen Bewegungen j 'Bot. Zeitung,' 1879, Deft 

der Blattorgane,' 1875, p. 1G1) by 19th, p. 830. 



284 MODIFIED CIRCUMNUTATION. CHAI-. VI 

ance of such movements to the plant. There is anothei 
difference between the two sets of cases, namely, that 
there is never, or very rarely, any torsion of the 
leaves, excepting when a pulvinus is present ; * but 
this statement applies only to periodic and nyctitropic 
movements, as may be inferred from other cases given 
by Frank.f 

The fact that the leaves of many plants place 
themselves at night in widely different positions from 
what they hold during the day, but with the one 
point in common, that their upper surfaces avoid 
facing the zenith, often with the additional fact that 
they come into close contact with opposite leaves or 
leaflets, clearly indicates, as it seems to us, that the 
object gained is the protection of the upper sur- 
faces from being chilled at night by radiation. There 
is nothing improbable in the upper surface needing 
protection more than the lower, as the two differ in 
function and structure. All gardeners know that 
plants suffer from radiation. It is this and not 
cold winds which the peasants of Southern Europe 
fear for their olives.J Seedlings are often protected 
from, radiation by a very thin covering of straw ; and 
fruit-trees on walls by a few fir-branches, or even by a 
fishing-net, suspended over them. There is a variety 
of the gooseberry, the flowers of which from being 
produced before the leaves, are not protected by 
them from radiation, and consequently often fail to 
yield fruit. An excellent observer | has remarked 

* Pfeffcr, ' Die Period. Beweg. Dew,' remarks that an exposed 

der Blatlurijane.' 1875, p. 15J). thermometer rises as soon as even 

t ' Die Nat. Wagerechte Rich- a fleecy cloud, hi^li in the fcky, 

tung von Pflanzentheik-n,' 1870, passes over the zenith, 
p. 52. 'Loudoa's Gardener's Mag .,' 

$ Martins in 'Bull. SOP. Bot. vol. iv. 1828, p. 112. 
do France,' torn. xix. 1872. || Mr. Rivers in 'Gardener's 

Wells, in his famous 'Essay on Chron.,' 1866, p. 73'2. 



CUAP. VI. USE OF SLEEP MOVEMENTS. 285 

that one variety of the cherry has the petals of its 
flo \vers much curled backwards, and after a severe 
frost all the stigmas were killed ; whilst at the same 
time, in another variety with incurved petals, tho 
stigmas were not in the least injured. 

This view that the sleep of leaves saves them from 
being chilled at night by radiation, would no doubt 
have occurred to Linnneus, had the principle of radia- 
tion been then discovered ; for he suggests in many 
parts of his * Somnus Plantarum ' that the position of 
the leaves at night protects the young stems and 
buds, and often the young inflorescence, against cold 
winds. We are far from doubting that an additional 
advantage may be thus gained ; and we have observed 
with several plants, for instance, Desmodium gyrans, 
that whilst the blade of the leaf sinks vertically down at 
night, the petiole rises, so that the blade has to move 
through a greater angle in order to assume its vertical 
position than would otherwise have been necessary ; but 
with the result that all the leaves on the same plant 
are crowded together as if for mutual protection. 

We doubted at first whether radiation would affect 
in any important manner objects so thin as are many 
cotyledons and leaves, and more especially affect dif- 
ferently their upper and lower surfaces ; for although 
the temperature of their upper surfaces would un- 
doubtedly fall when freely exposed to a clear sky, yet 
we thought that they would so quickly acquire by 
conduction the temperature of the surrounding air, 
that it could hardly make any sensible difference 
to them, whether they stood horizontally and radiated 
into the open sky, or vertically and radiated chiefly 
in a lateral direction towards neighbouring plants and 
other objects. We endeavoured, therefore, to ascer- 
tain something on this head by preventing the leaves 



286 MODIFIED ClftCUMNUTATION. CHAP. VI 

of several plants from going to sleep, and by exposing 
to a clear sky when the temperature was beneath 
the freezing-point, these, as well as the other leaves 
on the same plants which had already assumed their 
nocturnal vertical position. Our experiments show 
that leaves thus compelled to remain horizontal at 
night, suffered much more injury from frost than 
those which were allowed to assume their normal 
vertical position. It may, however, be said that 
conclusions drawn from such observations are not 
applicable to sleeping plants, the inhabitants of 
countries where frosts do not occur. But in every 
country, and at all seasons, leaves must be exposed to 
nocturnal chills through radiation, which might be in 
some degree injurious to them, and which they would 
escape by assuming a vertical position. 

In our experiments, leaves were prevented from 
assuming their nyctitropic position, generally by 
being fastened with the finest entomological pins 
(which did not sensibly injure them) to thin sheets 
of cork supported on sticks. But in some instances 
they were fastened down by narrow strips of card, 
and in others by their petioles being passed through 
slits in the cork. The leaves were at first fastened 
close to the cork, for as this is a bad conductor, and as 
the leaves were not exposed for long periods, we thought 
that the cork, which had been kept in the house, would 
very slightly warm them ; so that if they were injured 
by the frost in a greater degree than the free vertical 
leaves, the evidence would be so much the stronger 
that the horizontal position was injurious. But we 
found that when there was any slight difference in the 
result, which could be detected only occasionally, the 
leaves which had been fastened closely down suffered 
rather more than those fastened with very long and 



CHAP. VI. USE OF SLEEP MOVEMENTS. 287 

thin pins, so as to stand from J to f inch above the 
cork. This difference in the result, which is in itself 
curious as showing what a very slight difference in 
the conditions influences the amount of injury in- 
flicted, may be attributed, as we believe, to the sur- 
rounding warmer air not circulating freely beneath the 
closely pinned leaves and thus slightly warming them. 
This conclusion is supported by some analogous facts 
hereafter to be given. 

We will now describe in detail the experiments 
which were tried. These were troublesome from our 
not being able to predict how much cold the leaves of 
the several species could endure. Many plants had 
every leaf killed, both those which were secured in 
a horizontal position and those which were allowed to 
sleep that is, to rise up or sink down vertically. 
Others again had not a single leaf in the least in- 
jured, and these had to be re-exposed either for a 
longer time or to a lower temperature. 

Oxalis acetosella. A very large pot, thickly covered with 
between 300 and 400 leaves, had been kept all winter in the 
greenhouse. Seven leaves were pinned horizontally open, 
and were exposed on March 16th for 2 h. to a clear sky, the 
temperature on the surrounding grass being 4 C. (24 to 
25 F.). Next morning all seven leaves were found quite 
killed, so were many of the free ones which had previously 
gone to sleep, and about 100 of them, either dead or browned 
and injured, were picked off. Some leaves showed that they 
had been slightly injured by not expanding during the whole 
of the next day, though they afterwards recovered. As all the 
leaves which were pinned open were killed, and only about a 
third or fourth of the others were either killed or injured, we 
had some little evidence that those which were prevented from 
assuming their vertically dependent position suffered most. 

The following night (17th) was clear and almost equally cold 
( - 3 D to - 4 0. on the grass), and the pot was again exposed 
but this time for only 30 m. Eight leaves had been pinned out, 



288 MODIFIED CIKCUMNUTATION. CHAP. VI. 

and in the morning two of them were dead, whilst not a single 
other leaf on the many plants was even injured. 

On the 23rd the pot was exposed for 1 h. 30 m., the tempera- 
ture on the grass being only 2 C., and not one leaf was 
injured: the pinned open leaves, however, all stood from 
i to f of an inch above the cork. 

On the 24th the pot was again placed on the ground and 
exposed to a clear sky for between 35 m. and 40 m. By a mis- 
take the thermometer was left on an adjoining sun-dial 3 feet 
high, instead of being placed on the grass ; it recorded 25 to 
26 F. (- 33 to - 3-8 C.), but when looked at after 1 h. had 
fallen to 22 F. (- 5'5 C.); so that the pot was perhaps exposed 
to rather a lower temperature than on the two first occasions. 
Eight leaves had been pinned out, some close to the cork and 
some above it, and on the following morning five of them (i.e. 
63 per cent.) were found killed. By counting a portion of the 
leaves we estimated that about 250 had been allowed to go to 
sleep, and of these about 20 were killed (i.e. only 8 per cent.), 
and about 30 injured. 

Considering these cases, there can be no doubt that the 
leaves of this Oxalis, when allowed to assume their normal 
vertically dependent position at night, suffer much less from 
frost than those (23 in number) which had their upper surfaces 
exposed to the zenith. 

Oxalis carnosa. A plant of this Chilian species was exposed 
for 30 m. to a clear t-ky, the thermometer on the grass standing 
at 2 C , with some of its leaves pinned open, and not one leaf 
on the whole bushy plant was in the least injured. On the 
16th of March another plant was similarly exposed for 30 m., 
when the temperature on the grass was only a little lower, viz , 
3 to 4 C. Six of the leaves had been pinned open, and 
next morning five of them \\ere found much browned. The 
plant was a large one, and none of the free leaves, which 
were asleep and depended vertically, were browned, excepting 
four very young ones. But three other leaves, though not 
browned, were in a rather flaccid condition, and retained their 
nocturnal position during the whole of the following day. In 
this case it was obvious that the leaves which were exposed hori- 
zontally to the zenith suffered most. This same pot was after- 
wards exposed for 35-40 m. on a slightly colder night, and 
every leaf, both the pinned open and the free ones, was killed 
It may be added that two po!s of corniculata (var. Atr'o 



CHAP. VI. USE OF SLEEP MOVEMENTS. 289 

purpurea) were exposed for 2h. and 3h. to a clear sky with the 
temp, on grass 2 C., and none of the leaves, whether free or 
pinned open, were at all injured. 

Arachis hypogcea. Some plants in a pot were exposed at night 
for 30m. to a clear sky, the temperature on the surrounding 
grass being 2 C., and on two nights afterwards they were again 
exposed to the same temperature, but this time during 1 h. 30 m. 
On neither occasion was a single leaf, whether pinned open or 
free, injured ; and this surprised us much, considering its native 
tropical African home. Two plants were next exposed (March 
16th) for 30 m. to a clear sky, the temperature of the surrounding 
grass being now lower, viz., between 3 and 4 C., and all 
four pinned-open leaves were killed and blackened. These two 
plants bore 22 other and free leaves (excluding some very young 
bud-like ones) and only two of these were killed and three some- 
what injured; that is, 23 per cent, were either killed or injured, 
whereas all four pinned open leaves were utterly killed. 

On another night two pots with several plants were exposed 
for between 35m. .and 40m. to a clear sky, and perhaps to a 
rather lower temperature, for a thermometer on a dial, 3 feet 
high, close by stood at - 3"3 to - 3'8 C. In one pot three 
leaves were pinned open, and all were badly injured ; of the 
44 free leaves, 26 were injured, that is, 59 per cent. In the 
other pot 3 leaves were pinned open and all were killed; four 
other leaves were prevented from sleeping by narrow strips of 
stiff paper gummed across them, and all were killed ; of 24 free 
leaves, 10 were killed, 2 much injured, and 12 unhurt; that is, 
50 per cent, of the free leaves were either killed or much in- 
jured. Taking the two pots together, we may say that rather 
more than half of the free leaves, which were asleep, were either 
killed or injured, whilst all the ten horizontally extended leaves, 
which had been prevented from going to sleep, were either killed 
or much injured. 

Cassia floribunda. A bush was exposed at night for 40 m. to 
a clear sky, the temperature on the surrounding grass being 
- 2 C., and not a leaf was injured.* It was again exposed on 



* Cassia Ifemcjaia was exposed injured. But when C. Ixviguta 

to a clear sky for 35 m., and C. was exposed fur 1 h., the temp. 

calliantha (a Guiana species) for on the surrounding grass being 

(50 m., the temperature on the between 3 and 4 C., every 

surrounding grass being 2 C., leaf was killed, 
and neither were in the least 



290 MODIFIED CIRCUMNUTATION. CHAP. VI. 

another night for 1 h., when the temperature of the grass waa 
4 C. ; and now all the leaves on a large bush, whether pinned 
fiat open or free, were killed, blackened, and shrivelled, with 
the exception of those on one small branch, low down, which 
was very slightly protected by the leaves on the branches 
above. Another tall bush, with four of its large compound 
leaves pinned out horizontally, was afterwards exposed (temp, 
of surrounditg grass exactly the same, viz., 4 C.), but only 
for 30 m. On the following morning every single leaflet on 
these four leaves was dead, with both their upper and lower 
surfaces completely blackened. Of the many free leaves on the 
bush, only seven were blackened, and of these only a single one 
(which was a younger and more tender leaf than any of the 
pinned ones) had both surfaces of the leaflets blackened. The 
contrast in this latter respect was well shown by a free leaf, which 
stood between two pinned-open ones ; for these latter had the 
lower surfaces of their leaflets as black as ink, whilst the inter- 
mediate free leaf, though badly injured, still retained a plain 
tinge of green on the lower surface of the leaflets. This bush 
exhibited in a striking manner the evil effects of the leaves not 
being allowed to assume at night their normal dependent posi- 
tion; for had they all been prevented from doing so, assuredly 
every single leaf on the bush would have been utterly killed by 
this exposure of only 30 m. The leaves whilst sinking down- 
wards in the evening twist round, so that the upper surface is 
turned inwards, and is thus better protected than the outwardly 
turned lower surface. Nevertheless, it was always the upper 
surface which was more blackened than the lower, whenever 
any difference could be perceived between them ; but whether this 
was due to the cells near the upper surface being more tender, 
or merely to their containing more chlorophyll, we do not know. 
Melilotus officinalis. A largo pot with many plants, which 
had been kept during the winter in the greenhouse, was exposed 
during 5 h. at night to a slight frost and clear sky. Four 
leaves had been pinned out, and these died after a few days ; 
but so did many of the free leaves. Therefore nothing certain 
could be inferred from this trial, though it indicated that the 
Horizontally extended leaves suffered most. Another large pot 
with many plants was next exposed for 1 h., the temperature on 
the surrounding grass being lower, viz., - 3 to - 4 C. Ten 
leaves had been pinned out, and the result was striking, for 
on the following morning all these were found much injured o* 



CHAP. VI. USE OF SLEEP MOVEMENTS. 291 

killed, and none of the many free leaves on the several plants 
were at all injured, with the doubtful exception of two or 
three very young ones. 

Melilotus Jtalica. Six leaves were pinned out horizontally, 
three with their upper and three with their lower surfaces turned 
to the zenith. The plants were exposed for 5 h. to a clear sky, 
the temperature on ground being about - 1 C. Next morning 
the six pinned-open leaves seemed more injured even than the 
younger and more tender free ones on the same branches. The 
exposure, however, had been too long, for after an interval of 
some days many of the free leaves seemed in almost as bad a 
condition as the pinned-out ones. It was not possible to decide 
whether the leaves with their upper or those with their lower 
surfaces turned to the zenith had suffered most. 

Melilotus suavtolet/s. Some plants with 8 leaves pinned out 
were exposed to a clear sky during 2 h., the temperature on the 
surrounding grass being - 2 C. Next morning 6 out of these 
8 leaves were in a flaccid condition. There were about 150 free 
leaves on the plant, and none of these were injured, except 2 or 3 
very young ones. But after two days, the plants having been 
brought back into the greenhouse, the 6 pinned-out leaves all 
recovered. 

Melilotw Taurica. Several plants were exposed for 5 h, during 
two nights to a clear sky and slight frost, accompanied by some 
wind ; and 5 leaves which had been pinned out suffered more 
than those both above and below on the same branches which 
had gone to sleep. Another pot, which had likewise been kept 
in the greenhouse, was exposed for 35-40 m. to a clear sky, 
the-temperature of the surrounding grass being between - 3 and 
- 4 C. Nine leaves had been pinned out, and all of these were 
killed. On the same plants there were 210 free leaves, which 
had been allowed to go to sleep, and of these about 80 were 
killed, i.e. only 38 per cent. 

Melilotus Petitpitrrtana. The plants were exposed to a clear 
sky for 35^0 m. : temperature on surrounding grass 3 to 
4 C. Six leaves had been pinned out so as to stand about 
i inch above the cork, and four had been pinned close to it. 
These 10 leaves were all killed, but the closely pinned ones 
suffered most, as 4 of the 6 which stood above the cork still 
retained small patches of a green colour. A considerable 
number, but not nearly all, of the free leaves, were killed or 
much injured, whereas all the pinned out ones were killed. 



292 MODIFIED CIRCUMXUTATION. CHAI> VT 

Melilotus macrorrhiza. The plants were exposed in the same 
manner as in the last case. Six leaves had been pinned out 
horizontally, and five of them were killed, that is, 83 per cent. 
We estimated that there were 200 free leaves on the plants, and 
of these about 50 were killed and 20 badly injured, so that about 
85 per cent, of the free leaves were killed or injured. 

Lotus aristata.Si-x. plants were exposed for nearly 5 h. to a 
clear sky; temperature on surrounding grass 1'5 C. Four 
leaves had been pinned out horizontally, and 2 of these suffered 
more than those above or below on the same branches, which 
had been allowed to go to sleep. It is rather a remarkable fact 
that some plants of Lotus Jacobceus, an inhabitant of so hot a 
country as the Cape Verde Islands, were exposed one night to a 
clear sky, with the temperature of the surrounding grass - 2 C., 
and on a second night for 30 m. with the temperature of 
the grass between - 3 and - 4 C., and not a single leaf, either 
the pinned-out or free ones, was in the least injured. 

Marsilea quadrifoliata. A large plant of this species the 
only Cryptogamic plant known to sleep with some leaves pinned 
open, was exposed for 1 h. 35 m. to a clear sky, the temperature 
on the surrounding ground being - 2 C., and not a single leaf 
was injured. After an interval of some days the plant was again 
exposed for 1 h. to a clear sky, with the temperature on the 
surrounding ground lower, viz., 4 C. Six leaves had been 
pinned out horizontally, and all of them were utterly killed. 
The plant had emitted long trailing stems, and these had been 
wrapped round with a blanket, so as to protect them from the 
frozen ground and from radiation; but a very large number 
of leaves were left freely exposed, which had gone to sleep, 
and of these only 12 were killed. After another interval, the 
plant, with 9 leaves pinned out, was again exposed for 1 h., the 
temperature on the ground being again - 4 C. Six of the leaves 
were killed, and one which did not at first appear injured after- 
wards became streaked with brown. The trailing branches, which 
rested on the frozen ground, had one-half or three-quarters of their 
leaves killed, but of the many other leaves on the plant, which 
alone coiild be fairly compared with the pinned-out ones, none 
appeared at; first sight to have been killed, but on careful search 
12 were found in this state. After another interval, the plant 
with 9 leaves pinned out, was exposed for 35-40 rn. to a cleai 
sky and to nearly the same, or perhaps a rather lower, tempera- 
ture Cfor the thermometer by an accident liad been left on 6 



CIIAI-. VI. USE OF SLEEP MOVEMENTS. 293 

Bun-dial close by), and 8 of these leaves were killed. Of the free 
leaves (those on the trailing branches not being considered), a 
good many were killed, but their number, compared with the 
uninjured ones, was small. Finally, taking the three trials 
together, 24 leaves, extended horizontally, were exposed to the 
zenith and to unobstructed radiation, and of these 20 were 
killed and 1 injured ; whilst a relatively very small proportion 
of the leaves, which had been allowed to go to sleep with their 
leaflets vertically dependent, were killed or injured. 

The cotyledons of several plants were prepared for trial, but 
the weather was mild and we succeeded only in a single instance 
in having seedlings of the proper age on nights which were 
clear and cold. The cotyledons of 6 seedlings of Mimosa pudica 
were fastened open on cork, and were thus exposed for 1 h. 45 m. 
to a clear sky, with the temperature on the surrounding ground 
at 29 F. ; of these, 3 were killed. Two other seedlings, after 
their cotyledons had risen up and had closed together, were 
bent over and fastened so that they stood horizontally, with the 
lower surface of one cotyledon fully exposed to the zenith, and 
botli were killed. Therefore of the 8 seedlings thus tried 5, or 
more than half, were killed. Seven other seedlings, with their 
cotyledons in their normal nocturnal position, viz., vertical and 
closed, were exposed at the same time, and of these only 2 were 
killed.* Hence it appears, as far as these few trials tell anything, 
that the vertical position at night of the cotyledons of Mimosa 
pudica protects them to a certain degree from the evil effects of 
radiation and cold. 

Concluding Remarks on the Radiation from Leaves 
at Niglit. We exposed on two occasions during the 
summer to a clear sky several pinned-open leaflets 
of Triffilium pratense, which naturally rise at night, 
and of Oxalis purpurea, which naturally sink at night 
(the plants growing out of doors), and looked at 

* We were surprised that It m:iy be added thnt seedlings c/ 

yo-.mg seedlings of so tropical a the Indian CaKgia pubescem were 

plant us Miiuowi pudica were able exposed fur 1 It. 30 m. to a clear 

to res! t, as well as they did, ex- sky, with the temp, on the sur- 

posure for 1 br. 45 in. to a clear rounding ground at - 2 C., and 

Bky, the temperature on the sur- they were not in the least injured 
rounding ground being 2'J F. 



294 MODIFIED CIECUMNUTATIOy. CHAP. VL 

them early on several successive mornings, after they 
had assumed their diurnal positions. The difference 
in the amount of dew on the pinned-open leaflets 
and on those which had gone to sleep was generally 
conspicuous ; the latter being sometimes absolutely 
dry, whilst the leaflets which had been horizontal 
were coated with large beads of dew. This shows how 
much cooler the leaflets fully exposed to the zenith 
must have become, than those which stood almost 
vertically, either upwards or downwards, during the 
night. 

From the several cases above given, there can be no 
doubt that the position of the leaves at night affects 
their temperature through radiation to such a degree, 
that when exposed to a clear sky during a frost, it is a 
question of life and death. We may therefore admit 
as highly probable, seeing that their nocturnal posi- 
tion is so well adapted to lessen radiation, that the 
object gained by their often complicated sleep move- 
ments, is to lessen the degree to which they are 
chilled at night. It should be kept in mind that 
it is especially the upper surface which is thus pro- 
tected, as it is/never directed towards the zenith, and 
is often brotfght into close contact with the upper 
surface of an opposite leaf or leaflet. 

We failed to obtain sufficient evidence, whether 
the better protection of the upper surface has been 
gained from its being more easily injured than the 
lower surface, or from its injury being a greater evil 
to the plant. That there is some difference in consti- 
tution between the two surfaces is shown by the foil >w 
ing cases. Cassia floribunda was exposed to a clear sky 
on a sharp frosty night, and several leaflets which 
had assumed their nocturnal dependent position with 
their lower surfaces turned outwards so as to be 



CHAP. VL USE OF SLEEP MOVEMENTS. 295 

exposed obliquely to the zenith, nevertheless had these 
lower surfaces less blackened than the upper surfaces 
which were turned inwards and were in close contact 
with those of the opposite leaflets. Again, a pot 
full of plants of Trifolium resupinatum, which had 
been kept in a warm room for three days, was turned 
out of doors (Sept. 21st) on a clear and almost frosty 
night. Next morning ten of the terminal leaflets were 
examined as opaque objects under the microscope. 
These leaflets, in going to sleep, either turn vertically 
upwards, or more commonly bend a little over the 
lateral leaflets, so that their lower surfaces are more 
exposed to the zenith than their upper surfaces. 
Nevertheless, six of these ten leaflets were distinctly 
yellower on the upper than on the lower and more 
exposed surface. In the remaining four, the result 
was not so plain, but certainly whatever difference 
there was leaned to the side of the upper surface 
having suffered most. 

It has been stated that some of the leaflets experi- 
mented on were fastened close to the cork, and others 
at a height of from | to f of an inch above it ; and 
that whenever, after exposure to a frost, any difference 
could be detected in their states, the closely pinned 
ones had suffered most. We attributed this difference 
to the air, not cooled by radiation, having been pre- 
vented from circulating freely beneath the closely 
pinned leaflets. That there was really a difference in 
the temperature of leaves treated in these two dif- 
ferent methods, was plainly shown on one occasion ; 
for after the exposure of a pot with plants of Melilotus 
dentaia for 2 h. to a clear sky (the temperature on the 
surrounding grass being - 2 C.), it was manifest that 
more dew had congealed into hoar-frost on the closely 
pinned leaflets, than on those which stood horizontally 

20 



296 MODIFIED CIRGUMNUTATION. CHAP. VL 

a little above the cork. Again, the tips of some few 
leaflets, which had been pinned close to the cork, pro- 
ected a little beyond the edge, so that the air could 
circulate freely round them. This occurred with six 
leaflets of Oxalis acetosella, and their tips certainly 
Buffered rather less than the rest of the same leaflets ; 
for on the following morning they were still slightly 
green. The same result followed, even still more 
clearly, in, two cases with leaflets of Melilotus officinalis 
which projected a little beyond the cork ; and in two 
other cases some leaflets which were pinned close to 
the cork were injured, whilst other free leaflets on 
the same leaves, which had not space to rotate and 
assume their proper vertical position, were not at all 
injured. 

Another analogous fact deserves notice : we observed 
on several occasions that a greater number of free 
leaves were injured on the branches which had been 
kept motionless by some of their leaves having been 
pinned to the corks, than on the other branches. This 
was conspicuously the case with those of Melilotus 
Petitpierreana, but the injured leaves in this instance 
were not actually counted. With Arachis liypogsea, a 
young plant with 7 stems bore 22 free leaves, and of 
these 5 were injured by the frost, all of which were on 
two stems, bearing four leaves pinned to the cork- 
supports. With Oxalis carnosa, 7 free leaves were 
injured, and every one of them belonged to a cluster 
of leaves, some of which had been pinned to the cork. 
We could account for these cases only by supposing 
that the branches which were quite free had been 
slightly waved about by the wind, and that their 
leaves had thus been a .little warmed by the sur- 
rounding warmer air. If we hold our hands motion 
less before a hot fire, and then wave them about, we 



CHAP. VI. SLEEP OF COTYLEDONS. 297 

immediately feel relief; and this is evidently an 
analogous, though reversed, case. These several facts 
in relation to leaves pinned close to or a little above 
the cork-supports to their tips projecting beyond it 
and to the leaves on branches kept motionless seem 
to us curious, as showing how a difference, apparently 
trifling, may determine the greater or less injury of 
"the leaves. We may even infer as probable that the 
less or greater destruction during a frost of the leaves 
on a plant which does not sleep, may often depend on 
the greater or less degree of flexibility of their petioles 
and of the branches which bear them. 

JSYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS. 

We now come to the descriptive part of our work, 
and will begin with cotyledons, passing on to leaves 
in the next chapter. We have met with only two 
brief notices of cotyledons sleeping. Hofmeister,* 
after stating that the cotyledons of all the observed 
seedlings of the Caryophylleee (Alsiuese and Sileneae) 
bend upwards at night (but to what angle he does not 
state), remarks that those of Stellaria media rise up so 
as to touch one another ; they may therefore safely be 
said to sleep. Secondly, according to Ramey,f the 
cotyledons of Mimosa pudica and of Clianihus Dam- 
pieri rise up almost vertically at night and approach 
each other closely. It has been shown in a previous 
chapter that the cotyledons of a largo number of 
plants bend a little upwards at night, and we here 
have to meet the difficult question at what inclination 
may they be said to sleep? According to the view 
Ave maintain, no movement deserves to be called 



* 'Die Lehre von der Pflanzenzelle,' 18G7, p. 327. 
t ' Adansonia,' March 10th, 18C9. 



298 MODIFIED CIKCUMNUTATION. CHAP. VI 

ayctitropic, unless it has been acquired for the sake of 
lessening radiation ; but this could be discovered only 
by a long series of experiments, showing that the 
leaves of each species suffered from this cause, if pro- 
vented from sleeping. We must therefore take an 
arbitrary limit. If a cotyledon or leaf is inclined at 
60 above or beneath the horizon, it exposes to the 
zenith about one-half of its area ; consequently the 
intensity of its radiation will be lessened by about 
half, compared with what it would have been if the 
cotyledon or leaf had remained horizontal. This 
degree of diminution certainly would make a great 
difference to a plant having a tender constitution. 
We will therefore speak of a cotyledon and hereafter 
of a leaf as sleeping, only when it rises at night to 
an angle of about 60, or to a still higher angle, above 
the horizon, or sinks beneath it to the same amount. 
Not but that a lesser diminution of radiation may be 
advantageous to a plant, as in the case of Datura 
stramonium, the cotyledons of which rose from 31 at 
noon to 55 at night above the horizon. The Swedish 
turnip may profit by the area of its leaves being 
reduced at night by about 30 per cent., as estimated 
by M r. A. S. Wilson ; though in this case the angle 
through which the leaves rose was not observed. On 
the other hand, when the angular rise of cotyledons or 
of leaA r es is small, such as less than 30, the diminution 
of radiation is so slight that it probably is of no sig- 
nificance to the plant in relation to radiation. For 
instance, the cotyledons of Geranium Ibericum rose at 
night to 27 above the horizon, and this would lessen 
radiation by only 11 per cent. : those of Linutn Beren- 
dieri rose to 33, and this would lessen radiation by 
10 per cent. 

There are, however, some other sources of doubt witn 



CHAP. VI. SLEEP OF COTYLEDONS. 299 

respect to the sleep of cotyledons. In certain cases, 
the cotyledons whilst young diverge during the day to 
only a very moderate extent, so that a small rise at 
night, which we know occurs with the cotyledons of 
many plants, would necessarily cause them to assume 
a vertical or nearly vertical position at night ; and in 
this case it would be rash to infer that the movement 
was effected for any special purpose. On this account 
we hesitated long whether we should introduce several 
Cucurbitaceous plants into the following list ; but from 
reasons, presently to be given, we thought that they 
had better be at least temporarily included. This 
same source of doubt applies in some few other cases ; 
for at the commencement of our observations we did 
not always attend sufficiently to whether the cotyle- 
dons stood nearly horizontally in the middle of the day. 
With several seedlings, the cotyledons assume a highly 
inclined position at night during so short a period of 
their life, that a doubt naturally arises whether this 
can be of any service to the plant. Nevertheless, in 
most of the cases given in the following list, the coty- 
ledons may be as certainly said to sleep as may the 
leaves of any plant. In two cases, namely, with the 
cabbage and radish, the cotyledons of which rise almost 
vertically during the few first nights of their life, it 
was ascertained by placing young seedlings in the 
klinostat, that the upward movement was not due to 
apogeotropism. 

The names of the plants, the cotyledons of which 
stand at night at an angle of at least 60 J with the 
horizon, are arranged in the appended list on the same 
system as previously followed. The numbers of the 
Families, and with the Leguminosre the numbers of 
the Tribes, have been added to show how widely 
the plants in question are distributed throughout the 



300 



MODIFIED CIECUMNUTATTON. 



CHAP. VI. 



dicotyledon ous series. A few remarks will have to 
be made about many of the plants in the list. In 
doing so, it will be convenient not to follow strictly 
any systematic order, but to treat of the Oxalida) 
and the Leguminosfc at the close ; for in these 
two Families the cotyledons are generally provided 
with a pulvinus, and their movements endure for a 
much longer time than those of the other plants in 
the list. 



List of Seedling Plants, the cotyledons of which rise or Sink at 
night to an angle of at leant 60 above or beneath the horizon. 

guminosse (Tribe 13) accord- 
ing to Mr. K. I. Lynch. 

Cassia mimosoides. Leguminos;e 
(Tribe 14). 

glauca. 

florida. 



Brassica oleracea. Crucifera3 (Fam. 

14). 
napus (as we are informed 

by Prof. Pfeffer). 
Eaphauus sativus. Crucifera?. 
Githago segetum. Caryophylleac 

(Fam. 26). 
Stellaria media (according to Hof- 

meister, as quoted). Caryophyl- 

lea;. 
Anoda Wrightii. Malvaceae (Fam. 

36). 
Gossypium (var. Nankin cotton). 

Malvaceae. 
Oxalis rosea. Oxalida; (Fam. 41). 

floribunda. 

articulata. 

Valdiviana. 

sensitiva. 

Geranium rotundifolium. Gera- 

niaceae (Fam. 47). 
Trifolium subterraneum. Legu- 

minosae (Fain. 75, Tribe 3). 

strictum. 

leucanthemum. 

Lotus ornithopopoides. Leguini- 

nosse (Tribe 4). 

peregrinus. 

Jacobaeus. 

Cliauthus Dampieri. Legumi- 

nosae (Tribe 5)- according to M. 
Ratney. 
Smithia sensitiva. Leguminosaj 

(Tribe 6). 
Haematoivlon Campechianum. Le- 



-- corymbosa. 

-- pubescens. 

- - tora. 

-- neglecta. 

-- 3 other Brazilian 



spocies. 
Bauhinia (ip. ?). 

(Tribe 15). 
Neptunia oleracea. 

(Tribe 20). 
Mimosa pudica. 

(Tribe 21). 
albida. 



named 
Leguminosap 
Leguminosse 
Learn minosaj 



Cucurbita ovifera. Cucurbitacea? 
(Fam. 106). 

Lagenaria vulgaris. Cucurbitacese. 
Cucumis dudaini. Cucurbitaceas. 
Apium petroselinum. Umbelliferae 

(Fam. 113). 

graveolens. 

Lactuca scariola. Compositae (Fam. 

122). 

Helianthus annuus (?). CompositaB, 
Ipomoea casrulea. ConvoK ulacea 

(Fara. 151). 

purpurea. 

bona-nox. 

cocciuea. 



CHAP. YI. SLEEP OF COTYLEDONS 301 

List of Seedling Plants (continued). 

Rolanam lycopersicum. Solane* * Mirabilis longiflora. 

(Fam. 157). ! Beta vulgaris. Polygonese fjesati. 



Miinulus, (sp. ?) Scrophularinese 
(Fiim. 159) from information 
given us by Prof. Pfefter. 

llirabilis jalapa. Nyctaginese 
(Fam. 177). 



179). 
Amaranthus caudatus. Am HMD 

thacese (Fam. 180). 
Cannabis sativa (?). Cannabineas 

(Fam. 195). 



Brassica, oleracea (Crueiferae). It was shown in the first chapter 
that the cotyledons of the common cabbage rise in the evening 
and stand vertically up at night with their petioles in contact- 
But as the two cotyledons are of unequal height, they frequently 
interfere a little with each other's movements, the shorter one 
often not standing quite vertically. They awake early in the 
morning; thus at 6.45 A.M. on Nov. 27th, whilst it was still 
dark, the cotyledons, which had been vertical and in contact on 
the previous evening, were reflexed, and thus presented a very 
different appearance. It should be borne in mind that seedlings 
in germinating at the proper season, would not be subjected to 
darkness at this hour in the morning. The above amount of 
movement of the cotyledons is only temporary, lasting with plants 
kept in a warm greenhouse from four to six days ; how long it 
would last with seedlings growing out of doors we do not know. 

Eaphanus sativus.In the middle of the day the blades of 
the cotyledons of 10 seedlings stood at right angles to their 
hypocotyls, with their petioles a little divergent ; at night the 
blades stood vertically, with their bases in contact and with 
their petioles parallel. Next morning, at 6.45A.M., whilst it 
was still dark, the blades were horizontal. On the following 
night they were much raised, but hardly stood sufficiently ver- 
tical to be said to be asleep, and so it was in a still less degree 
on the third night. Therefore the cotyledons of this plant (kept 
in the greenhouse) go to sleep for even a shorter time than 
those of the cabbage. Similar observations were made, but only 
during a single day and night, on 13 other seedlings likewise 
raised in the greenhouse, with the same result. 

The petioles of the cotyledons of 11 young seedlings of 
Sinapis nigra were slightly divergent at noon, and the blades 
stood at right angles to the hypocotyls ; at night the petioles 
were in close contact, and the blades considerably raised, 
with their bases in contact, but only a few stood sufficiently 
upright to be called asleep. On the following morning, 



302 MODIFIED CIKCUMNUTATION. CHAI>. VI 

the petioles diverged before it was light. The hypocotyl u 1 
slightly sensitive, so that if rubbed with a needle it bends 
towards the rubbed side. In the case of Lepidium sativum, tha 
petioles of the cotyledons of young seedlings diverge during 
the day and converge so as to touch each other during the 
night, by which means the bases of the tripartite blades are 
brought into contact ; but the blades are so little raised that 
they cannot be said to sleep. The cotyledons of several other 
cruciferous plants were observed, but they did not rise sufficiently 
during the night to be said to sleep. 

Oithago srgetum (Caryophyllese). On the first day after the 
cotyledons had burst through the seed-coats, they stood at noon 
at an angle of 75 above the horizon ; at night they moved 
upwards, each through an angle .of 15 so as to stand quite 
vertical and in contact with one another. On the second day 
they stood at noon at 59 above the horizon, and again at 
night were completely closed, each having risen 31. On the 
fourth day the cotyledons did not quite close at night. The 
first and succeeding pairs of young true leaves behaved in 
exactly the same manner. We think that the movement in this 
case may be called nyctitropic, though the angle passed through 
was small. The cotyledons are very sensitive to light and will 
not expand if exposed to an extremely dim one. 

Anoda Wrightii (Malvaceae). The cotyledons whilst moderately 
young, and only from -2 to '3 inch in diameter, sink in the 
evening from their mid-day horizontal position to about 35 
beneath the horizon. But when the same seedlings were older 
and had produced small true leaves, the almost orbicular 
cotyledons, now '55 inch in diameter, moved vertically downwards 
at night. This fact made us suspect that their sinking might 
be due merely to their weight ; but they were not in the least 
flaccid, and when lifted up sprang back through elasticity into 
their former dependent position. A pot with some old seedlings 
was turned upside down in the afternoon, before the noc- 
turnal fall had commenced, and at night they assumed in op- 
position to their own weight (and to any geotropic action) an 
upwardly directed vertical position. When pots were thus 
reversed, after the evening fall had already commenced, the 
sinking movement appeared to be somewhat disturbed; but all 
their movements were occasionally variable without any apparent 
cause. This latter fact, as well as that of the young cotyledons 
not sinking nearly so much as the older ones, deserves notice. 



CHAP. VL SLEEP OF COTYLEDONS. 303 

Although the movement of the cotyledons endured for a long 
time, no pulvinus was exteriorly visible; but their growth 
continued for a long time. The cotyledons appear to be only 
slightly heliotropic, though the hypocotyl is strongly so. 

(jrossypiumarbortumC!) (var. Nankin cotton) (Malvaceae). Ths 
cotyledons behave in nearly the same manner as those of tho 
Anoda. On June 15th the cotyledons of two seedlings were 
65 inch in length (measured along the midrib) and stood hori- 
zontally at noon ; at 10 P.M. they occupied the same position 
and had not fallen at all. On June 23rd, the cotyledons of one 
of these seedlings were I'l inch in length, and by 10 P.M. they 
had fallen from a horizontal position to 62 beneath the horizon. 
The cotyledons of the other seedling were 1*3 inch in length, and 
a minute true leaf had been formed; they had fallen at 10 P.M. 
to 70 beneath the horizon. On June 25th, the true leaf of this 
latter seedling was '9 inch in length, and the cotyledons occu- 
pied nearly the same position at night. By July 9th the cotyle- 
dons appeared very old and showed signs of withering ; but they 
stood at noon almost horizontally, and at 10 P.M. hung down 
vertically. 

Gossypium herbiceum. It is remarkable that the cotyledons of 
this species behave differently from those of the last. They were 
observed during 6 weeks from their first development until 
they had grown to a very large size (still appearing fresh and 
green), viz. 2 inches in breadth. At this age a tvue leaf had 
been formed, which with its petiole was 2 inches long. During 
the whole of these 6 weeks the cotyledons did not sink at night ; 
yet when old their weight was considerable and they were borne 
by much elongated petioles. Seedlings raised from some seed 
sent us from Naples, behaved in the same manner ; as did those 
of a kind cultivated in Alabama and of the Sea-island cotton. 
To what species these three latter forms belong we do not know. 
We could not make out in the case of the Naples cotton, that 
the position of the cotyledons at night was influenced by tho 
soil being more or less dry ; care being taken that they wero 
not rendered flaccid by being too dry. The weight of the large 
cotyledons of the Alabama and Sea-island kinds caused them to 
hang somewhat downwards, when the pots in which they grew 
were left for a time upside down. It should, however, bo 
observed that these three kinds were raised in the middle of 
the winter, which sometimes greatly interferes with the proper 
nyctitropic movements of leaves and cotyledons. 



304 MODIFIED CIRCUMNUTATIOX. CRAP VI. 

Oucurbitacece. The cotyledons of Cucurbit aurantia and ovi- 
fera, and of Lagenuriavulguris, stand from the 1st lo the 3rd day 
of their life at about 60 above the horizon, and at night rise up 
so as to become vertical and in close contact with one another. 
With Cucumis dudaim they stood at noon at 45 above the hori- 
zon, and closed at night. The tips of the cotyledons of all these 
species are, however, reflexed, so that this part is fully exposed 
to the zenith at night ; and this fact is opposed to the belief 
that the movement is of the same nature as that of sleeping 
plants. After the first two or three days the cotyledons 
diverge more during the day and cease to close at night. 
Those of Trichosauthes anguina are somewhat thick and fleshy, 
and did not rise at night ; and they could perhaps hardly be 
expected to do so. On the other hand, those of Acatdhosicyos, 
horrida * present nothing in their appearance opposed to their 
moving at night in the same manner as the preceding species ; 
yet they did not rise up in any plain manner. This fact leads 
to the belief that the nocturnal movements of the above-named 
species has been acquired for some special purpose, which may 
be to protect the young plumule from radiation, by the close 
contact of the whole basal portion of the two cotyledons. 

Gtranium rotundifolium (Geraniace;e). A single seedling came 
up accidentally in a pot, and its cotyledons were observed to 
bend perpendicularly downwards during several successive 
nights, having been horizontal at noon. It grew into a fine 
plant but died before flowering : it was sent to Kew and pro- 
nounced to be certainly a Geranium, and in all probability the 
above-named species. This case is remarkable because the 
cotyledons of G. cinereum, Endressii, Ibericum, Ricliardsoni, and 
subcauli-scens were observed during some weeks in the winter, 
and they did not sink, whilst those of G. Ibericum rose 27 at 
night. 

Apium petroselinum (Umbelliferae). A seedling had its coty- 
ledons (Nov. 22nd) almost fully expanded during the day ; by 
8.30 P.M. they had risen considerably, and at 10.30 P.M. were 
almost closed, their tips being only ^ of an inch apart. On 
the following morning (23rd) the tips were T 5 ^ of an inch apart, 



* This plant, from Dammara climber; it has been doHcribed 

Land in S. Africa, ia rermirkable in 'Transact. Linn. 8r>c.,' xxyii 

from being the ono known mem- p. 30. 
her of the Family which is not a 



CHAP. VI. SLEEP OF COTYLEDONS. 305 

or more than seven times as much. On the next night the 
cotyledons occupied nearly the same position as before. On the 
morning of the 21th they stood horizontally, and at night were 
60 above the horizon ; and so it was on the night of the 25th. 
But four days afterwards (on the 29th), when the seedlings 
were a week old, the cotyledons had ceased to rise at night to 
any plain degree. 

Apium graveolens The cotyledons at noon were horizontal, 
and at 10 P.M. stood at an angle of 61 above the hori/.on. 

Lactuci scariula (Composite). The cotyledons whilst young 
stood sub-horizontally during the day, and at night rose so as 
to be almost vertical, and some were quite vertical and closed ; 
but this movement ceased when they had grown old and large, 
after an interval of 11 days. 

Helianthus annuus (Composite). This case is rather doubtful ; 
the cotyledons rise at night, and on one occasion they stood at 
73 above the horizon, so that they might then be said to have 
been asleep. 

Jpomoea ccerulea vel Pharbitis nil (Convolvulaceae). The coty- 
ledons behave in nearly the same manner as those of the Anorla 
and Nankin cotton, and like them grow to a large size. Whilst 
young and small, so that their blades were from -5 to '6 of an 
inch in length, measured along the middle to the base of the 
central notch, they remained horizontal both during the middle 
of the day and at night. As they increased in size they began 
to sink more and more in the evening and early night ; and 
when they had grown to a length (measured in the above 
manner) of from J to 1'25 inch, they sank between 55 and 70 
beneath the horizon. They acted, however, in this manner only 
when they had been well illuminated during the day. Never- 
theless, the cotyledons have little or no power of bending 
towards a lateral light, although the hypocotyl is strongly helio- 
tropic. They are not provided with a pulvinus, but continue 
to grow for a long time. 

Jpomvea j/urpurea (vel Pharbitis hispida}. The cotyledons 
tehave in all respects like those of /. ccerulea. A seedling with 
cotyledons '75 inch in length (measured as before) and 1-65 
inch in breadth, having a small true leaf developed, was placed 
at 5.30 P.M. on a klinostat in a darkened box, so that neither 
weight nor geotropism could act on them. At 10 P.M. one coty- 
ledon stood at 77 and tho other at 82 beneath the horizon. 
Before being placed in tho klinostat they stood at 15 and 2 ( J 



306 MODIFIED CIRCUMNUTATION. CHAP. VI. 

beneath the horizon. The nocturnal position depends chiefij 
on the curvature of the petiole close to the blade, but the whole 
petiole becomes slightly curved downwards. It deserves notice 
that seedlings of this and the last-named species were raised at 
the end of February and another lot in the middle of March, 
and the cotyledons in neither case exhibited any nyctitropic 
movement. 

Jpomia bona-nox. The cotyledons after a few days grow to 
an enormous size, those on a young seedling being 3j inches 
in breadth. They were extended horizontally at noon, and at 
10 P.M. stood at 63 beneath the horizon. Five days . after- 
wards they were 4^ inches in breadth, and at night one stood at 
64 and the other 48 beneath the horizon. Though the blades 
are thin, yet from their great size and from the petioles being 
long, we imagined that their -depression at night might be 
determined by their weight ; but when the pot was laid hori- 
zontally, they became curved towards the hypocotyl, which 
movement could not have been in the least aided by their 
weight, at the same time they were somewhat twisted upwards 
through apogeotropism. Nevertheless, the weight of the coty- 
ledons is so far influential, that when on another night the pot 
was turned upside down, they were unable to rise and thus to 
assume their proper nocturnal position. 

Jpomva coccit<eu.T\ie cotyledons whilst young do not sink 
at night, but when grown a little older, but still only -4 inch in 
length (measured as before) and '82 in breadth, they became 
greatly depressed. In one case they were horizontal at noon, 
and at 10 P.M. one of them stood at 64 and the other at 47 
beneath the horizon. The blades are thin, and the petioles, 
which become much curved down at night, are short, so that 
here weight can hardly have produced any effect. With all the 
above species of Ipomoea, when the two cotyledons on the same 
seedling were unequally depressed at night, this seemed to 
depend on the position which they had held during the day 
with reference to the light. 

Solanum lycopersicum (Solaneae). The cotyledons rise so 
much at night as to come nearly in contact. Those of 8. palina- 
canthum were horizontal at noon, and by 10 P.M. had risen only 
'27 30' ; but on the following morning before it was light they 
stood at 59 above the horizon, and in the afternoon of the same 
ebiy were again horizontal. The behaviour of the cotyledons of 
this latter species seems, therefore, to be anomalous. 



CHAP. VL SLEEP OF COTYLEDONS. 307 

Mlrabilis jnlapa and longiflora (Nyctagineae). The cotyledons, 
which are of unequal size;, stand horizontally during the middle 
of the day, and at night rise up vertically and come into close 
contact with one another. But this movement with M. longijlur.i 
lasted for only the three first nights. 

Jltta vulyaris (Polygoneae). A large number of seedlings were 
observed on three occasions. During the day the cotyledons 
sometimes stood sub-horizontally, but more commonly at an 
angle of about 50 above the horizon, and for the first two or 
three nights they rose up vertically so as to be completely 
closed. During the succeeding one or two nights they rose 
only a Lttle, and afterwards hardly at all. 

Amaranthus cuudatus (Amaranthactss). At noon the coty- 
ledons of many seedlings, which had just germinated, stood at 
about 45 above the horizon, and at 10.15 P.M. some were nearly 
and others quite closed. On the following morning they were 
again well expanded or open. 

C'^innalis satiua (Canuabineae). We are very doubtful whether 
this plant ought to be here included. The cotyledons of a large 
number of seedlings, after being well illuminated during the 
day, were curved downwards at night, so that the tips of some 
pointed directly to the ground, but the basal part did not appear 
to be at all depressed. On the following morning they were 
again flat and horizontal. The cotyledons of many other seed- 
lings were at the same time not in any way affected.v Therefore 
this case seems very different from that of ordinary sleep, and 
probably comes under the head of epinasty, as is the case with 
the leaves of this plant according to Kraus. The cotyledons are 
heliotropic, and so is the hypocotyl in a still stronger degree. 

Vxalis. We now come to cotyledons provided with a pulvinus, 
all of which are remarkable from the continuance of the nocturnal 
movements during several days or even weeks, and apparently 
after growth has ceased. The cotyledons of 0. rosea, Jloribuuda 
and urticulata sink vertically down at night and clasp the upper 
part of the hypocotyl. Those of 0. Valdiviana and s usitiva, on 
the contrary, rise vertically up, so that their upper surfaces come 
into close contact; and after the young leaves are developed these 
are clasped by the cotyledons. As in the daytime they stand hori- 
zontally, or are even a little deflected beneath the horizon, they 
move in the evening through an angle of at least 90. Their 
3omp)icated circum nutating movements during the da.v have 



308 MODIFIED CIRCUMXUTATION. CHAP. VI 

been described in the first chapter. The experiment was a 
superfluous one, but pots with seedlings of 0. rosea and flonbum/a 
were turned upside down, as soon as the cotyledons began to 
show any signs of sleep, and this made no difference in their 
movements. 

],eguminosce. It may be seen in our list that the cotyledons 
of several species in nine genera, widely distributed through- 
out the Family, sleep at night ; and this probably is the case 
with many others. The cotyledons of all these species are pro- 
vided with a pulvinus; and the movement in all is continued 
during many days or weeks. In Cassia the cotyledons of the 
ten species in the list rise up vertically at night and como 
into close contact with one another. We observed that those 
of 0. florida, opened in the morning rather later than those of 
('. glauca and pubescens. The movement is exactly the same 
in C'. mimosoides as in the other species, though its subsequently 
developed leaves sleep in a different manner. The cotyledons 
of an eleventh species, namely, C. nodofa, are thick and fleshy, 
and do not rise up at night. The circunmutation of the coty- 
ledons during the day of 0. tora has been described in the first 
chapter. Although the cotyledons of Smithia scnsitica rose from 
a horizontal position in the middle of the day to a vertical one 
at night, those of S. Pfundii, which are thick and fleshy, did not 
sleep. When Mimosa pudica and ulbida have been kept at a 
sufficiently high temperature during the day, the cotyledons 
corne into close contact at night ; otherwise they merely rise up 
almost vertically. The circumnutation of those of M. pudica 
has been described. The cotyledons of a Bauhinia from St. 
Catharina in Brazil stood during the day at an angle of about 
&j above the horizon, and at night rose to 77; but it is pro- 
bable that they would have closed completely, if the seedlings 
had been kept in a warmer place. 

Lotus. In three species of Lotus the cotyledons were observed 
to sleep. Those of L. Jacobaeus present the singular case of not 
rising at night in any conspicuous manner for the first 5 or 
6 days of their life,* and the pulvinus is not well developed at 
this period. Afterwards the sleeping movement is well dis- 
played, though to a variable degree, and is long continued. 
We shall hereafter meet with a nearly parallel case with the 
leaves of Sida rhomb if dm. The cotyledons of L. Gebelii are 
only slightly raised at night, and differ much in this respec* 
from the threo species in our list. 



CHAP VI. SLEEP OF COTYLEDONS. 309 

Trifulium. The germination of 21 species was observed. In 
most of them the cotyledons rise hardly at all, or only slightly, 
at night; but those of T. glomeratum, striatum and incmnatum 
rose from 45 to 55 above the horizon. With T. subterraneum, 
leucanthcmum and strictum, they stood up vertically; and with 
T. strictum the rising-movement is accompanied, as we shall see, 
by another movement, which makes us believe that the rising 
is truly nyctitropic. We did not carefully examine the coty- 
ledons of all the species for a pulvinus, but this organ was 
distinctly present in those of T. subterraneum and striclum ; whilst, 
there was no trace of a pulvinus in some species, for instance, in 
T. resupinatum, the cotyledons of which do not rise at night. 

Trlfolium subterraneum. The blades of the cotyledons OD the 
first day after germination (Nov. 21st) were not fully expanded, 
being inclined at about 35 above the horizon ; at night they 
rose to about 75. Two days afterwards the blades at noon 
were horizontal, with the petioles highly inclined upwards; 
and it is remarkable that the nocturnal movement is almost 
wholly confined to the blades, being effected by the pulvinus at 
their bases; whilst the petioles retain day and night nearly tho 
same inclination. On this night (Nov. 23rd), and for some few 
succeeding nights, the blades rose from a horizontal into a 
vertical position, and then became bowed inwards at about an 
average angle of 10 ; so that they had passed through an angle 
of 100. Their tips now almost touched one another, their 
bases being slightly divergent. The two blades thus formed 
a highly inclined roof over the axis of the seedling. This 
movement is the same as that of the terminal leaflet of the 
tripartite leaves of many species of Trifolium. After an interval 
of 8 days (Nov. 29th) the blades were horizontal during the 
day, and vertical at night, and now they were no longer bowed 
inwards. They continued to move in the same manner for the 
following two months, by which time they had increased greatly 
in size, their petioles being no less than -8 of an inch in length, 
and two true leaves had by this time been developed. 

Trifolium strictum. On the first day after germination the 
cotyledons, which are provided with a pulvinus, stood at noon 
horizontally, and at night rose to only about 45 above the 
horizon. Four days afterwards the seedlings were again ob- 
served at night, and now the blades stood vertically and were 
in contact, excepting the tips, which were much deflexed, so 
that they faced tho zenitlL At this ago the petioles arc curved 



310 MODIFIED CIECUMNDTATION. CIIAP. VI. 

upwards, and at night, when the bases of the blades are in con- 
tact, the two petioles together form a vertical ring surrounding 
the plumule. The cotyledons continued to act iu nearly the same 
manner for 8 or 10 days from the period of germination ; but 
the petioles had by this time become straight and had increased 
much in length. After from 1'2 to 14 clays the first simple true 
leaf was formed, and during the ensuing fortnight a remarkable 
movement was repeatedly observed. At I. (Fig. 125) we have 
a sketch, made in the middle of the day, of a seedling about 
a fortnight old. The two cotyledons, of which Re is the 
light, and Lc the left one, stand directly opposite one another, 




Trifolium strictum : diurnal and nocturnal positions of the two cotyledons 
and of the first leaf. I. Seedling viewed obliquely from above, during 
the day: EC, right cotyledon; Lc, left cotyledon; F, first true leaf. 
II. A rather younger seedling, viewed at night: jRc, right cotyledon 
raised, but its position not otherwise changed ;. Lc, left cotyledon raised 
and laterally twisted; F, first leaf raised and twisted so as to face the 
left twisted cotyledon. III. Same seedling viewed at night from the 
opposite side. The back of the first leaf, F, is here shown instead of 
the front, as in 11. 

and the first true leaf (F) projects at right angles to them. At 
night (see II. and III.) the right cotyledon (7?c) is greatly 
raised, but is not otherwise changed in position. The left 
cotyledon (/.c) is likewise raised, but it is also twisted, so that 
its blade, instead of exactly facing the opposite one, now stands 
at nearly right angles to it This nocturnal twisting movement 
is effected not by means of the pulvinus, but by the twisting of 
the whole length of the petiole, as could be seen by the curved 
Jine of its upper concave surface. At the same time the true 
leaf (F) rises up, so as to stand vertically, or it even passes the 
vortical and is inclined a little inwards. It also twists a little, 
uy which means the upper surface of its blade fronts, and 
almost comes into contact with, the upper surface of the twisted 



CHAF VI. SLEEP OF COTYLEDONS. 311 

ieft cotyledon. This seems to be the object gained by these 
singular movements. Altogether 20 seedlings were examined on 
successive nights, and in 19 of them it was the left cotyledon 
alone which became twisted, with the true leaf always so twisted 
that its upper surface approached closely and fronted that of the 
left cotyledon. In only one instance was the right cotyledon 
twisted, with the true leaf twisted towards it; but this seedling 
was in an abnormal condition, as the left cotyledon did not riso 
up properly at night. This whole case is remarkable, as with 
the cotyledons of no other plant have we seen any nocturnal 
movement except vertically upwards or downwards. It is the 
more remarkable, because we shall meet with an analogous case 
in the leaves of the allied genus Melilotus, in which the ter- 
minal leaflet rotates at night so as to present one edge to the 
zenith and at the same time bends to one side, so that its upper 
surface comes into contact with that of one of the two now ver- 
tical lateral leaflets. 

Concluding Remarks on the Nyctitropic Movements of 
Cotyledons. The sleep of cotyledons (though this is a 
subject which has been little attended to), seems to be 
a more common phenomenon than that of leaves. We 
observed the position of the cotyledons during the day 
and night in 153 genera, widely distributed v through- 
out the dicotyledonous series, but otherwise selected 
almost by hazard; and one or more species in 26 of 
these genera placed their cotyledons at night so as 
to stand vertically or almost vertically, having gene- 
rally moved through an angle of at least 60. If we 
lay on one side the Leguminosae, the cotyledons of 
which are particularly liable to sleep, 140 genera 
remain ; and out of these, the cotyledons of at least one 
species in 19 genera slept. .Now if we were to select 
by hazard 140 genera, excluding the Leguininosee, and 
observed their leaves at night, assuredly not nearly 
so many as 19 would be found to include sleeping 
species. We here refer exclusively to the plants 
observed by ourselves. 



312 MODIFIED CIRCUMNUTATION. CHAP. VI 

In our entire list of seedlings, there are 30 genera, 
belonging to 16 Families, the cotyledons of which in 
some of the species rise or sink in the evening or 
early night, so as to stand at least 60 above or be- 
neath the horizon. In a large majority of the genera, 
namely, 24, the movement is a rising one ; so that 
the same direction prevails in these nyctitropic move- 
ments as in the lesser periodic ones described in the 
second chapter. The cotyledons move downwards 
during the early part of the night in only 6 of the 
genera; and in one of them, Cannabis, the curving 
down of the tip is probably due to epinasty, as Kraus 
believes to be the case with the leaves. The down- 
ward movement to the amount of 90 ' is very decided 
in Oxalis Valdimana and sensitiva, and in Geranium 
rotundifolium. It is a remarkable fact that with Anoda 
Wriglitii, one species of Gossypium and at least 3 
species of Ipomcea, the cotyledons whilst young and 
light sink at night very little or not at all ; although 
this movement becomes well pronounced as soon as 
they have grown large and heavy. Although the 
downward movement cannot be attributed to the 
weight of the cotyledons in the several cases which 
were investigated, namely, in those of the Anoda, 
Ipomcea purpurea and bona-nox, nor in that of J coc- 
sinea, yet bearing in mind that cotyledons are con- 
tinually circumnutating, a slight cause might at first 
have determined whether the great nocturnal move- 
ment should be upwards or downwards. We may 
therefore suspect that in some aboriginal member of 
the groups in question, the weight of the cotyledons 
first determined the downward direction. The fact of 
the cotyledons of these species not sinking down much 
whilst they are young and tender, seems opposed to 
the belief that the greater movement when they are 



CHAP. VI. SLEEP OF COTYLEDONS. 313 

grown older, has been acquired for the sake of pro* 
tecting them from radiation at night ; but then we 
should remember that there are many plants, the 
leaves of which sleep, whilst the cotyledons do not ; 
and if in some cases the leaves are protected from cold 
at night whilst the cotyledons are not protected, so in 
other cases it may be of more importance to the species 
that the nearly full-grown cotyledons should be better 
protected than the young ones. 

In all the species of Oxalis observed by us, the coty- 
ledons are provided with pulvini ; but this organ has 
become more or less rudimentary in 0. corniculata, 
and the amount of upward movement of its cotyledons 
at night is very variable, but is never enough to be 
called sleep. We omitted to ascertain whether the 
cotyledons of Geranium rotundifolium possess pulvini. 
In the Leguminosae all the cotyledons which sleep, as 
far as we have seen, are provided with pulvini. But 
with Lotus Jacobseus, these are not fully developed 
during the first few days of the life of the seedling, 
.ind the cotyledons do not then rise much at night. 
With Trifolium strictum the blades of the cotyledons 
rise at night by the aid of their pulvini ; whilst the 
petiole of one cotyledon twists half-round at the same 
time, independently of its pulvinus. 

As a general rule, cotyledons which are provided 
with pulvini continue to rise or sink at night during 
a much longer period than those destitute of this organ. 
In this latter case the movement no doubt depends on 
alternately greater growth on the upper and lower side 
of the petiole^ or of the blade, or of both, preceded 
probably by the increased turgescence of the growing 
cells. Such movements generally last for a very 
short period for instance, with Brassica and Githago 
fur 4 or 5 nights, with Beta for 2 or 3, and with 



314 MODIFIED CIRCUMNUTATION. CHAP. VI 

Raphanus for only a single night. There are, however, 
some strong exceptions to this rule, as the cotjledons 
of Gossypium, Anoda and Ipoincea do not possess pul- 
vini, yet continue to move and to grow for a long time. 
We thought at first that when the movement lasted for 
only 2 or 3 nights, it could hardly be of any servico 
to the plant, and hardly deserved to be called sleep ; 
but as many quickly-growing leaves sleep for only a 
few nights, and as cotyledons are rapidly developed 
and soon complete their growth, this doubt now seems 
to us not well-founded, more especially as these move- 
ments are in many instances so strongly pronounced. 
We may here mention another point of similarity 
between sleeping leaves and cotyledons, namely, that 
some of the latter (for instance, those of Cassia and 
Githago) are easily affected by the absence of light ; 
and they then either close, or if closed do not open ; 
whereas others (as with the cotyledons of Oxalis) are 
very little affected by light. In the next chapter it 
will be shown that the nyctitropic movements both 
of cotyledons and leaves consist of a modified form of 
circumnutation . 

As in the Leguminosae and Oxalidee, the leaves and 
the cotyledons of the same species generally sleep, the 
idea at first naturally occurred to us, that the sleep 
of the cotyledons was merely an early development of 
a habit proper to a more advanced stage of life. But 
no such explanation can be admitted, although there 
seems to be some connection, as might have been 
expected, between the two sets of cases. For the 
leaves of many plants sleep, whilst their cotyledons do 
not do so of which fact JJesmodium gyrans offers & 
good instance, as likewise do three species of Nico- 
tiana observed by us; also Sida rJiombifolia, Abutilon 
Darwinn, and Chenopodium album. On the other 



CUAI>. VI. SLEEP OF COTYLEDONS. 315 

hand, the cotyledons of some plants sleep and not the 
leaves, as with the species of Beta, Brassica, Geranium, 
Apium, Solanum, and Mirabilis, named in our list. 
Still more striking is the fact that, in the same genus, 
the leaves of several or of all the species may sleep, 
but the cotyledons of only some of them, as occurs 
with Trifolium, Lotus, Gossypium, and partially with 
Oxalis. Again, when both the cotyledons and the 
leaves of the same plant sleep, their movements may 
be of a widely dissimilar nature : thus with Cassia the 
cotyledons rise vertically up at night, whilst their 
leaves sink down and twist round so as to turn their 
lower surfaces outwards. With seedlings of Oxalis 
Valdiviana, having 2 or 3 well-developed leaves, it 
was a curious spectacle to behold at night each leaflet 
folded inwards and hanging perpendicularly down- 
wards, whilst at the same time and on the same plant 
the cotyledons stood vertically upwards. 

These several facts, showing the independence of 
the nocturnal movements of the leaves and cotyledons 
on the same plant, and on plants belonging to the 
same genus, lead to the belief that the cotyledons have 
acquired their power o movement for some special 
purpose. Other facts lead to the same conclusion, 
such as the presence of pulvini, by the aid of which 
the nocturnal movement is continued during some 
weeks. In Oxalis the cotyledons of some species 
move vertically upwards, and of others vertically 
downwards at night ; but this great difference within 
the same natural genus is not so surprising as it 
may at first appear, seeing that the cotyledons of all 
the species are continually oscillating up and down 
during the day, so that a small cause might determine 
whether they should rise or sink at night. Again, the 
peculiar nocturnal movement of the left-hand coty- 



318 MODIFIED CIRCUMNUTATION CHAP VI 

ledon of Trifolium strictum, in combination with thai 
of the first true leaf. Lastly, the wide distribution in 
the dicotyledonous series of plants with cotyledons 
which sleep. Reflecting on these several facts, our 
conclusion seems justified, that the nyctitropic move- 
ments of cotyledons, by which the blade is made to 
stand either vertically or almost vertically upwards 
or downwards at night, has been acquired, at least 
in most cases, for some special purpose ; nor can w& 
doubt that this purpose is the protection of the upper 
surface of the blade, and perhaps of the central bud 
or plumule, from radiation at uight. 



CHAF. VIL MODIFIED CIRCUilNUTATION. 317 



CHAPTER VII. 

MODITIED CIRCTMNUTATIOX : NvcTiTnopic OB SLEEP MOVEMEN-.S o 
LEAVES. 

Conditions necessary for these movements List of Genera and Families, 
which include sleeping plants Description of the movements in 
the several Genera Oxalis: leaflets folded at night Averrhoa : 
rapid movements of the leaflets Porlietia: leaflets close when 
plant kept very dry Tropscolum : leaves do not Bleep unless well 
illtiininatid during day Lupinus: various modes of sleeping 
Melilotus : singular movements of terminal leaflet Trifolium 
Desmodinm : rudimentary lateral leaflets, movements of, not de- 
veloped on young plants, state of their pulvini Cassia : complex 
movements of the leaflets Bauhinia: leaves folded at night 
Mimosa pudica: compounded movements of leaves, effect of dark- 
ness Mimosa albida, reduced leaflets of Schrankia: downward 
movement of the pinnae Marsile.i : the only cryptogam known to 
sleep Concluding remarks and summary Nyctitropism consists 
of modified circumnutation, regulated by the alternations of light 
and darkness Shape of first true leaves. 

WE now come to the nyctitropic or sleep move- 
ments of leaves. It should be remembered that we 
confine this term to leaves which place their blades 
at night either in a vertical position or not more than 
30 from the vertical, that is, at least 60 above or 
beneath the horizon. In some few cases this is 
effected by the rotation of the blade, the petiole not 
being either raised or lowered to any considerable 
extent. The limit of 30 from the vertical is obviously 
an arbitrary one, and has been selected for reasons 
previously assigned, namely, that when the blade 
approaches the perpendicular as nearly as this, only 
half as much of the surface is exposed at night to the 



318 MODIFIED CIECUMNUTATION. CHAP. VU 

zenith and to free radiation as when the blade ig 
horizontal. Nevertheless, in a few instances, leaves 
which seem to be prevented by their structure from 
moving to so great an extent as 60 above or beneath 
the horizon, have been included amongst sleeping 
plants. 

It should be premised that the nyctitropic move- 
ments of leaves are easily affected by the conditions 
to which the plants have been subjected. If the ground 
is kept too dry, the movements are much delayed 
or fail : according to Dassen,* even if the air is 
very dry the leaves of Impatiens and Malva are 
rendered motionless. Carl Kraus has also lately 
insisted f on the great influence which the quantity of 
water absorbed has on the periodic movements of 
leaves ; and he believes that this cause chiefly deter- 
mines the variable amount of sinking of the leaves of 
Polygonum convolvulus at night ; and if so, their move- 
ments are not in our sense strictly nyctitropic. Plants 
in order to sleep must have been exposed to a proper 
temperature : Erythrina crista-galli, out of doors and 
nailed against a wall, seemed in fairly good health, 
but the leaflets did not sleep, whilst those on another 
plant kept in a warm greenhouse were all vertically de- 
pendent at night. In a kitchen-garden the leaflets of 
Phaseolus vulgaris did not sleep during the early part 
of the summer. Ch. Boyer says,J referring I suppose 
to the native plants in France, that they do not sleep 
when the temperature is below 5 C. or 41 F. In 
the case of several sleeping plants, viz., species of 



* Dassen, ' Tijdschrift vor. Na- Bot.' (5th series \ ix. 18GS, p. 345. 

lurlijke Gcseh. en Physiologic,' f ' Beitriige zur Kentuiss der 

1837, vol. iv. p. 106. See also Bewt gungcn,' &c., in 'Flora,' 

Ch. Rover on the importance of a 1879, pp. 42, 43, G7, &c. 

proper state of turgesccnce of the J Annal. des Sc. Nat. Bot.' 

cells, in 'Aiinal. det Sc. Nat. (5th Series), ix. 1868 p.36G. 



CHAP. VII. SLEEP OF LEAVES. 319 

Tropaeoliun, Lupinus, Ipomoea, Abutilon, Siegesbeckia, 
and probably other genera, it is indispensable that 
the leaves should be well illuminated during the day 
in order that they may assume at night a vertical 
position ; and it was probably owing to this cause 
that seedlings of Chenopodium album and Siegcsbeckia 
orientalis, raised by us during the middle of the winter, 
though kept at a proper temperature, did not sleep. 
Lastly, violent agitation by a strong wind, during a 
few minutes, of the leaves of Maranto arundinacea 
(which previously had not been disturbed in the hot- 
house), prevented their sleeping during the two next 
nights. . 

We will now give our observations on sleeping 
plants, made in the manner described in the Intro- 
duction. The stem of the plant was always secured 
(when not stated to the contrary) close to the base of 
the leaf, the movements of which were being observed, 
so as to prevent the stem from circumnutating. As 
the tracings were made on a vertical glass in front of 
the plant, it was obviously impossible to Irace its 
course as soon as the leaf became in the evening 
greatly inclined either upwards or downwards; it 
must therefore be understood that the broken lines 
in the diagrams, which represent the evening and 
nocturnal courses, ought always to be prolonged to a 
much greater distance, either upwards or downwards, 
than appears in them. The conclusions which may be 
deduced from our observations will be given near tho 
end of this chapter. 

In the following list all the genera which include 
sleeping plants are given, as far as known to us. Tho 
same arrangement is followed as in former cases, and 
the number of the Family is appended. This list 
possesses some interest, as it shows that the habit of 



320 



MODIFIED CIRCUMXUTATION. 



CHAT. VII. 



sleeping is common to some few plants throughout 
the whole vascular series. The greater number of the 
genera in the list have been observed by ourselves 
with more or less care ; but several are given on the 
authority of others (whose names are appended in the 
list), and about these we have nothing more to say. 
No doubt the list is very imperfect, and several genera 
might have been added from the ' Sornnus Plantarum ' 
by Linnaeus ; but we could not judge, in some of his 
cases, whether the blades occupied at night a nearly 
vertical position. He refers to some plants as sleeping, 
for instance, Lathyrus odoratus and Vicia faba, in which 
we could observe no movement deserving to be called 
sleep, and as no one can doubt the accuracy of Linnaeus, 
we are left in doubt. 

List of Genera, including specie's the leaves of tvJiich sleep. 



CLASS I. DICOTYLEDON'S. 


Sub-class I. AXGIOSPERJIS continued. 


Sub-class I. 
Genus. 
Githago 


&.XGIOSPKRMS. 

Family. 
Caryophyllese (26). 


Genus. Family. 
Tropseolum. Tropseoleas (49). 
Crotolaria(Thisel-\ Legumiuosaj (75J 
ton Dyer). / Tribe 11. 


Stellaria (Bataliu). 


w 


Lupinus. i 


Portulaca (Ch.\ 
Royer). / 


Portulaceae (27). 


Cytisus. 
Trigonella. 


" Tr. 'ill. 


Sida. 


Malvaceae (36). 


Medicago. 


M 


Abutilon. 




Melilotus. 


. 


Malva (Linnaeus'! 




Trifolium. 




and Pfeffer). / 


" 


Securigera. 


! Tr."lV. 


Hibiscus (Lin-1 




Lotus. 




nacus). / 


" 


Psora lea. 


.' Tr.'V. 


Anoda. 


n 


Amorpha (Du-| 


Gossypium. 


n 


chartre). / " " 


Ayenia (Linnaeus). 


Sterculacezc (37). 


Da3lea. 


11 


Iriumfetta (Lin-\ 
naeus). / 
Linum (Batulin). 


Tiliacc-ffi (38). 
Linese (39). 


Indigofera. 
Tephrosia. 
Wistaria. 


" 


Oxalis. 


Oxalida: (41). 


Robinia. 


" !' 


Averrhoa, 




SphaDrophysa. 




Porlieria. 


Zygophylleae (45). 


Colutea. 


> >i 


Guiacum. 




Astragalus. 




Impatiens (Lin-j 


Glycyrrhiza. 


!! 


n:eus, Pfe Fer,V Balsamineae (48). 


Cpronilla. 


" Tr.VI 


Batalin) j , 


Hcdysarura. 





CHAP. VII. 



SLEEP OF LEAVES. 



List of Genera (continued). 


CtASS I. DICOTYLEDONS (continued). 


Sub-class I. ANGIOPF_1MS (conti),itccf) 


Sub-class I. ANGIGSPERMS. 


Genus. 


Family, 


Genus. j Family. 


^nothera (Lin-| 
use us). J 


Onagravieae (100), 


f. , , . (Lecjuminosae (75) 
Onobrychis. 1 < , . v v . ' 
\\ ir. vi. 


Passiflora. 
Siegesbeckia. 


Pass:floraccae(10 : > > '- 
Compositas (122). 


Smithia. 
Arachis. 




Ipomcea. 


JConvolvulaceae 
I (151). 


Dcsmodium. 





Nicotiana. 


Solaneaj (157). 


Urania. 





Mirabilis. 


Nyctaginecc (177). 


Vicia. 
Centrosema. 


Tr. VII. 
Tr. VIII. 


Polygonum (Ba-\ 
talin). / 


Polygoneae (179). 


Amphicarpaca. 
Glycine. 
Erythriua. 


" 


Amaranthus. 
Chenopodium. 


JAmaranthacesB 
I (180). 
Chenopodieae (181^. 


Apios. 
Phaseolus. 


" 


Pimelia (Douche"). 
Euphorbia. 


Thymetea) (188). 
Euphorbiaceac (2u2} 


Sophora. 
Ctcsalpinia. 


Tr.'x. 

Tr. XIII. 


Phyllanthus(Pfef-) 
fer). / 





Haematoxylon. 






Gleditsclna (Du-\ 




Sub-class II. GYM^OSPERMS. 


chartre). J 
Poinciana. 


" 


Abies <Chatiu). 


Cassia. 


11 Tr.XIV. 




Bauhinia. 


Tr. XV. 


CLASS II. MONOCOTYLEDONS. 


Tamarindu?. 


Tr. .XVI. 




Adenanthera. 


Tr. XX. 


Thalia. 


Cannaceae (21). 


Prosopis. 




Marantn. 


^ 


Neptunia. 




Colocasia. 


Aroideac (30). 


Mimosa. 


11 11 


Strephium. 


Grainineas (55). 


Schraukia. 






. 


Tr. x'xil 




Albizzia. 


11 Tr'.XXIli. 


CLASS III. ACOTYLEDOXS. 


AJclaleuca(Bouche). 


Jlyrtaceae (94). 


Marsilea. Marsileaceae (4). 



Githago segctum (Caryophyllefe). The first leaves produced 
by young seedlings, rise up and close together at night. On a 
rather older seedling, two young leaves stood at noon at 55 
above the horizon, and at night at 86, so each had risen 31. 
The angle, however, was less in some cases. Similar observations 
were occasionally made on young leaves (for the older ones moved 
very little) produced by nearly full-grown plants. Batalin 
says ('Flora,' Oct. 1st, 1873, p. 437) that the young leaves of 
Stellaria close up so completely at night that they form together 
great buds. 

f-ida (Malvaceae). The nyctitropic movements of the leaves 
in this genus are remarkable in some respects. Bataliu informs 



322 



MODIFIED CIRCUMNUTATIOX. 



CHAP. TIL 



as (see also 'Flora,' Oct. 1st, 1873, p. 437) that those ol 
S. napcea fall at night, but 
to what angle he cannot 
remember. The leaves of 
S. rhombifolia and retusa, on 
the other hand, rise up 
vertically, and are pressed 
against the stem. \\e have 
therefore here within the 
same genus, directly op- 
posite movements. Again, 
the leaves of <S. rhombifoliu 
are furnished with a pul- 
vinus, formed of a mass of 
small cells destitute of chlo- 
rophyll, and with their 
longer axes perpendicular 
to the axis of the petiole. 
As measured along this 
latter line, these cells are 
only -Jth of the length of 
those of the petiole; but 
instead of being abruptly 
separated from them (as is 
usual with the pulvinus in 
most plants), they graduate 
into the larger cells of the 
petiole. On the other hand, 
S. napcea, according to Ba- 
talin, does not possess a 
pulvinus; and he informs 
us that a gradation may be 
traced in the several species 
of the genus between these 
two states of the petiole. 
>^ida rhombifolia presents 
another peculiarity, of which 
we have seen no other in- 
stance with leaves that 
sleep: for those on very 
young plants, though they 

rise somewhat in the evening, do not go to sleep, as we observed 




Qidt rhom'nfolia : circumnutation and 
nyttitropic (or sleep) movements of 
a leaf on a young plant, 9 inches 
high; filament fixed to midrib of 
nearly full-grown leaf, 2jj inches in 
length ; movement traced under a sky- 
light. Apex of leaf 5| inches from 
the vertical glass, so diagram not 
greatly enlarged. 



CHAP. VII. SLEEP OF LEAVES 323 

on several occasions; whilst those on rather older plants sleep 
in a conspicuous manner. For instance, a leaf (-85 of an inch 
in length) on a very young seedling 2 inches high, stood at noon 
9 above the horizon, and at 10 P.M. at 28, so it had risen only 
19; another leaf (1 '4' inch in length) on a seedling of the 
same height, stood at the same two periods at 7 and 32, and 
therefore had risen 25. These leaves, which moved so little, 
had a fairly well- developed pulviuus. After an interval of some 
weeks, when the same seedlings were 2 5 and 3 inches in height, 
some of the young leaves stood up at night quite vertically, and 
others were highly inclined ; and so it was with bushes which 
were fully grown and were flowering. 

The movement of a leaf was traced from 9.15 A.M. on 
May 28th to 8.30 A.M. on the 30th. The temperature was too 
low (15 16 C.), and the illumination hardly sufficient ; con- 
sequently the leaves did not become quite so highly inclined at 
night, as they had done previously and as they did subse- 
quently in the hot-house ; but the movements did not appear 
otherwise disturbed. On the first day the leaf sank till 
5.15 P.M. ; it then rose rapidly and greatly till 10.5 P.M., and 
only a little higher during the rest of the night (Fig. 126). 
Early on the next day (29th) it fell in a slightly zigzag line 
rapidly until 9 A.M., by which time it had reached nearly the 
same place as on the previous morning. During the remainder 
of the day it fell slowly, and zigzagged laterally. The evening 
rise began after 4 P.M. in the same manner as before, and on 
the second morning it again fell rapidly. The ascending and 
descending lines do not coincide, as may be seen in the diagram. 
On the 30th a new tracing was made (not here given) on a 
rather enlarged scale, as the apex of the leaf now stood 9 inches 
from the vertical glass. In order to observe more carefully the 
course pursued at the time when the diurnal fall changes into 
the nocturnal rise, dots were made every half-hour between 
4 P.M. and 10.30 P.M. This rendered the lateral zigzagging 
movement during the evening more conspicuous than in the 
diagram given, but it was of the same nature as there shown. 
The impression forced on our minds was that the leaf was 
expending superfluous movement, so that the great nocturnal 
rise might not occur at too early an hour. 

Alutilon Darwinii (Malvaceae). The leaves on some very 
young plants stood almost horizontally during the day, and 
hung down vertically at night. Very fine plants kept in a 



324 MODIFIED CIKCUMNUTATION. CHAP. VII. 

large hall, lighted only from the roof, did not sleep at night, 
for in order to do so the leaves must be well illuminated during 
the day. The cotyledons do not sleep. Linnaeus says that the 
leaves of his 'Sid-t abutilon sink perpendicularly down at night, 
though the petioles rise. Prof. Pfeffer informs us that the 
leaves of a Malva, allied to M. sylvestris, rise greatly at night; 
and this genus, as well as that of Hibiscus, are included by 
Linnaeus in his list of sleeping plants. 

Anoda Wriglitii (Malvaceae). The leaves, produced by very 
young plants, when grown to a moderate size, sink at night 
either almost vertically down or to an angle of about 45 beneath 
the horizon; for there is a considerable degree of variability in 
the amount of sinking at night, which depends in part on the 
degree to which they have been illuminated during the day. 
But the leaves, whilst quite young, do not sink down at night, 
and this is a very unusual circumstance. The summit of the 
petiole, where it joins the blade, is developed into a pulvinus, 
and this is present in very young leaves which do not sleep ; 
though it is not so well denned as in older leaves. 

Qossyjrium (var. Nankin cotton, Malvaceae). Some young 
leaves, between 1 and 2 inches in length, borne by two seedlings 
6 and 7h inches in height, stood horizontally, or were raised a 
little above the horizon at noon on July 8th and 9th ; but by 
10 P.M. they had sunk down to between 68 and 90 beneath 
the horizon. When the same plants had grown to double 
the above height, their leaves stood at night almost or quite 
vertically dependent. The leaves on some large plants of 
G. maritimum and JBruzilense, which were kept in a very badly 
lighted hoMiouse, only occasionally sank much downwards 
at night, and hardly enough to be called sleep. 

Oxalis (Oxalidae). In most of the species in this large genus 
the three leaflets sink vertically down at night; but as their 
sub-petioles are short the blades could not assume this position 
from the want of space, unless they were in some manner ren- 
dered narrower; and this is effected by their becoming more 
or less folded (Fig. 127;. The angle formed by the two halves 
of the same leaflet was found to vary in different individuals of 
several species between 92 and 150; in three of the best 
folded leaflets of 0. fragrans it was 76, 74, and 54. The 
angle is often different in the three leaflets of the same leaf. 
As the leaflets sink down at night and become folded, their 
lower surfaces are brought near together (jsee B), or even into 



CHAP. VII. SLEEP OF LEAVES. 325 

close contact; and from this circumstance it might be thought 
that the object of the folding was the protection of their lower 
surfaces. If this had been the case, it would have formed 
a strongly marked exception to the rule, that when there is any 
difference in the degree of protection from radiation of the two 
surfaces of the leaves, it is always the upper surface which is 
the best protected. But that the folding of the leaflets, and 
consequent mutual approximation of their lower surfaces, 
serves merely to allow them to sink down vertically, may be 




A. B. 

Oxalis acetosclla : A, leaf seen from vertically above ; B, diagram of leaf 
asleep, also seen from vertically above. 

inferred from the fact that when the leaflets do not radiate 
from the summit of a common petiole, or, again, when there is 
plenty of room, from the sub-petioles not being very short, the 
leaflets sink down without becoming folded. This occurs with 
the leaflets of 0. sensitiva, Plumierii, and bupleurifolia. 

There is no use in giving a long list of the many species 
which sleep in the above described manner. This holds good 
with species having rather fleshy leaves, like those of 0. carnosrt, 
or large leaves like those of 0. Ortrgesii, or four leaflets like 
those of 0. variabilis. There are, however, some species which 
show no signs of sleep, viz., 0. pextophyUa, enneuphylla, hirta, 
and rubella. We will now describe the nature of the movements 
in some of the species. 

Oxulis acttosclla.The movement of a leaflet, together with 
that of the main petiole, are shown in the following dia- 
gram (Fig. 128), traced between 11 A.M. on October 4th and 
7.45 A.M. on the 5th. After 5.30 P.M. on the 4th the leaflet sank 
rapidly, and at 7 P.M. depended vertically. Fr some time 
before it assumed this latter position^ its movements could, of 
course, no longer be traced on the vertical glass, and the 
broken line in the diagram ought to l>e extended much further 



326 



MODIFIED CIRCUMNUTATIOX. CHAP. VH 



Fig. 128. 




down in this and all other 
cases. By 6.45 A.M. on the 
following morning it had 
risen considerably, and con- 
tinued to rise for the next 
hour; but, judging from 
other observations, it would 
soon have begun to fall again. 
Between 11 A.M. and 5.30 P.M. 
the leaflet moved at least four 
times up and four times 
down before the great noc- 
turnal fall commenced; it 
reached its highest point at 
noon. Similar observations 
were made on two other 
leaflets, with nearly the same 
results. Sachs and Pfeffer 
have also described briefly* 
the autonomous movements 
of the leaves of this plant. 

On another occasion the 
petiole of a leaf was secured 
to a little stick close beneath 
the leaflets, and a filament 
tipped with a bead of sealing- 
wax was affixed to the mid- 
rib of one of them, and a 
mark was placed close behind. 
At 7 P.M., when the leaflets 
were asleep, the filament de- 
pended vertically down, and 
the movements of the bead 
were then traced till 10.40 
P.M., as shown in the fol- 
lowing diagram (Fig. 129). 



full-grown leaf, with filament at- moved a little from Side to 
tached to the midrib of one of the s {^ e> as we n as a Httle up 
leaflets; traced on vertical glass dur- d d ^ m&i ^ 

ing 20 h. 45 m. 

* Siiclis in 'Flora,' 1863, p. 470, Ac.; Pfeffer, 'Die Period. Bewe- 
' &c., 1875, p. 53. 



CHAP. VII. SLEEP OF LEAVES. 327 

Oxalis Valdiviana. The leaves resemble those of the last 
species, and the movements of two leaflets (the main petioles of 
both having been secured) were 
traced during two days; but the F'g. 129. 

tracings are hot given, as they 
resembled that of 0. acetusella, with 
the exception that the up and 
down oscillations were not so fre- 
quent during the day, and there Oxalis acetusella: circumnuta- 

was more lateral movement, so that tion , f leariet when 1 asle P 5 

, . -i ! i traced on vertical glass 

broader ellipses were described. during 3 h. 40m. 
The leaves awoke early in the morn- 
ing, for by 6.45 A.M. on June 12th and 13th they had not only 
risen to their full height, but had already begun to fall, that is, 
they were circumnutating. We have seen in the last chapter 
that the cotyledons, instead of sinking, rise up vertically at 
night. 

Oxalis Ortegesii. The large leaves of this plant sleep like 
those of the previous species. The main petioles are long, and 
that of a yourg leaf rose 20 between noon and 10 P.M., whilst 
the petiole of an older leaf rose only 13. Owing to this rising 
of the petioles, and the vertical sinking of the large leaflets, 
the leaves become crowded together at night, and the whole 
plant then exposes a much smaller surface to radiation than 
during the day. 

Oxalis Plumierii. In this species the three leaflets do not 
surround the summit of the petiole, but the terminal leaflet 
projects in the line of the petiole, with a lateral leaflet on each 
side. They all sleep by bending vertically downwards, but 
do not become at all folded. The petiole is rather long, and, 
one having been secured to a stick, the movement of the terminal 
leaflet was traced during 45 h. on a vertical glass. It moved 
in a very simple manner, sinking rapidly after 5 P.M., and 
rising rapidly early next morning. During the middle of the day 
it moved slowly and a little laterally. Consequently the ascend- 
ing and descending lines did not coincide, and a single great 
ellipse was formed each day. There was no other evidence of 
circumnutation, and this fact is of interest, as we shall here- 
after see. 

Oxalis sensitiva. The leaflets, as in the last species, bend 
vertically down at night, without becoming folded. The much 
elongated main petiole rises considerably in the evening, but in 
22 



328 



MODIFIED CIRCUMNUTATION. CHAP. VII 



Rome very young plants the rise did not commence until late 
at night. We have seen that the cotyledons, instead of sink- 
ing like the leaflets, rise up vertically at night. 

Oxalis bupleurifolia, This speciaa 
is rendered remarkable by the petiolea 
being foliaceous, like the phyllcdes 
of many Acacias. The leaflets are 
bmall, of a paler green and more 
tender consistence than the folia- 
ceous petioles. The leaflet which was 
observed was '55 inch in length, and 
was borne by a petiole 2 inches long 
and '3 inches broad. It may be 
suspected that the leaflets are on the 
road to abortion or obliteration, as 
has actually occurred with those of 
another Brazilian species, 0. rusci- 
formis. Nevertheless, in the present 
species the nyctitropic movements 
are perfectly performed. The folia- 
ceous petiole was first observed 
during 48 h., and found to be in 
continued circumnutation, as shown 
in the accompanying figure (Fig. 
130). It rose during the day and 
early part of the night, and fell 
during the remainder of the night 
and early morning; but the move- 
ment was not sufficient to be called 
sleep. The ascending and descend- 
ing lines did not coincide, so that an 
ellipse was formed each day. There 
was but little zigzagging; if the 
filament had been fixed longitudi- 
nally, we should probably have seen 
that there was more lateral move- 
ment than appears in the diagram. 

A terminal leaflet on another leaf was next observed (the 
petiole being secured), and its movements are shown in 
Fig. 131. During the day the leaflets are extended horizon- 
tally, and at night depend vertically ; and as the petiole rises 
during the day the leaflets have to bend down in the evening 



Osdii bupleurifoUa : circum- 
nutation of foliaceous pe- 
tiole, filament fixed ob- 
liquely across end of petiole; 
movements traced on ver- 
tical glass from 9. A.M. .June 
26th to 8.50 A.M. 28th. 
Apex of leaflet 4 inches 
from the glass, so movement 
not much magnified. Plant 
9 inches high, illuminated 
from above. Temp. 23i- 
24| C. 



CHAP. VII. 



SLEEP OF LEAVES. 



329 



more than 90, so as to assume at night their vertical position. 
On the first day the leaflet simply moved np and down ; on tUa 



Fig. 131. 




second day it plainly circumnutated between 8 A.M. and 4.30 P.M. 
after which hour the great evening fall commenced. 



330 MODIFIED CIRCUMNUTATION. CHAP. VII 

Averrhoa bilimbi (Oxalidae). It has long beer: known,* firstly. 
that the leaflets in this genus sleep ; eecondly, that they move 
spontaneously during the day ; and thirdly, that they are sensi- 
tive to a touch; but in none cf these respects do they diffei 
essentially from the species of Oxalis. They differ, however, as 
Mr. K. I. Lynch f has lately shown, in their spontaneous move- 
ments being strongly marked. In the case of A. bilimbi, it is a 
wonderful spectacle to behold on a warm sunny day the leaflets 
one after the other sinking rapidly downwards, and again 
ascending slowly. Their movements rival those of Desmodium 
gyrans. At night the leaflets hang vertically down ; and now 

Fig. 132. 




Averrkoi, bilimbi : loaf asleep; drawing reduced. 

they 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 arc placed close behind the leaflet. A fine glass filament 
was fixed to the leaf, so as to project like a continuation of the 



* Dr. Bruce, < Philosophical Trans.,' 1785, p. 35G. 
'Journal Linn. Soc.,' vol. xvi. 1877, p. 281. 



CHAP. VII. SLEEP OF LEAVES. 331 

midrib. This filament acted as an index; and as the leaf rose 
and fell, rotating about its basal joint, its angular movement 

Fig. 133. 




Atcrrlwa bilimhi: angular movements ot a IwiHet during its evening 
descent, when going to sleep. Temp. 78-81 F. 

could be recorded by reading off at short intervals of time the 
position of the glass filament on the graduated arc. In order 



332 MODIFIED CIRCUMNUTATION. CHAP. VII 

to avoid errors of parallax, all readings were made by looking 
through a small ring painted on the vertical glass, in a lina 
with the joint of the leaflet and the centre of the graduated aro. 
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 p M. the leaflet 
formed an angle of 85 with the vertical, or was only 5 below 
the horizontal; but in order that the diagram might get into 
our page, the leaflet is represented falling from 75 instead 
of 85. Shortly after 6 P.M. it hung vertically down, and had 
attained its nocturnal position. Between 6.10 and 6.35 P.M. it 
performed a number of minute oscillations of about 2 each, 
occupying periods of 4 or 5 m. The complete state of rest of 
the leaflet which ultimately followed is not shown in the dia- 
gram. It is manifest that each oscillation consists of a gradual 
rise, followed by a sudden fall. Each time the leaflet fell, it 
approached nearer to the nocturnal position than it did on the 
previous fall. The amplitude of the oscillations diminished, 
while the periods of oscillation became shorter. 

In bright sunshine the leaflets assume a highly inclined de- 
pendent position. A leaflet in diffused light was observed rising 
for 25 m. A blind was then pulled up so that the plant was 
brightly illuminated (BE 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 
s-imilar 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 BE', when the sun 
was again admitted. In this experiment cool air was allowed 
to enter by the windows being opened at the same time that 
the blinds were pulled up, so that in spite of the sun shining 
on the plant the temperature was not raised. 

The effect of an increase of temperature in diffused light is 

* In nil the diagrams 1 mm. in ment. In Figs. 133 and 134 the 

the horizontal direction represents temperature is represented (along 

one minute of time. Each mm. the ordinates) in the scale of 1 

in the vertical direction repre- mm. to each 0'1G. In Fig. 

ents one degree of angular movu- 135 each mm. equals 0'2 F. 



CHAP. VII. 



SLEEP OF LEAVES. 



333 



shown in Fig. 135. The temperature began to rise at 11.35 
A.M. (in consequence of the fire being lighted), but by 12.42 a 
marked fall had occurred. It may be seen in the diagram that 
when the temperature was highest there were rapid oscillations 

Fig. 134. 




A-errhoa bilimbi: angular movements of leaflet during a change from 
bright illumination to shade ; temperature (broken line) remaiuing 
nearly the same. 

of small amplitude, the mean position of the leaflet being at the 
time nearer the vertical. When the temperature began to fall, 
the oscillations became slower and larger, and the mean position 
of the leaf again approached the horizontal. The rate of oscil- 
lation was sometimes quicker than is represented in the above 
diagram. Thus, when the temperature was between 31 end 



334 MODIFIED CIRCUMNUTATION. CHAP. VII. 

Fig. 135. 




CliAP. VII. 



SLEEP OF LEAVES. 



335 



Fig. 136. 



32 C., 14 oscillations of a few degrees occurred in 19m. On 
the other hand, an oscillation may be much slower ; thus a leafle) 
was observed (temperature 25 C.) to 
rise during 40 m. before ifc fell and 
completed its oscillation. 

Porlieria hygrometrica (Zygophyllese) 
The leaves of this plant (Chilian 
form) are from 1 to l inches in length, 
and bear as many as 1G or 17 small 
leaflets on each side, which do not 
stand opposite one another. They are 
articulated to the petiole, and the 
petiole to the branch by a pulvinus. 
We must premise that apparently two 
forms are confounded under the same 
name : the leaves on a bush from Chili, 
which was sent to us from Kew, bore 
many leaflets, whilst those on plants 
in the Botanic Garden at Wiirzburg 
bore only 8 or 9 pairs ; and the whole 
character of the bushes appeared some- 
what different. We shall also see that 
they differ in a remarkable physio- 
logical peculiarity. On the Chilian 
plant the petioles of the younger leaves 
on upright branches, stood horizontally 
during the day, and at night sank 
down vertically so as to depend parallel 
and close to the branch beneath. The 
petioles of rather older leaves did not 
become at night vertically depressed, 
but only highly inclined. In one 
instance we found a branch which had Policria hygrometrica 
grown perpendicularly downwards, 
and the petioles on it moved in the same 
direction relatively to the branch as 
just stated, and therefore moved up- 
wards. On horizontal branches the 
younger. petioles likewise move at night 
in the same direction as before, that is, 

towards the branch, and are consequently then extended hori 
jsontally; but it is remarkable that the older petioles on the 




tiole of leaf, traced from 
9.35 A.M. July 7th to 
about midnight on the 
8th. Apex of leaf 7 
inches from tho vertical 
glass. Temp. 19-20.j C. 



336 MODIFIED CIRCUMNUTATION. CHAP. VII 

same branch, though moving a little in the same direction, also 
l>end downwards ; they thus occupy a somewhat different posi- 
tion, relatively to the centre of the earth and to the branch, from 
that of the petioles on the upright branches. With respect to 
the leaflets, they move at night towards the apex of the petiole 
until their midribs stand nearly parallel to it ; and they then 
lie neatly imbricated one over the other. Thus half of the upper 
surface of each leaflet is in close contact with half of the lower 
surface of the one next in advance ; and all the leaflets, except- 
ing the basal ones, have the whole of their upper surfaces and 
half of their lower surfaces well protected. Those on the oppo- 
site sides of the same petiole do not come into close contact 
at night, as occurs with the leaflets of so many Leguminosse, 
but are separated by an open furrow ; nor could they exactly 
coincide, as they stand alternately with respect to one another. 

The circumnutation of the petiole of a leaf f of an inch in 
length, on an upright branch, was observed during 36 h., 
and is shown in the preceding diagram (Fig. 136). On the 
first morning, the leaf fell a little and then rose until 1 P.M , 
and this was probably due to its being now illuminated through 
a skylight from above; it then circumnutated on a very small 
scale round the same spot until about 4 P.M., when the great 
evening fall commenced. During the latter part of the night or 
very early on the next morning the leaf rose again. On the 
second day it fell during the morning till 1 P.M., and this no 
doubt is its normal habit. From 1 to 4 P.M. it rose in a zigzag 
line, and soon afterwards the great evening fall commenced. It 
thus completed a double oscillation during the 24 h. 

The specific name given to this plant by Euiz 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 Vog. Florae Peru- about its power of foretelling 

vianso et Chilensis,' torn. i. p. 95, changes in the weather-; ami it 

1798. We cannut understand the appears as if the brightness of the 

tu-count given by the authors of sky largely di termined tho open- 

the behaviour of this plant in its ing uiul closing of the leuileta. 
native homo There is much 



CHAT. VII. SLEEP OF LEAVES. 337 

or even quite, closed during the day. But twigs cut from thi? 
bush, with their ends standing in water, or wholly immersed in 
it, or kept in damp air under a bell-glass, opened their leaves 
though exposed to a blazing sun; whilst those on the plant 
in the ground remained closed. The leaves on this same plant, 
after some heavy rain, remained open for two days; they then 
became half closed during two days, and after an additional 
day were quite closed. This plant was now copiously watered, 
and on the following morning the leaflets were fully ex- 
panded. The other plant growing in a pot, after having been 
exposed to heavy rain, was placed before a window in the Labo- 
ratory, with its leaflets open, and they remained so during the 
daytime for 48 h. ; but after an additional day were half closed. 
The plant was then watered, and the leaflets on the two following 
days remained open. On the third day they were again half 
closed, but on being again watered remained open during the 
two next days. From these several facts we may conclude that 
the plant soon feels the want of water ; and that as soon as this 
occurs, it partially or quite closes its leaflets, which in their 
then imbricated condition expose a small surface to evaporation. 
It is therefore probable that tliis sleep-like movement, which 
occurs only when the ground is dry, is an adaptation against 
the loss of moisture. 

A bush about 4 feet in height, a native of Chili, which was 
thickly covered with leaves, behaved very differently, for during 
the day it never closed its leaflets. On July 6th the earth ir 
the small pot in which it grew appeared extremely dry, and 
it was given a very little water. After 21 and 22 days (on 
the 27th and 28th), during the whole of which time the plant 
did not receive a drop of water, the leaves began to droop, but 
they showed no signs of closing during the day. It appeared 
almost incredible that any plant, except a fleshy one, could 
have kept alive in soil so dry, which resembled the dust on 
a road. On the 29th, when the bush was shaken, some leaves 
fell off, and the remaining ones were unable to sleep at night. 
Jt was therefore moderately watered, as well as syringed, late in 
the evening. On the next morning (30th) the bush looked as fresh 
as ever, and at night the leaves went to sleep. It may be added 
that a small branch while growing on the bush was enclosed, 
by means of a curtain of bladder, during 13 days in a large 
bottle half full of quicklime, so that the air within must have been 
intensely dry ; yet the leaves on this branch did not suffer in the 



338 MODIFIED CIRCUMNUTATION. CHAP. VIJ 

least, and did not close at all during the hottest days. Anothei 
trial was made with the same bush on August 2nd and 6th (the soil 
appearing at, this latter date extremely dry), for it was exposed 
out of doors during the whole day to the wind, but the leaflets 
showed no signs of closing. The Chilian form therefore differs 
widely from the one at Wurzburg, in not closing its leaflets 
when suffering from the want of water; and it can live for a 
surprisingly long time without water. 

Tropceolum majus (?) (cultivated var.) (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 difference in their positions was in some manner 
due to heliotropism, for the leaves are highly heliotropic. The 
true explanation, however, is that unless they are well 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 J and 1 inch in length ; but the part close to the blade, 
for about i 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 vdth several plants was brought on the morning 
of Sept. 3rd out of the greenhouse and placed before a north-east 
window, in the same position as before with respect to the light, 
as far as that was possible. On the front of the plants, 24 leaves 
were marked with thread, some of which had their blades hori- 
zontal, but the greater number were inclined at about 45, 
beneath the horizon; at night all these, without exception, 
became vertical. Early on the following morning (4th) they 
reassumed their former positions, and at night again became 
ver+jcal. On the 5th the shutters were opened at 0.15 A.M., and 



CHAP. VII. SLEEP OF LEAVES. 339 

by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m., 
and had acquired their diurnal position, they were placed in a 
dark cupboard. They were looked at twice during the day and 
thrice in the evening, the last time at 10 30 P.M., and not one had 
become vertical. At 8 A.M. on the following morning (6th) they 
still retained the same diurnal position, and were now replaced 
before the north-east window. At night all the leaves which 
had faced the light had their petioles curved and their blades 
vertical ; whereas none of the leaves on the back of the plants, 
although they had been moderately illuminated by the diffused 
light of the room, were vertical. They were now at night placed 
in the same dark cupboard; at 9 A.M. on the next morning (7th) 
all those which had been asleep had reassumed their diurnal 
position. The pot was then placed for 3 h. in the sunshine, so 
as to stimulate the plants ; at noon they were placed before the 
same north-east window, and at night the leaves slept in the 
usual manner and awoke on the following morning. At noon on 
this day (8th) the plants, after having been left before the north- 
east window for 5 h. 45 m. and thus illuminated (though not 
brightly, as the sky was cloudy during the whole time), were 
replaced in the dark cupboard, and at 3 P. M. the position of the 
leaves was very little, if at all, altered, so that they are not 
quickly affected by darkness; but by 10.15 P.M. all the leaves 
which had faced the north-east sky during the 5h. 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 thorn 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. (llth) 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. CHAP. 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 (l'2th) 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. 

Leguminosce. This Family includes many more genera with 
sleeping species than all the other families put together. The 
number of the tribes to which each genus belongs, according to 
Bentham and Hooker's arrangement, has been added. 

Orotolaria (sp. ?) (Tribe 2). This plant is monophyllous, and 
we are informed by Mr. T. Thiselton Dyer that the leaves rise 
up vertically at night and press against the stem. 

Lupinus (Tribe 2). The palmate or digitate leaves of the 
species in this large genus sleep in three different manners. 
One of the simplest, is that all the leaflets become steeply in- 
clined downwards at night, having been during the day ex- 
tended horizontally. This is shown in the accompanying 
figures (Fig. 137), of a leaf of L. pilosus, as seen during the 
day from vertically above, and of another leaf asleep with the 
leaflets inclined downwards. As in this position they are 
crowded together, and as they do not become folded like those 
in the genus Oxalis, they cannot occupy a vertically dependent 
position ; but they are often inclined at an angle of 50 beneath 
the horizon. In this species, whilst the leaflets are sinking, 
the petioles rise up, in two instances when the angles were 
measured to the extent of 23. The leaflets of L. sub-carnosus and 
arboreus, which were horizontal during the day, sank down at 
night in nearly the same manner ; the former to an angle of 38, 
and the latter of 36, beneath the horizon : but their petioles 



CHAP. VII. 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 specios 

Fig. 137. 





B. 

Lupinus piloaus : A, leaf seen from vertically above in daytime; B, lea? 
asleep, seen laterally at night. 

were to be observed at all seasons, some of the leaves would be 
found to sleep in a different manner. 

In the two following species the leaflets, instead of moving 
downwards, rise at night. With L. Hartwegii some stood at 
noon at a mean angle of 36 above the horizon, and at night 
at 51, thus forming together a hollow cone with moderately 
steep sides. The petiole of one leaf rose 14 and of a second 
11 at night. With L. luteus a leaflet rose from 47 at noon to 
65 above the horizon at night, and another on a distinct leaf 
rose from 45 to 69. The petioles, however, sink at night to 
a small extent, viz., in three instances by 2, 6, and 9 30 7 . 
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 
Trhen asleep, which is common to several species of Lupines. 
On the same leaf the shorter leaflets, which generally face the 
centre of the plant, sink at night, whilst the longer ones 
on the opposite side rise; the intermediate and lateral ones 
merely twisting on their own axes. But there is some variability 
with respect to which leaflets rise or fall. As might have been 
expected from such diverse and complicated movements, the 



342 



MODIFIED CIRCUMNUTATION. 



CHAP. 



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 
qnite vertically, forming in this latter case a vertical star. This 
occurs with the leaves of a species purchased under the name c/ 

Fig. 138 




Lupinus pubescens: A, leaf viewed laterally during the day; B, same-leaf 
at night ; C, another leaf with the leaflet forming a vertical star at 
night. Figures reduced. 

//. pubcscens ; 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 



CHAP. VII. SLEEP OF LEAVES. 343 

is much variability in the position of the leaves at night on the 
same plant ; some remaining with their leaflets almost horizontal, 
others forming more or less highly inclined or vertical stars, and 
some with all their leaflets sloping downwards, as in our first 
class of cases. It is also a remarkable fact, that although all the 
plants produced from the same lot of seeds were identical 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 30. The 
petiole of another leaf, the leaflets of which occupied a similar 
position at night, rose only 6. On the other hand, the petiole 
of a leaf with all its leaflets sloping down at night, fell at this 
time 4. The petioles of two rather older leaves were subse- 
quently observed ; both of which stood during the day at exactly 
the same angle, viz., 50 above the horizon, and one of these rose 
7 8, and the other fell 3 4 at night. 

We meet with cases like that of L. pubescens with some other 
species. On a single plant of L. mutabilis some leaves, which 
stood horizontally during the day, formed highly inclined stars 
at night, and the petiole of one rose 7. Other leaves which 
likewise stood horizontally during the day, had at night all theii 
leaflets sloping downwards at 46 beneath the horizon, but 
their petioles had hardly moved. Again, L. luttus 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 thfe leaves on this one plant assumed 
at night three different positions. Though we cannot account 
for this fact, we can see that such a stock might readily give 
birth to species having widely different nyctitropic habits. 

Little more need be said about the sleep of the species of Ln- 
pinus; several, namely, L. polyphyllus, nunus, Menziesii, spectostw, 



344 



MODIFIED CIRCUMNUTAT10N. 



CHAP. VII. 



and albifrons, though observed out of doors and in the green- 
house, did not change the position of their leaves sufficiently at 
night to be said to sleep. From observations made on two 
Bleeping species, it appears that, as with Tropceolum majus, the 
leaves must be well illuminated during the day in order to sleep 
at night. For several plants, kept all day in a sitting-room 
with north-east windows, did not sleep at night ; but when the 
pots were placed on the following day out of doors, and were 
brought in at night, they slept in the usual manner. The trial 
was repeated on the following day and night with the same 
result. 

Some observations were made on the circumnutation of the 
leaves of L. lufeus and arb -reus. It will suffice to say that the 
leaflets of the latter exhibited a double oscillation in the course 
of '24: la. for they fell from the early morning until 10 15 A.M., 
then rose and zigzagged greatly till 4 P.M., after which hour the 
great nocturnal fall commenced. By 8 A.M. on the following 
morning the leaflets had risen to their proper height. We have 
seen in the fourth chapter, that the leaves of l.upinus 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 

Fig. 139. 




A. B. 

Medicago marina : A. leaves during the day ; B, leaves asleep at night. 

a few observations were made on these three genera. The 
petioles on a young plant, about a foot in height, of Cytisus 
fragrans rose at night, on one occasion 23 and on another 33. 
The three leaflets also bend upwards, and at the same time 



CHAP. VII. SLEEP OF LEAVES. 345 

approach each other, so that the base of the central leaflet 
overlaps the bases of the two lateral leaflets. They bend 
up so much that they press against the stem ; and on looking 
down on one of these 3 oung plants from vertically above, tho 
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. Eoyer,-* the leaves 
of Medicago maculata rise up at night, and " se renversent uu 
peu de maniere a presenter obliquement au ciel leur face in- 
ferieure." A drawing is here given (Fig. 139) of the leaves 
of M. marina awake and asleep ; and this would almost serve 
for Oytisus fragrans in the same two states. 

Melilotus (Tribe 3). The species in this gemis sleep in a 
remarkable manner. The three leaflets of each leaf twist through 
an angle of 90 Q , 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 of 
the other N.N.E. The terminal leaflet behaves differently, for 
it twists to either side, the upper surface facing sometimes east 
and sometimes west, but rather more commonly west than east. 
The terminal leaflet also moves in another and more remarkable 
manner, for whilst its blade is twisting and becoming vertical, 
the whole leaflet bends to one side, and invariably to the side 
towards which the upper surface is directed; so that if this 
surface Jkces 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 



* Annalcs des So. Nut Bot.' (5th scries), ix. 1868, p. 308. 



340 



MODIFIED CIRCUMNUTATION'. 



CIIAP. VII. 



side and afterwards Lending 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 us 

Fig. 140. 




iftlilotus offidnalis : 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 ihe same direction; for 



CHAP. VII. SLEEP OF LEAVES. 347 

one leaflet, which had originally faced east, was observed after 
9 days to face west. The seat of both the twisting and bending 
movement is in the pulvinus of the sub-petioles. 

We believe that the leaflets, especially the two lateral ones, 
in performing the above described complicated movements 
generally bend a little downwards ; but we are not sure of this, 
for, as far as the main petiole is concerned, its nocturnal move- 
ment is largely determined by the position which the leaf 
happens to occupy during the day. Thus one main petiole was 
observed to rise at night 59, whilst three others rose only 7 
and 9. The petioles and sub-petioles are continually circum- 
nutating during the whole 24 h., as we shall presently see. 

The leaves of the following 15 species, M. officinalis, suaveolens, 
parviflora, alba, infesta, dtntata, gracilfs, sulcata, elegans, cccrulea, 
petitpierreana, macrorrhiza, Italica, secundiflora, and Taurica, 
sleep in nearly the same manner as just described; but the 
bending to one side of the terminal leaflet is apt to fail unless 
the plants are growing vigorously. With M. petitpierreana and 
sccundiftora the terminal leaflet was rarely seen to bend to one 
side. In young plants of M. lialica 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, 
BO that the position of the leaflets even at this late hour was far 
from the ordinary one. Again, with M. Taurica the terminal 
leaflets were never seen to bend towards either of the two lateral 
leaflets, though these, whilst becoming vertical, had bent towards 
the terminal one. The sub-petiole of the terminal leaflet in 
this species is of unusual length, and if the leaflet had bent to 
one side, its upper surface could have come into contact only 
with the apex of either lateral leaflet; and this, perhaps, is the 
meaning of the loss of the lateral movement. 

The cotyledons do not sleep at night. The first leaf consists of 
a single orbicular kaflet, which twists at night so that the blade 
stands vertically. It is a remarkable fact that with M. Taurica, 
and in a somewhat less degree with M. macrorrhiza and petit- 
pierreana, all the many bmall and young leaves produced during 



348 MODIFIED CIRCUMNUTATION. CHAP. YH 

the early spring from shoots on some cut-down plants in the 
greenhouse, slept in a totally different manner from the normal 
one ; for the three leaflets, instead of twisting on their own axes 
so as to present their lateral edges to the zenith, turned upwards 
and stood vertically with their apices pointing to the zenith. 
They thus assumed nearly the same position as in the allied 
genus Trifolium ; and on the same principle that embryological 
characters reveal the lines of descent in the animal kingdom, so 
the movements of the small leaves in the above three species of 
Melilotus, perhaps indicate that this genus is descended from 
a form which was closely allied to and slept like a Trifolium. 
Moreover, there is one species, M. m<-st.anr,nsis, the leaves of 
which, on full-grown plants between 2 and 3 feet in height, 
sleep like the foregoing small leaves and like those of a Trifolium. 
We were so much surprised at this latter case that, until the 
flowers and fruit were examined, we thought that the seeds of 
some Trifolium had been sown by mistake instead of those of a 
Melilotus. It appears therefore probable that M. messanensis 
has either retained or recovered a primordial habit. 

The circumnutation of a leaf of M. officinalis was traced, 
the stem being left free; and the apex of the terminal leaflet 
described three laterally extended ellipses, between 8 A.M. and 
4 P.M. ; after the latter hour the nocturnal twit-ting movement 
commenced. It was afterwards ascertained that the above 
movement was compounded of the circumnutation of the stem 
on a small scale, of the main petiole which moved most, and of 
the sub-petiole of the terminal leaflet. The main petiole of a 
leaf having been secured to a stick, close to the base of the sub- 
petiole of the terminal leaflet, the latter described two small 
ellipses between 10.30 A.M., and 2 P.M. At 7.15 P.M., after this 
same leaflet (as well as another) had twisted themselves into 
their vertical nocturnal position, they began to rise slowly, and 
continued to do so until 10.35 P.M., after which hour they were 
no longer observed. 

As M. messanensis sleeps in an anomalous manner, unlike that 
of any other species in the genus, the circumnutation of a 
terminal leaflet, with the stem secured, was traced during two 
days. On each morning the leaflet fell, until about noon, and 
then began to rise very slowly ; but on the first day the rising 
movement was interrupted between 1 and 3 P.M. by the formation 
of a laterally extended ellipse, and on the second day, at the 
Bamo time, by two smaller ellipses. The rising movement then 



CHAP. VII. SLEEP OF LEAVES. 349 

recommenced, and became rapid late in the evening, when 
the leaflet was beginning to go to sleep. The awaking or 
sinking movement had already commenced by 6.45 A.M. on both 
mornings. 

Trifolium (Tribe 3). The nyctitropic movements of 11 
species were observed, and were found to be closely similar. If 
we select a leaf of T. repens having an upright petiole, and with 
the three leaflets expanded horizontally, the two lateral leaflets 
will be seen in the evening to twist and approach each other, 
until their upper surfaces come into contact. At the same time 
they bend downwards in a plane at right angles to that of their 
former position, until their midribs form an angle of about 45 
with the upper part of the petiole. This peculiar change of 
position requires a considerable amount of torsion in the pul- 
vinus. The terminal leaflet merely rises up without any twist- 
Fig. 141. 




A. B. 

Trifolium repens: A, leaf during the day ; B, leaf asleep at night. 

ing, and tends over until it rests on and forms a roof over the 
edges of the now vertical and united lateral leaflets. Thus the 
terminal leaflet always passes through an angle of at least 90, 
generally of 130 or 140, and not rarely as was often observed 
with T. subterraneum of 180. In this latter case the terminal 
leaflet stands at night horizontally (as in Fig. 141), with its 
lower surface fully exposed to the zenith. Besides the difference 
in the angles, at which the terminal leaflets stand at night in 
the individuals of the same species, the degree to which tha 
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. CHAP. VII 

With T. Pannonicum the first true leaf was generally unifoliate, 
but sometimes trifoliate, or again partially lobed and in an 
intermediate condition. 

Circumnutation. Sachs described in 1863* the spontaneous 
up and down movements of the leaflets of T. incarnatum, when 
kept in darkness. Pfeffer made many observations on the 
similar movements in T. pratensej He states that the terminal 
leaflet of this species, observed at different times, passed through 
angles of from 30 to 120 in the course of from l to 4h. We 
observed the movements of T. subterraneum, resupinatum, and 
repens. 

Trifolium sulterranenm. A petiole was secured close to the 
base of the three leaflets, and the movement of the terminal 
leaflet was traced during 26j h., as shown in the figure on the 
next page. 

Between 6.45 A.M. and 6 P.M. the apex moved 3 times up 
and 3 times down, completing 3 ellipses in 11 h. 15 m. The 
ascending and descending lines stand nearer to one another 
than is usual with most plants, yet there was some lateral 
motion. At 6 P.M. the great nocturnal rise commenced, and 
on the next morning the sinking of the leaflet was continued 
until 8.30 A.M., after which hour it circumnutated in the manner 
just described. In the figure the great nocturnal rise and 
the morning fall are greatly abbreviated, from the want of 
space, and are merely represented by a short curved line. The 
leaflet stood horizontally when at a point a little beneath the 
middle of the diagram; so that during the daytime it oscillated 
almost equally above and beneath a horizontal position. At 
.8.30 A.M. it stood 48 beneath the horizon, and by 11.30 A.M. it 
had risen 50 above the horizon ; so that it passed through 98 
in 3 h. By the aid of the tracing we ascertained that the 
distance travelled in the 3 h. by the apex of this leaflet was 
I '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 prolongation of one of the diurnal 
ellipses. The same leaflet had been observed on the previous 
day, and the course then pursued was almost identically the 
same as that here described. 



* 'Flora,' 18K3, p. 497 
t ' Die Period. Bewegu 



*17f . 

regungen," 1875, pp. 35. 52. 



CHAP. VII. 



SLEEP OF LEAVES. 



351 



Fig. 142. 



Trifolium resupinatum. A plant left entirely free 
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 wero 
closely similar. Those 
executed on the second 
day are shown in Fig. 
143. The obliquity of 
the several lines is due 
partly to the manner in 
which the leaflet was- 
viewed, and partly to its 
having moved a little to- 
wards the light. From 
7.50 A.M. to 8.40 A.M. the 
leaflet fell, that is, the 
awakening movement was 
continued. It then rose 
and moved a little late- 
rally towards the light. 
At 12.30 it retrograded, 
and at 2.30 resumed its 
original course, having 
thus completed a small 
ellipse during the middle 
of the day. In the even- 
ing it rose rapidly, and 
by 8 A.M. on the following 
morning had returned to 
exactly the same spot as 
on the previous morning. 
The line representing the 
nocturnal course ought 
to be extended much 
higher up, and is here ;/ 
abbreviated into a short. 



was placed 




352 



MODIFIED CIRCUMNUTAT10N. 



CHAP. VII 



Fig. 143. 




Tnfohum resupinatum : circtnnntitatio 
and nyetitropic movements of the ter- 
minal leaflet during 24 hours. 



curved, broken line. The terminal leaflet, therefore, of this 
species described during the daytime only a single additional 

ellipse, instead of two ad- 
ditional ones, as in the 
case of T. subterraneum. 
But we should remember 
that it was shown in the 
fourth chapter that the 
stem circumnutates, as no 
doubt does the main petiole 
and the sub- petioles ; so 
that the movement 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 
began to go to sleep after 

_ ' \ 

h. ID m., and this was 

well pronounced after 4 h. 

30m. 

Trifdium repens. A stem was secured close to the base of 
a moderately old leaf, and the movement of the terminal leaflet 
was observed during two days. This case is interesting solely 
from the simplicity of the movements, in contrast with those of 
the two preceding species. On the first day the leaflet fell 
between 8 A.M. and 3 P.M., and on the second between 7 A.M. 
and 1 P.M. On both days the descending course was somewhat 
zigzag, and this evidently represents the circumnutating move- 
ment of the two previous species during the middle of the day. 
After 1 P.M., Oct. 1st (Fig. 144), the leaflet began to rise, but 
the movement was slow on both days, both before and after 
this hour, until 4 P.M. The rapid evening and nocturnal rise 
then commenced. Thus in this species the course during 24 h. 
consists of a single great ellipse; in T. resupinatum of two 
ellipses, one of which includes the nocturnal movement and is 
much elongated; and in T. subterraneum of three ellipses, of 
which the nocturnal one is likewise of great length. 

Securigera coronilla (Tribe 4). The leaflets, which stand 
opposite one another and are numerous, lise up at night, come 
into close contact, and bend backwards at a moderate angle 
lowards the base of the petiole. 



OJIAP VII. 



SLEEP OF LEAVES. 



353 



Fig. 



Lotus (Tribe 4). The nyctitropic movements of 10 species 
in this genus were observed, and found to be the same. The 
main petiole rises a little at night, and 
the three leaflets rise till they become 
vertical, and at the same time approach 
each other. This was conspicuous with 
L. Jacobceus, in which the leaflets are 
almost linear. In most of the species 
the leaflets rise so much as to press 
against the stem, and not rarely they 
become inclined a little inwards with 
their lower surfaces exposed obliquely 
to the zenith. This was clearly the 
case with L. major, as its petioles are 
unusually long, and the leaflets are thus 
enabled to bend further inwards. The 
young leaves on the summits of the 
stems close up at night so much, as 
often to resemble large buds. Tl<e 
stipule-like leaflets, which are often of 
large size, rise up like the other leaflets, 
and press against the stem (Fig. 145). 
All the leaflets of L. Gebelii, and pro- 
bably of the other species, are provided 
at their bases with distinct pulvini, of 
a yellowish colour, and formed of very 
small cells. The circumnutation of a 
terminal leaflet of L. periyrinus (with 
the stem secured) was traced during 
two days, but the movement was so 
simple that it is not worth while to 
give the diagram. The leaflet fell 
slowly from the early morning till 
about 1 P.M. It then rose gradually 
at first, but rapidly late in the evening. 

It occasionally stood still for about 20 m. during the day, and 
sometimes zigzagged a little. The movement of one of the 
basal, stipule-like leaflets was likewise traced in the same 
manner and at the same iime, 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 1'2 genera have been observed by ourselves and 




Trifolium rcpens : 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. Nocturnil course, 
represented by curved 
broken line, much ab- 
breviated. 



354 



MODIFIED CIKCUMNUTATION. 



CHAP VII 



others, but only in Eobinia with any care. Psoraka acanlii 
raises its three leaflets at night ; whilst Amorpha fruticosa* 
Dalea alopecuroides, and Indigofera tinctoria depress them. 
Duchartre f states that Tephrosia caribcea is the sole example 
>f " folioles couchees le long du petiole et vers la base ; " but a 

Fig. 145. 




Lotus CiCticus: 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, arid shall 
again see in other cases. Wistaria Sinensis, according tc 
"Royer.J " abaisse les folioles qui par une disposition bizarre 
Bont inclinees dans la meme fcuille, les sup6rieures vers le 



* Ducliarte, Klcmenta 
olanique,' 1867, p. H4U. 
t Ibid., p. 347. 



Jfl 



J Ann. dee Srienccs, Nats 
Dot/ (5th series), ix. 1808. 



CHAP. VII. SLEEP OF LEAVES. 355 

Bommet, les inferieures vers la base du petiole comrmm ; " but 
the leaflets on a young plant observed by us in the green- 
house merely sank vertically downwards at night. The leaflets 
are raised in Sphcerophysa salsola, Colutea arborea, and Astra- 
galus uliyinosus, but are depressed, according to Linnaeus, in 
.Glycyrrhiza. The leaflets of Bobinia psrvdu-acacia likewise sink 
vertically down at night, but the petioles rise a little, viz., in 
one case 3, and in another 4. The circumnutating move- 
ments of a terminal leaflet on a rather old leaf were traced 
during two days, and were simple. The leaflet fell slowly, in a 
slightly zigzag line, from 8 A.M. to 5 P.M., and then more 
rapidly ; by 7 A.M. on the following morning it had risen to its 
diurnal position. There was only one peculiarity in the move- 
ment, namely, that on both days there was a distinct though 
small oscillation up and down between 8.30 and 10 A.M., and 
this -would probably have been more strongly pronounced if 
the leaf had been younger. 

Coronilla rosea (Tribe 6). The leaves bear 9 or 10 pairs of 
opposite leaflets, which during the day stand horizontally, with 

Fig. 146. 




Coronilla rosca : leaf asleep. 

their midribs at right angles to the petiole. At night they rise 
up, so that the opposite leaflets come nearly into contact, and 
those on the younger leaves into close contact. At the same 
time they bend back towards the base of the petiole, until their 
midribs form with it angles of from 40 to 50 in a vertical 
plane, as here figured (Fig. 146). The leaflets, however, some- 
times bend FO much back that their midribs become parallel to 
and lie on the petiole. They thus occupy a reversed position 
to what they do in several Lcguminossc, for instance, in Mimosa 



356 



MODIFIED CIRCUMNUTATION. 



CHAP. VII. 



pudtca ; but, from standing further apart, they do not overlap 
one another nearly so much as in this latter plant. The main 
petiole is curved slightly downwards during the day, but 
straightens itself at night. In three cases it rose from 3 above 
the horizon at noon, to 9 at 10 P.M. ; from 11 to 33 ; and from 
5 to 33 the amount of angular movement in this latter case 
amounting to 28. In several other species of Coronilla the 
leaflets showed only feeble movements of a similar kind. 

Ledysarum coronarium (Tribe 6). The small lateral leaflets 
on plants growing out of doors rose up vertically at night, but 
the large terminal one became only moderately inclined. The 
petioles apparently did not rise at all. 

Smithia Pfundii (Tribe 6). The leaflets rise up vertically, 
and the main petiole also rises considerably. 

Arachis hypuyona (Tribe 6). The shape of a leaf, with its two 
pairs of leaflets, is shown at A (Fig. 147) ; and a leaf asleep, 



Fig. 147. 




Arachis hypogcea: A, leaf during the day. seen from vertically above ; B, 
leaf asleep, seen laterally; copied from a photogr.iph. Figures much 
reduced. 

traced from a photograph (made by the aid of aluminium 
light), is given at B. The two terminal leaflets twist round at 
night until their blades stand vertically, and approach each 
other until they meet, at the same time moving a little upwards 
and backwards. The two lateral leaflets meet each other in the 
same manner, but move to a greater extent forwards, that is, in 
a contrary direction to the two terminal leaflets, which they 
partially embrace. Thus all four leaflets form together a single 
packet, with their edges directed to the zenith, and with their 
lower surfaces turned outwards. On a plant which was not 
growing vigorously the closed leaflets seemed too heavy for the 



CHAI-. VII. 



SLEEP OF LEAVES. 



357 



Fig. 148. 



petioles to support them in a vertical position, so that each 
night the main petiole became twisted, and all the packets were 
extended horizontally, with the lower surfaces of the leaflets on 
one side directed to the zenith in a most anomalous manner. 
This fact is mentioned solely as a caution, as it surprised us 
greatly, until we discovered that it was an anomaly. The 
petioles are inclined upwards during the day, but sink at night, 
so as to stand at about right angles with the stem. The amount 
of sinking was measured only on one occasion, and found to be 
39. A petiole was secured to a stick at the base of the two 
terminal leaflets, and the circumnutating movement of one of 
these leaflets was traced from 6.40 A.M. to 10.40 P.M., the plant 
being illuminated from above. The temperature was 17 174 C., 
and therefore rather too low. During the 16 h. the leaflet moved 
thrice up and thrice down, and as the ascending and descend- 
ing lines did not coincide, three ellipses were formed. 

Desmodium gyrans (Tribe 6). A large and full-grown leaf of 
this plant, so famous for the spontaneous 
movements of the two little lateral leaflets, 
is here represented (Fig. 148). The large 
terminal leaflet sleeps by sinking vertically 
down, whilst the petiole rises up. The coty- 
ledons do not sleep, but the first -formed leaf 
sleeps equally well as the older ones. The 
appearance presented by a sleeping branch 
and one in the day-time, copied from two 
photographs, are shown at A and B (Fig. 
149), and we see how at night the leaves are 
crowded together, as if for mutual pro- 
tection, by the rising of the petioles. The 
petioles of the younger leaves near the sum- 
mits of the shoots rise up at night, so as to 
stand vertical and parallel to the stem ; 
whilst those on the sides were found in four 
cases to have risen respectively 46^, 36, D esmndiumqyran8 . 
20, and 19-5 above the inclined positions 
which they had occupied during the day. 
For instance, in the first of these four cases 
the petiole stood in the day at 23, and at 
night at 69 2 L above the horizon. In the 
evening the rising of the petioles is almost 
completed before the leaflets sink perpendicularly downwards. 




leaf seen from 
above, reduced 
to one-half na- 
tural size. The 
minute stipule* 
unusually lar^n 



358 MODIFIED CIRCUMNUTATION. CHAP. Ml. 

Circumnutation. The circumnutating movemeuts of four 
young shoots were observed during 5 h. 15 ru. ; and in this time 
each completed an oval figure of small size. The main petiole 
also circumnutates rapidly, for in the course of 31 m. (temp, 
91 F.) it changed its course by as much as a rectangle six times, 
describing a figure which apparently, represented two ellipsea 



Fig. 149 




Desmodium gyrans: A, stem during the day; B, stem with leaves asleep. 
Copied from a photograph ; figures reduced. 

The movement of the terminal leaflet by means of its sub- 
petiole or pulvinus is quite as rapid, or even more so, than that 
of the main petiole, and has much greater amplitude. Pfeffer 
has seen* these leaflets move through an angle of 8 in the 
course of from 10 to 30 seconds. 

A fine, nearly full-grown leaf on a young plant, 8 inches in 
height, with the stem secured to a stick at the base of the leaf, 
was observed from 8.30 A.M. June 22nd to 8 A.M. June 24th. 



Die Period. Beweg.,' p. 35. 



CHAP. VII. SLEEP OF LEAVES. 359 

In the diagram given on the next page (Fig. 150), the two 
curved broken lines at the base, which represent the nocturnal 
courses, nught to be prolonged far downwards. On the first 
day the leaflet moved thrice down and thrice up, and to a con- 
siderable distance laterally ; the course was also remarkably 
crooked. The dots were generally made every hour; if they 
had been made every few minutes all the lines would have been 
zigzag to an extraordinary degree, with here and there a loop 
formed. We may infer that this would have been the case, 
because five dots were made in the course of 31m. (between 
32.34 and 1.5 P.M.), and we see in the upper part of the diagram 
how crooked the course here is : if only the first and last dots 
had been joined we should have had a straight line. Exactly 
the same fact may be seen in the lines representing the course 
between 2.24 P.M. and 3 P.M., when six intermediate dots were 
made ; and again at 4.46 and 4.50. But the result was widely 
different after 6 P.M., that is, after the great nocturnal descent 
had commenced ; for though nine dots were then made in the 
course of 32 in., when these were joined (see Figure) the line thus 
formed was almost straight. The leaflets, therefore, begin to 
descend in the afternoon by zigzag lines, but as soon as the 
descent becomes rapid their whole energy is expended in thus 
moving, and their course becomes rectilinear. After the leaflets 
are completely asleep they move very little or not at all. 

Had the above plant been subjected to a higher temperature 
than 67 70 F., the movements of the terminal leaflet would 
probably have been even more rapid and wider in extent than 
those shown in the diagram ; for a plant was kept for some time 
in the hot-hou.-e at from 92 93 F., and in the course of 35 m. 
the apex of a leaflet twice descended and once ascended, travelling 
over a space of 1*2 inch in a vertical direction and of '82 inch in 
a horizontal direction. Whilst thus moving the leaflet also 
rotated on its own axis (and this was a point to which no atten- 
tion had been before paid), for the plane of the blade differed by 
41 after an interval of only a few minutes. Occasionally the 
leaflet stood still for a short time. There was no jerking move- 
ment, which is so characteristic of the little lateral leaflets. A 
sudden and considerable fall of temperature causes the terminal 
leaflet to sink downwards ; thus a cut-off leaf was immersed in 
water at 95 F., which was slowly raised to 103 F., and after- 
wards allowed to sink to 70 F., and the sub-petiole of the ter- 
minal leaflet then curved downwards. The water was afterwards 
24 



B60 MODIFIED CIRCUMNUTATION. CHAP YU 

Fig. L50. 




CHAP. VII. SLEEP OF LEAVES 361 

raised to 120 F., and the sub-petiole straightened itself. Similar 
experiments with leaves in water were twice repeated, with 
nearly the same result. It should be added, that water raised 
to even 122 F. does not soon kill a leaf. A plant was placed 
in darkness at 8.37 A.M., and at 2 P.M. (i.e. after 5 h. 23 m.), though 
the leaflets had sunk considerably, they had by no means ac- 
quired their nocturnal vertically dependent position. Pfeffer, on 
the other hand, says * that this occurred with him in from I h. 
to 2 h. ; perhaps the difference in our results may be due to 
the plant on which we experimented being a very young and 
vigorous seedling. ' 

The Movements of the little Lateral Leaflets. These have been so 
often described, that we will endeavour to be as brief as possible 
in giving a few new facts and conclusions. The leaflets some- 
times quickly change their position by as much as nearly 180; 
and their sub-petioles can then be seen to become greatly curved. 
They rotate on their own axes, so that their upper surfaces are 
directed to all points of the compass. The figure described by 
the apex is an irregular oval or ellipse. They sometimes re- 
main stationary for a period. In these several respects there is 
no difference, except in rapidity and extent, between their move- 
ments and the lesser ones performed by the large terminal 
leaflet whilst making its great oscillations. The movements of 
the little leaflets are much influenced, as is well known, by 
temperature. This was clearly shown by immersing leaves with 
motionless leaflets in cold water, which was slowly raised to 
103 F., and the leaflets then moved quickly, describing about a 
dozen little irregular circles in 40 m. By this time the water 
had become much cooler, and the movements became slower or 
almost ceased ; it was then raised to 100 F., and the leaflets 
again began to move quickly. On another occasion a tuft of 
fine leaves was immersed in water at 53 F., and the leaflets 
were of course motionless. The water was raised to 99, and 
the leaflets soon began to move ; it was raised to 105, and the 
movements became much more rapid ; each little circle or oval 
being completed in from 1 m. 30 s. to 1 m. 45 s. There was, 
however, no jerking, and this fact may perhaps be attributed to 
the resistance of the water. 

Sachs statwe that the leaflets do not move until the surround- 
ing air is as high as 71 72 F., and this agrees with our 



* Die Period. Beweg.,' p. 39. 



362 MODIFIED CIRCUMNUTATION. CHAP. Vfl. 

experience on full-grown, or nearly full-grown, plants. But the 
leaflets of young seedlings exhibit a jerking movement at much 
lower temperatures. A seedling was kept (April 16th) in a room 
for half the day where the temperature was steady at 64 F., 
and the one leaflet which it bore was continually jerking, but 
not so rapidly as in the hot-house. The pot was taken in the 
evening into a bed-room where the temperature remained at 
62 during nearly the whole night ; at 10 and 11 P.M. and at 
1 A.M. the leaflet was still jerking rapidly ; at 3.30 A M. it was not 
seen to jerk, but was observed during only a short time. It was, 
however, now inclined at a much lower angle than that occupied 
at 1 A.M. At 6.30 A.M. (temp. 61 F.) its inclination was still 
less than before, and again less at 6.45 A.M. ; by 7.40 A.M. it had 
risen, and at 8.30 A.M. was again -seen to jerk. This leaflet, 
therefore, was moving during the whole night, and the move- 
ment was by jerks up to 1 A.M. (and possibly later) and again at 
8.30AM., though the temperature was only 61 to 62 F. We 
must therefore conclude that the lateral leaflets produced by 
young plants differ somewhat in constitution from those on 
older plants. 

In the large genus Desmodium by far the greater number 
of the species are trifoliate ; but some are unifoliate, and even 
the same plant may bear uni- and trifoliate leaves. In most 
of the species the lateral leaflets are only a little smaller than 
the terminal one. Therefore the lateral leaflets of I), gyrana 
(see former Fig. 148) must be considered as almost rudi- 
mentary. They are also rudimentary in function, if this ex- 
pression may be used ; for they certainly do not sleep like the 
full-sized terminal leaflets. It is, however, possible that the 
sinking down of the leaflets between 1 A.M. and 6.45 A.M., as 
above described, may represent sleep. It is well known that 
the leaflets go on jerking during the early part of the night; 
but my gardener observed (Oct. 13th) a plant in the hot-house 
between 5 and 5.30 A.M., the temperature having been kept up 
to 82 F., and found that all the leaflets were inclined, but he 
saw no jerking movement until 6.55 A.M., by which time tho 
terminal leaflet had risen and was awake. Two days after- 
wards (Oct. 15th) the same plant was observed by him ut 
4.47 A.M. (temp. 77 F.), and he found that the large terminal 
leaflets were awake, though not quite horizontal ; and the only 
cause which we could assign for this anomalous wake&ilness was 
that the plant had been kept for experimental purposes during 



CHAF. VII. SLEEP OF LEAVES. 

the previous day at an unusually high temperature ; the little 
lateral leaflets were also jerking at this hour, but whether 
there was any connection between this latter fact and the sub- 
horizontal position of the terminal leaflets we do not know. 
Anyhow, it is certain that the lateral leaflets do not sleep like 
the terminal leaflets; and in so far they may be scid to be 
in a functionally rudimentary condition. They are in a similar 
condition in relation to irritability; for if a plant be shaken 
or syringed, the terminal leaflets sink down to about 45 be- 
neath the horizon ; but we could never detect any effect thus 
produced on the lateral leaflets; yet we are not prepared to 
assert positively that rubbing or pricking the pulvinus produces 
no effect. 

As in the case of most rudimentary organs, the leaflets are 
variable in size; they often depart from their normal position 
and do not stand opposite one another ; and one of the two is 
frequently absent. This absence appeared in some, but not in 
all the cases, to be due to the leaflet having become completely 
confluent with the main petiole, as might be inferred from tho 
presence of a slight ridge along its upper margin, and from the 
course of the vessels. In one instance there was a vestige of 
the leaflet, in the shape of a minute point, at the further end of the 
ridge. The frequent, sudden, and complete disappearance of one 
or both of the rudimentary leaflets is a rather singular fact ; but 
it is a much more surprising one that the leaves which are first 
developed on seedling plants are not provided with them. Thus, 
on one seedling the seventh leaf above the cotyledons was the 
first which bore any lateral leaflets, and then only a single one. 
On another seedling, the eleventh leaf first bore a leaflet ; of the 
nine succeeding leaves five bore a single lateral leaflet, and 
lour bore none at all ; at last a leaf, the twenty-first above tho 
cotyledons, was provided with two rudimentary lateral leaflets. 
From a widespread analogy in the animal kingdom, it might 
have been expected that these rudimentary leaflets would have 
been better developed and more regularly present on very young 
than on older plants. But bearing in mind, firstly, that long- 
lost characters sometimes reappear late in life, and secondly, 
that the species of Desmodium are generally trifoliate, but that 
Borne are unifoliate, the suspicion arises that D. gyrans is 
descended from a unifoliate species, and that this was descended 
from a trifoliate one ; for in this case both the absence of tho 
little lateral leaflets on very young seedlings, and their sub- 



8(54 MODIFIED CIECUMNUTATION CHAP. VII 

sequent appearance, may be attributed to reversion to more 01 
less distant progenitors.* 

. No one supposes that the rapid movements of the lateral 
leaflets of D. yyrans are of any use to the plant; and why 
they should behave in this 1 manner is quite unknown. Wo 
imagined that their power of movement might stand in somo 
relation with their rudimentary condition, and therefore ob- 
served the almost rudimentary leaflets of Mimosa albida vel 
scnsitiva (of which a drawing will hereafter be given, Fig. 159); 
but they exhibited no extraordinary movements, and at night 
they went to sleep like the full-sized leaflets. There is, how- 
ever, this remarkable difference in the two cases ; in Dcsmo- 
dium the pulvinus of the rudimentary leaflets has not been 
reduced in length, in correspondence with the reduction of the 
blade, to the same extent as has occurred in the Mimosa ; and it 
is on the length and degree of curvature of the pulvinus that the 
amount of movement of the blade depends. Thus, the average 
length of the pulvinus in the large terminal leaflets of Desmo- 
diuni is 3 mm., whilst that of the rudimentary leaflets is 2 - 86 mm. ; 
BO that they differ only a little in length. But in diameter they 
differ much, that of the pulvinus of the little leaflets being only 
0'3 mm. to 0'4 mm.; whilst fhat of the terminal leaflets is 
I "33 mm. If we now turn to the Mimosa, we iind that the 
average length of the pulvinus of the almost rudimentary 
leaflets is only 0'466 mm., or rather more than a quarter of the 
length of the pulvinus of the fall-sized leaflets, namely, 1 66 mm. 
In this small reduction in length of the pulvinus of the rudi- 
mentary leaflets of Desmodium, we apparently have the proxi- 
mate cause of their great and rapid circumnutating movement, 
in contrast with that of the almost rudimentary leaflets of the 
Mimosa. The small size and weight of the blade, and the little 
resistance opposed by the air to its movement, no doubt also come 
into play ; for we have seen that these leaflets if immersed in 
water, when the resistance would be much greater, were pre- 
vented from jerking forwards. Why, during the reduction of 
the. lateral leaflets of Desmodium, or during their reappearance 
T-if they owe their origin to reversion the pulvinus should 
have been so much less affected than the blade, whilst with the 



* Desmodium rfizpertilionis is rudimentary lateral leaflets. Du- 
closely allied to D. (ji/rant>, and chart re, ' Kle'mentsde Botanique, 
it seems only occasionally to bear 1SG7, p. 3oo. 



CHAP. 



SLEEP OF LEAVES. 



365 



Mimosa the pulvinus has been greatly reduced, we do not 
know. Nevertheless, it deserves notice that the reduction of 
the leaflets in these two genera has apparently been effected by 
a different process and for a different end ; for with the Mimosa 
the reduction of the inner and basal leaflets was necessary from 
the want of space; but no such necessity exists with Desmo- 
dium, and the reduction of its lateral leaflets seems to have 
been due to the principle of compensation, in consequence of 
the great size of the terminal leaflet. 

Uraria (Tribe 6) and ('ei.trofema (Tribe 8). The leaflets of 
L'raria lai/opus and the leaves of a Centrosema from Brazil 
both sink vertically down at night. In the latter plant the 
petiole at the same time rose 16 i. 

Amphicarpcea monoica (Tribe 8). The leaflets sink down ver- 
tically at night, and the petioles likewise fall considerably. 

Fig. 151. 




Amplticarpcea monoica : cireumnutation nnd nyctitropic movement of left} 
luring 48 h. ; its apex 9 inches from the vertical glass. Figure reduced 
to one-third of original scale. Plant illuminated from above* temp. 
17-18 C. 

A petiole, which was carefully observed, stood during the day 
^5 above the horizon and at night 32 below it; it therefore 
fell 57. A filament was fixed transversely across the terminal 
leaflet of a fine young leaf (2 inches in length including the 



ii66 MODIFIED CIRCUMNUTATION. CHAP. VII, 

petiole), and the movement of the whole leaf was traced on a 
vertical glass. This was a bad plan in some respects, because 
the rotation of the leaflet, independently of its rising or falling, 
raised and depressed the filament ; but it was the best plan for 
our special purpose of observing whether the leaf moved much 
after it had gone to sleep. The plant had twined closely round 
a thin stick, so that the circumnutation of the stem was pre- 
vented. The movement of the leaf was traced during 48 h., 
from 9 A.M. July 10th to 9 A.M. July 12th. In the figure given 
(Fig. 151) we see how complicated its course was on both days : 
during the second day it changed its course greatly 13 times. 
The leaflets began to go to sleep a little after 6 P.M., and by 
7.15 P.M. hung vertically down and were completely asleep; 
but on both nights they continued to move from 7.15 P.M. 
to 10.40 and 10.50 P.M., quite as much as during the day ; and 
this was the point which we wished to ascertain. We see in 
the figure that the great sinking movement late in the evening 
does not differ essentially from the circumnutation during 
the day. 

Glycine Idxpida (Tribe 8). The three leaflets sink vertically 
down at night. 

Erythrina (Tribe 8). Five species were observed, and the 
leaflets of all sank vertically down at night ; with K. caffra and 
with a second unnamed species, the petioles at the same time 
rose slightly. The movements of the terminal leaflet of K. crista- 
galli (with the main petiole secured to a stick) were traced 
from 6.40 A.M., June 8th, to 8 A.M. on the 10th. In order to 
observe the nyctitropic movements of this plant, it is necessary 
that it should have grown in a warm greenhouse, for out of 
doors in our climate it does not sleep. We see in the tracing 
(Fig. 152) that the leaflet oscillated twice up and down between 
early morning and noon ; it then fell greatly, afterwards rising 
till 3 P.M. At this latter hour the great nocturnal fall com- 
menced. On the second day (of which the tracing is not given) 
there was exactly the same double oscillation before noon, but 
only a very small one in the afternoon. On the third morning 
the leaflet moved laterally, which was due to its beginning to 
assume an oblique position, as seems 'invariably to occur with 
the leaflets of this species as they grow old. On both nights after 
the leaflets were asleep and hung vertically down, they continued 
to move a little both up and down, and from side to side. 

Erytliriua cajj'ra. A filament was fixed transversely across 



CHAP. Til. 



SLEEP OF LEAVES. 



367 



a terminal leaflet, as we wished 
to observe its movements when 
asleep. The plant was placed 
in the morning of June 10th 
under a skylight, where the 
light was not bright; and we 
do not know whether it was 
owing to this cause or to the 
plant having been disturbed, 
but the leaflet hung vertically 
down all day; nevertheless it 
circumnutated in this posi- 
tion, describing a figure which 
represented two irregular el- 
lipses. On the next day it 
circumnutated in a greater 
degree, describing four irre- 
gular ellipses, and by 3 P.M. 
had risen into a horizontal po- 
sition. By 7.15 P.M. it was 
asleep and vertically depen- 
dent, but continued to circum- 
nutate as long as observed, 
until 11 P.M. 

Erythrina corallo'It-n/L on . 
The movements of a terminal 
leaflet were traced. During 
the second day it oscillated 
four times up and four times 
down between 8 A.M. and 4 
P.M., after which hour the great 
nocturnal fall commenced. On 
the third day the movement 
was equally great in ampli- 
tude, but was remarkably 
simple, for the leaflet rose in 
an almost perfectly straight 
line from 6.50 A.M. to 3 P.M., 
and then sank down in an 
equally straight line until 
vertically dependent and 
asleep. 



Fig. 152. 



6'40'a.M 




9-f.tn. 



of terminal leaflet, 3J inches in 
length, traced during 25 h. ; apex 
of leaf 3 J inches from the vertical 
glass. Figure reduced to one-half 
of original scale. Plant illumi- 
nated from above; temp. 17A- 
18* C. 



{JG8 MODIFIED CIRCUMNUTATION. CIIAF. TH. 

Apios tuberosa (Tribe 8). The leaflets sink vertically down 
at night. 

Phaseolus vulyaris (Tribe 8). The leaflets likewise sink verti- 
cally down at night. In the greenhouse the petiole of a young 
leaf rose 16, and that of an older leaf 10 at night. With 
plants growing out of doors the leaflets apparently do not sleep 
until somewhat late in the season, for on the nights of July llth 
and 12th none of them were asleep ; whereas on the night of 
August 15th the same plants had most of their leaflets verti- 
cally dependent and asleep. With Ph. caracalla and Ilernan- 
dtsii, the primary unifoliate leaves and the leaflets of the 
secondary trifoliate leaves sink vertically down at night. This 
holds good with the secondary trifoliate leaves of Ph. Rox- 
lurylrii, but it is remarkable that the primary unifoliate leaves, 
which are much elongated, rise at night from about 20 to 
about 60 above the horizon. With older seedlings, however, 
having the secondary leaves just developed, the primary leaves 
stand in the middle of the day horizontally, or are deflected 
a little beneath the horizon. In one such case the primary 
leaves ro?e from 26 beneath the horizon at noon, to 20 above 
it at 10 P.M. ; whilst at this same hour the leaflets of the 
secondary leaves were vertically dependent. Here, then, we 
have the extraordinary case of the primary and secondary 
leaves on the same plant moving at the same time in opposite 
directions. 

We have now seen that the leaflets in the six genera of Pha- 
seolcse observed by us (with the exception of the primary leaves 
of Phaseolus Rvxburyliii} all sleep in the same manner, namely, 
by sinking vertically down. The movements of the petioles 
were observed in only three of these genera. They rose in 
Centrosema and Phaseolus, and sunk in Amphicarpaea. 

Suphora chrysophylla (Tribe 10). The leaflets rise at night, 
and are at the same time directed towards the apex of the leaf, 
as in Mimoi-a pudica. 

Ccesalpinia, Hcem'ito.rylon, Gleditschia, Poinciana. The leaflets 
of two species of Csesalpinia (Tribe 13) rose at night. With 
Hccmatoxyloii Cumpechianum (Tribe 13) the leaflets move for- 
wards at night, so that their midribs stand parallel to the 
petiole, and their now vertical lower surfaces are turned out- 
wards (Fig. 153). The petiole sinks a little. In Gleditschia, if 
we understand correctly Duchartre's description, and in /"oil*- 



CHAF. VIL SLEEP OF LEAVES. 369 

cfona Gillicsii (both belonging to Tribe 13), the leaves behave 
in the same manner. 

Fig. 153. 




ffcematoxylon Campechianum : A, branch during daytime ; B, branch with 
leaves asleep, reduced to two-thirds of natural scale. 

Cassia (Tribe 14). The nyctitropic movements of the leaves 
in many species in this genus are closely alike, and are highly 
complex. They were first briefly described by Linnaeus, and since 
by Duchartre. Our observations were made chiefly on C.flori- 
lunda * and corymbofa, but several other species were casually 
observed. The horizontally extended leaflets sink down verti- 
cally at night; but not simply, as in so many other genera, for 
each leaflet rotates on its own axis, so that its lower surface 
faces outwards. The upper surfaces of the opposite leaflets are 
thus brought into contact with one another beneath the petiole, 
and are well protected (Fig. 154). The rotation and other move- 
ments are eifected by means of a well-developed pulvinus at the 
base of each leaflet, as could be plainly seen when a straight 
narrow black line had been painted along it during the day. 
The two terminal leaflets in the daytime include rather less than 
a right angle : but their divergence increases greatly whilst they 



* I am informed by Mr. Dyer near to C. lavigati. It is no doubt 

that Mr. Benthimi believes that the same as the form desciibed by 

C. flori'wdi (a common green- Lindley (' Bot. Reg.,' Tab. K22; 

house bush) is a hybrid raised in <w> C. Ikrbertiana. 
France, and that it comes very 



370 



MODIFIED CIRCUMNUTATION. 



CHAP. Vi 



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 coryinbcsa: A, plant during day ; C, same plant at night. 
Both figures copied from photographs. 

in one instance we found that the midrib of a terminal 
leaflet formed at night an angle of 36, with a line dropped 



CHAP. VII. 



SLEEP OF LEAVES. 



371 



perpendicularly from the end of the petiole. The second pair 
of leaflets likewise moves a little backwards, but less than the 
terminal pair; and the third pair moves vertically downwards, 
or even a little forwards. Thus all the leaflets, in those species 
which bear only 3 or 4 pairs, tend to form a single packet, with 
their upper surfaces in contact, and their lower surfaces turned 
outwards. Lastly, the main petiole rises at night, but with 
leaves of different ages to very different degrees, namely, some 
rose through an angle of only 12, and others as much as 41. 

Cassia call tan tha. 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. 




Ihssia pulxscens: 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; 



372 MODIFIED CIKCUMNUTATION. CHAP. 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 ne.trly or quite vertically. This, together with tho 
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 mimosoides. At night the numerous leaflets on each 
leaf rotate on their axes, and their tips move towards the apex 
of the leaf; they thus become imbricated with their lower 
surfaces directed upwards, and with their midribs almost 
parallel to the petiole. Consequently, this species differs from 
all the others seen by us, with the exception of the following 
one, in the leaflets not sinking down at night. A petiole, tho 
movement of which was measured, rose 8 at night. 

Cassia Bardayana. 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 1 P.M. on August 13th to 8.30 A.M. 17th ; but thosa 
during the last 2 h. are alone given in Fig. 156. From 8 A.M. on 
each day (by which hour the leaf had assumed its diurnal posi- 
tion) to 2 or 3 P.M., it either zigzagged or circumnutated over 
nearly tho same small space ; at between 2 and 3 P.M. the great 
evening fall commenced. The lines representing this fall and 
the early morning rise are oblique, owing to the peculiar manner 
in which the leaflets sleep, as already described. After the 
leaflet was asleep at 6 P.M., and whilst the glass filament hung 



CHAP. VII 



SLEEP OF LEAVES. 



873 




.*. 



perpendicularly down, the movement of its apex was traced 
until 10.30 P.M.; and during this whole time it swayed from 
side to side, completing more than one ellipse. 

Bauhiitia (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 
night the two halves rise 
up and close completely 
together, like the opposite 
leaflets of many Legurui- 
nosse. With very young 
plants the petioles rise con- 
siderably at the same time ; 
one, which was inclined at 
noon 45 above the hori- 
zon, at night stood at 75 ; 
it thus rose 30; another 
rose 34. Whilst the two 
halves of the leaf are closing, 
the midrib at first sinks 
vertically downwards and 
afterwards bends back- 
wards, so as to pass close 
along one side of its own 
upwardly inclined petiole; 
the midrib being thus di- 
rected towards the stem or 
axis of the plant. The angle 
which the midrib formed 
with the horizon was mea- //' ^ 

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= oj 

s I 









I'Se 

^=3 



374 MODIFIED CIECUMNUTATION. CIIAP. VII. 

Owing to this movement to the leaves being folded and to 
the petioles rising, the whole plant is as much more compact at 
night than during the day, as a fastigiate Lombardy poplar is 
compared with any other species of poplar. It is remarkable 
that when our plants had grown a little older, viz., to a height 
of 2 01 3 feet, the petioles did not rise at night, and the midribs 
of the folded leaves were no longer bent back along one side of 
the petiole. We have noticed in some other genera that the 
petioles of very young plants rise much more at night than do 
those of older plants. 

Tamarindus Indica (Tribe 16). The leaflets approach or 
meet each other at night, and are all directed towards the apex 
of the leaf. They thus become imbricated with their midribs 
parallel to the petiole. The movement is closely similar to 
that of Hsematoxylon (see former Fig. 153), but more striking 
from the greater number of the leaflets. 

Adenanthera, Prosopis, and Neptunia (Tribe 20). With Ade- 
nanthera pavonia the leaflets turn edgeways and sink at night. 
In Prosopis they turn upwards With Neptunia oleracea the 
leaflets on the opposite sides of the same pinna come into 
contact at night and are directed forwards. The pinnae 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 pinnae also approach each other closely, and the whole leaf 
is thus rendered very compact. The main petiole sinks down- 
wards during the day till late in the evening, and rises until 
very early in the morning. The stem is continually circumnu- 
tating at a rapid rate, though not to a wide extent. Some very 
young plants, kept in darkness, were observed during two days, 
and although subjected to a rather low temperature of 57 59 F., 
the stem of one described four small ellipses in the course ol 
12 h. We shall immediately see that the main petiole is like- 
wise continually circumnutating, as is each separate pinna and 
each separate leaflet. Therefore, if the movement of the apex 
of any one leaflet were to be traced, the course described would 
be compounded of the movements of four separate parts. 



CHAP. VII. 



SLEEP OF LEAVES. 



375 



Fig. 157. 



A filament had been fixed on the previous evei jng, longi- 
tudinally to the main petiole of a nearly full-grown, highly- 
sensitive leaf (four inches in length), the stem having been 
secured to a stick at its base ; and a tracing was made on a 
vertical glass in the hot-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 hues do not coincide, the 
petiole manifestly circuninu- 
tates ; the great evening fall 
and nocturnal rise being an 
exaggeration of one of the cir- 
cumuutations. It should, how- 
ever, be observed that the pe- 
tiole fell much lower down in 
the evenings than could be 
seen on the vertical glass or is 
represented in the diagram. 
After 7 P.M. on the 3rd (when 
the last dot in Fig. 157 was 
made) the pot was earned into 
a bed-room, and the petiole was 
found at 12.50 A.M. (i.e. after 
midnight) standing almost, up- 
right, and much more highly 
inclined than it was at 10.40 

P.M. When observed again at Mimosn ? udica : circram f nta . 

... , ,, , nvcmropic movement or main p- 

4 A.M. it had begun to fall, and tio i e) traccd during 34 h. ao m. 
continued falling till 6.15 A.M., 

after which hour it zigzagged and again circumnutated. Similar 
observations were made on another petiole, with nearly the 
same result. 

On two other occasions the movement of the main petiole 




376 MODIFIED CIRCUMNUTATION. CIAP. VII. 

was observed every two or three minutes, the plants being kept 
at a rather high temperature, viz., on the first occasion at 
77 81 F., and the filament then described 2| ellipses in 69 m. 
On the second occasion, when the temperature was 81 86 F., 
it made rather more than 3 ellipses in 07 m. Therefore, 
"Fig. 157, though now sufficiently complex, would have been in- 
comparably more so, if dots had been made on the glass every 
2 or 3 minutes, instead of every hour or half-hour. Although 
the main petiole is continually and rapidly describing small 
ellipses during the day, yet after the great nocturnal rising 
movement has commenced, if dots are made every 2 or 3 
minutes, as was done for an hour between 9.30 and 10.30 P.M. 
(temp. 84 F.), and the dots are then joined, au almost abso- 
lutely straight line is the result. 

To show that the movement of the petiole is in all proba- 
bility due to the varying turgescence of the pulvinus, and not 
to growth (in accordance with the conclusions of Pfeffer), a very 
old leaf, with some of its leaflets yellowish and hardly at all 
sensitive, was selected for observation, and the plant was kept 
at the highly favourable temp, of 80 F. The petiole fell from 
8 A.M. till 10.15 A.M., it then rose a little in a somewhat zigzag 
line, often remaining stationary, till 5 P.M., when the great 
evening fall commenced, which was continued till at least 
10 P.M. By 7 A.M. on the following morning it had risen to tho 
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 ouly twice 
down and twice up in the course of 24 h., with the ascending 
and descending lines not coincident. 

It has already l>een stated that the pinna3 move independently 
of the main petiole. The petiole of a leaf was fixed to a cork 
support, close to the point whence the four pinnae diverge, with 
a short fins filament cemented longitudinally to one of the two 
terminal pinnae, and a graduated semicircle was placed close 
beneath it. By looking vertically down, its angular or lateral 
movements could be measured with accuracy. Between noon 
and 4.15 TM. tho pinna changed its position to one side by only 
7; but not continuously in the same direction, as it moved 
four times to one side, and three times to the opposite side, 



CHAP. VII. SLEEP OF LEAVES. 377 

in one instance to the extent of 16. This pinna, therefore; 
circumnutated. Later in the evening the four pinnae approach 
each other, and the one which was observed moved inwards 
5 ( J between noon and 6.45 P.M. Ten observations were made 
in the course of 2 h. 20 m. (at average intervals of 14 in.), 
between 4.25 and 6.45 P.M. ; and there was now, wiaen the lea! 
Avas going to sleep, HO swaying from side to side, but a steady 
inward movement. Here therefore there is in the evening the 
same conversion of a circumnutating into a steady movement 
in one direction, as in the case of the main petiole. 

It has also been stated that each separate leaflet circum- 
nutates. A pinna was cemented with shellac on the summit 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 tho 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 circumuutation 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 tho 
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 periol the leaflets 
showed no signs of sleeping, and were only slightly sensitive to 
a touch. On the following day the stein was cemented to a 



378 



MODIFIED CIKCUMNUTATION. 



CHAP. Vli. 



stick, and the movements of two leaves were traced on a vertical 
glass during 72 h. The plants were still kept in the dark, ex- 
cepting that at each observation, which lasted 3 or 4 minutes, 



158. 




Mimnsa pudica: circumnutation and nyctitropic movement of a leaflet 
(vr:th pinna secured), traced on a vertical glass, from 8 A.M. Sept. 14tL 
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 circumnutato distinctly, 



CHAP. VII. SLEEP OF LEAVES. 379 

although the proper order of their movements in relation to the 
clay and night was wholly lost. Thus, one leaf descended during 
the first two nights (i.e. between 10 P.M. and 7 A.M. next morn- 
ing) instead of ascending, and on the third night it moved 
chiefly in a lateral direction. The second leaf behaved in an 
equally abnormal manner, moving laterally during the first 
night, descending greatly during the second, and ascending to 
an unusual height during the third night. 

With plants kept at a high temperature and exposed to the 
light, the most rapid circumnutating movement of the apex 
of a leaf which was observed, amounted to ^ of an inch in 
one second; and this would have equalled of an inch in a 
minute, had not the leaf occasionally stood still. The actual 
distance travelled by the apex (as ascertained by a measure 
placed close to the leaf) was on one occasion nearly ^ of an inch 
in a vertical direction in 15 m.; and on another occasion | 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 f- of the natural size, present some 

Fig. 159. 




Mimosa albida : leaf seen from vertically above. 

interesting peculiarities. It consists of a long petiole bearing 
only two pinnae (here represented as rather more divergent 
than is usual), each with two pairs of leaflets. But the inner 



* Mr. Thistleton Dyer informs Linn. Soc.,' vol. xxx. p. 390) to 

us that this Peruvian plant (which be " the species or variety which 

was sent to us from Kew) is con- most commonly represents the M 

eidered by Mr. Bentham (' Trans. sensitiva of our gardens." 



380 MODIFIED CIECUMNUTATION. CHAP. VII. 

basal leaflets are greatly reduced in size, owing probably to the 
want of space for their full development, so that they may be 
considered as almost rudimentary. They vary somewhat in 
size, and both occasionally disappear, or only one. Neverthe- 
less, they are not in. the least rudimentary in function, for they 
are sensitive, extremely heliotropic, circumnutate at nearly the 
same rate as the fully developed leaflets, and assume when 
asleep exactly the same position. With M. pudica the inner 
leaflets at the base and between the pinnae are likewise much 
shortened and obliquely truncated ; this fact was well seen in 
some seedlings of M. pudica, in which the third leaf above the 
cotyledons bore only two pinnse, each with only 3 or 4 pairs of 
leaflets, of which the inner basal one was less than half as long 
as its fellow; so that the whole leaf resembled pretty closely 
that of M. albida. In this latter species the main petiole termi- 
nates in a little point, and on each side of this there is a pair 
of minute, flattened, lancet-shaped projections, hairy on their 
margins, which drop off and disappear soon after the leaf is 
fully developed. There can hardly be a doubt that these littlo 
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. y^ of an 
inch, whilst the inner one is only f ^- long. Now if the basal 
pair of leaflets of the existing leaves were to become rudimen- 
tary, we should expect that the rudiments would still exhibit 
some trace of their present great inequality of size. The con- 
clusion that the pinnse of the parent-form of M. albida possessed 
at least three pairs of leaflets, instead of, as at present, only two, 
- is supported by the structure of the first true leaf; for this 
consists of a simple petiole, often bearing three pairs of leaflets. 
This latter fact, as well as the presence of the rudiments, both 
lead to the conclusion that M. albida is descended from a form 
the leaves of which bore more than two pairs of leaflets. The 
second leaf above the cotyledons resembles in all respects the 
leaves on fully developed plants. 

When the leaves go to sleep, each leaflet twists half round, 
so as to present its edge to the zenith, and comes into dose 
contact with its fellow. The pinnae also approach each other 
closely, so that the four terminal leaflets come together. The 
large basal leaflets (with the little rudimentary ones in contact 
with them) move inwards and forwards, so as to embrace the 
outside of the united terminal leaflets, and thus all eight leaflets 



CHAP. VII. SLE.EP OF LEAVES. 381 

(the rudimentary ones included) form together a single vertical 
packet. The two pinnae at the same time that they approach 
each other sink downwards, and thus instead of extending hori- 
zontally in the same line with the main petiole, as during the 
day, they depend at night at about 45, or even at a greater 
angle, beneath the horizon. The movement of the main petiole 
seems to be variable; we have seen it in the evening 27 lower 
than during the day ; but sometimes in nearly the same position. 
Nevertheless, a sinking movement in the evening and a rising 
one during the night is probably the normal course, for this 
was well-marked in the petiole of the first-formed true leaf. 

The circumnutation of the main petiole of a young leaf was 
traced during 21 days, and was considerable in extent, but less 
complex than that of M. pudica. The movement was much 
more lateral than is usual with circumnutating leaves, and this 
was the sole peculiarity which it presented. The apex ol 
one of the terminal leaflets was seen under tho microscope to 
travel ^ of an inch in 3 minutes. 

Mimosn 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. 

tichrankia urtcinata (Tribe 20). A leaf consists of two or three 
pairs of pinnso, each bearing many small leaflets. These, when 
the plant is asleep, are directed forwards and become imbricated. 
The angle between the two terminal pinnae was diminished at 
night, in one case by 15 ; and they sank almost vertically down- 
wards. The hinder pairs of pinnae likewise sink downwards, 
but do not converge, that is, move towards the apex of the leaf. 
The main petiole does not become depressed, at least during the 
evening. In this latter respect, as well as in the sinking of the 
I innae, there is a marked difference between tho nyctitropio 
movements of the present plant and of Mimosa pudica. It 
should, however, be added that our specimen was not in a very 
vigorous condition. The pinnae of Sclirankia 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 name leaf in the two states is shown in the following 
figure (Fig. 160"). The leaflets move towards the apex of the 
pinna and become imbricated, and the pinnae then look like bits 
of dangling string. The following remarks and measurements 



382 



MODIFIED CIKCUMNUTATION. 



CHAP. VIL 



do not fully apply to the small leaf here figured. The pinme 
move forwards and at the same time sink downwards, whilst 
the main petiole rises considerably. With respect to the degree 
of movement : the two terminal pinnae of one specimen formed 
together an angle of 100 during the day, and at night of only 
88, so each had moved 31 forwards. The penultimate pinnae 
during the day formed together an angle of 180, that is, they 
stood in a straight line opposite one another, and at night each 
had moved 65 forwards. The basal pair of pinnae were directed. 

Fig. 160. 




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 pinnso at 
the same time sink greatly, and sometimes hang almost perpen- 
dicularly downwards. The main petiole, on the other hand, 
rises much : by 8.30 P.M. one stood 34 higher than at noon, 
and by 6.40 A.M. on the following morning it was still higher 
by 10; shortly after this hour the diurnal sinking move- 
ment commenced. The course of a nearly full-grown leaf was 
traced during 14 h. ; it was strongly zigzag, and apparently 



CHAP. VII. SLEEP OF LEAVES. 383 

represented five ellipses, with their longer axes differently 
directed. 

Albizzia lopltantha (Tribe 23). The leaflets at night come into 
contact with one another, and are directed towards the apex of 
the pinna. The pinnae approach one another, but remain in the 
same plane as during the day ; and in this respect they differ 
much from those of tbs above Schrankia and Acacia. The main 
petiole rises but little. The lirst-formed leaf above the coty- 
ledons bore 11 leaflets on each side, and these slept like thoso 
on the subsequently formed leaves ; but the petiole of this first 
leaf was curved downwards during the day and at night 
straightened itself, so that the chord of its arc then stood 16 
higher than in the day-time. 

Melaleuca ericcefolia (Myrtacese). According to Bouche" (' Bot. 
Zeit.,' 1874, p. 359) the leaves sleep at night, in nearly the same 
manner as those of certain species of Pimelia. 

(Enothtra mollissima (Onagrariese). According to Linmeus 
(' Somnus Plantarum '), the leaves rise up vertically at night. 

Passiflora gracilis (Passifloracse). 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 yoiing leaf, the blade of 
which was inclined at 40 below the horizon. From its position 
the leaf had to be viewed obliquely, consequently the vertically 
ascending and descending movements appeared when traced 
oblique. On the first day (Oct. 12th) the leaf descended in a 
zigzag Line until late in the evening ; and by 8.15 A.M. on the 
13th had risen to nearly the . same level as on the previous 
morning. A new tracing was now begun (Fig. 161). The 
leaf continued to rise until 8.50 A.M., then moved a little to the 
right, and afterwards descended. Between 11 A.M. and 5 P.M. it 
circumnutated, and after the latter hour the great nocturnal 
fall commenced. At 7.15 P.M. it depended vertically. The 
dotted line ought to have been prolonged much lower down in 
the figure. By 6.50 A.M. on the following morning (14th) the 



384 



MODIFIED CIECUMNUTATION. 



CHAP. VII. 



leaf had risen greatly, and continued to rise till 7.50 A.M., after 
which hour it redescended. It should be observed that the lines 
traced on this second morning would have coincided with and 
confused those previously traced, had not the pot been slided 
a very little to the left. In the evening (14th) a mark was 
placed behind the filament attached to the apex of the leaf, and 
its movement was carefully traced from 5 P.M. to 10.15 P.M. 

Fig. 161. 




Passiflora gracilis: circumnutation and nyctitropic movement of leaf, 
traced on vertical glass, from 8.20 A.M. Oct. i3th to 10 A.M. 14th 
Figure reduced to two-thirds of original scale. 

Between 5 and 7.15 P.M. the leaf descended in a straight line, 
and at the latter hour it appeared vertically dependent. But 
between 7.15 and 10.15 P.M. the line consisted of a succession 
of steps, the cause of which we could not understand ; it was, 
however, manifest that the movement was no longer a simple 
descending one. 

Siegesbeckia orientalis (Compositse). Some seedlings were 
raised in the middle of winter and kept in the hot-house ; they 
flowered, but did not grow well, and their leaves never showed 
any signs of sleep. The leaves on other seedlings raised in May 
were horizontal at noon (June 22nd), and depended at a consi 



CHAP. VII. 



SLEEP OF LEAVES. 



385 



derable angle beneath the horizon at 10 P.M. In the case of four 
youngish leaves, which were from 2 to 2 inches in length, 
these angles were found to be 50, 56, 60, and 65. At the 
end of August, when the plants had grown to a height of 10 to 11 
inches, the younger leaves were so much curved downwards at 
night that they might truly be said to be asleep. This is oue 
- 

Fig. 162. 




AYrtrfwmT g7(i>ica : shoots with leaves expanded during the day, and aslcqi 
at night. Figures copied from photographs, and reduced. 

of the species which must be well illuminated during the day 
in order to sleep, for on two occasions when plants were kept 
all day in a room with north-east windows, the leaves did not 
sleep at night. The same cause probably accounts for the 
leaves on our seedlings raised in the dead of the winter not 
bleeping. Professor Pfeffer informs us that the leaves of 
another species (S. Jorulknsis ?) hang vertically down at night. 



386 



MODIFIED CIRCUMNUTATION. 



CHAP. VIL 



Ipomcea ccerulea (aidpurptirea (Convolvulaceae). The haves on 
very young pi ants, a foot or two in height, are depressed at night 

to between 68 and 80 
beneath tho horizon ; 
and some hang quite 
vertically downwards. 
On the following morn- 
ing they again rise into 
a horizontal position. 
The petioles become 
at night downwardly 
curved, either through 
their entire length or in 
the upper part alone ; 
and this apparently 
causes the depression 
of the blade. It seems 
necessary that the 
leaves should be well 
illuminated during the 
day in order to sleep, 
for those which stood 
on the back of a plant 
before a north-east 
window did not sleep. 

Nicotiana tabacum 
(var. Virginian) and 
glauca (Solaneae). The 
young leaves of both 
these species sleep by 
bendinh vertically up- 
wards. Figures of two 
shoots of N. sjZomco* 
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 




Nicotiana tabacum : circumnutation and nyc- 
titropic movement of a leaf (5jj inches in 
Isagth), 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. 17 J- 
18 C. Figure reduced to one-half 
original scale. 



CHAP. VII. SLEEP OF LEAVES. 387 

have their longer axes differently directed from the ceHs of the 
parenchyma, and may therefore be considered as forming a sort 
of pulvinus. A young plant of N. tabacum was selected, and 
the circumnutation of the fifth leaf above the cotyledons was 
observed during three days. On the first morning (July 10th) 
the leaf fell from 9 to 10 A.M., which is its normal course, but 
rose during the remainder of the day ; and this no doubt was 
due to its being illuminated exclusively from above; for properly 
the evening rise does not commence until 3 or 4 P.M. In the 
figure as given on p. 386 (Fig. 163) the first dot was made at 
3 P.M. ; and the tracing was continued for the following 65 h. 
When the leaf pointed to the dot next above that marked 3 P.M. 
it stood horizontally. The tracing is remarkable only from its 
simplicity and the straightness of the lines. The leaf each day 
described a single great ellipse ; for it should be observed that 
the ascending and descending lines do not coincide. On the 
evening of the llth the leaf did not descend quite so low as 
usual, and it now zigzagged a little. The diurnal sinking move- 
ment had already commenced each morning by 7 A.M. The broken 
lines at the top of the figure, representing the nocturnal vertical 
position of the leaf, ought to be prolonged much higher up. 

Mirabilis longiflora and jalapa (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 (Polygoneae). Professor Batalin informs 
us that the young leaves rise up vertically at night. This is 
likewise the case, according to Linnaeus, with several species 
of Amaraiithus (Amaranthacese) ; and we observed a sleep move- 
ment of this kind in one member of the genus. Again, with 
Chenopodium album (Chenopodiese), the upper young leaves ot 
some seedlings, about 4 inches in height, were horizontal or 
Bub-horizoutal 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 Bouche (' JBot. Zeitung/ 1874, 
p. 359; the leaves of Pimelia lino-ides and spedabilis (Thymelese) 
sleep at night. 



388 MODIFIED CIRCUMNUTATION. CHAP. VIT. 

Euphorbia jacquiniceflora (Euphorbiacese). 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 llth) 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 tlio 
following morning. The fourth and fifth leaves from the summit 
stood horizontally during the day, and sank down at night only 
38. The sixth leaf did not sensibly alter its position. The 
sinking movement is due to the downward curvature of the 
petiole, no part of which exhibits any structure like that of 
a pulvinus. Early on the morning of June 7th a filament was 
fixed longitudinally to a young leaf (the third from the summit, 
and 2f inches in length), and its movements were traced on 
a vertical glass during 72 h., the plant being illuminated from 
alx)ve through, a skylight. Each day the leaf fell in a nearly 
straight line from 7 A.M. to 5 P.M., after which hour it was so 
much inclined downwards that the movement could no longer 
be traced ; and during the latter part of each night, or early in 
the morning, the leaf rose. It therefore circumnutated in a 
very simple manner, making a single large ellipse every 24 h., 
for the ascending and descending lines did not coincide. On 
each successive morning it stood at a less height than on the 
previous one, and this was probably due, partly to the increasing 
age of the leaf, and partly to the illumination being insufficient ; 
for although the leaves are very slightly heliotropic, yet, accord- 
ing to Mr. Lynclrs 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. 

PliyllantliUK Kiruri (Euphorbiacefe). The leaflets of this 
plant sleep, as described by Pfeft'er,* 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 



Die Period. Bewcg.,' p. 151). 



CHAP. VII. SLEEP OF LEAVES 389 

outwards. They are furnished, .as might have been expected 
from tliis complex kind of movement, with a pulvinus. 

GYMNOSPERMS. 

Pinus Nordmanniana (Coniferse). M. Chatin states* that the 
leaves, which are horizontal during the day, rise up at night, so 
a- to assume a position almost perpendicular to the branch from 
which they arise ; we presume that he here refers to a horizontal 
branch. He adds : " En meme temps, ce mouvement direction 
est accompagne d'un mouvement de torsion imprime a la partie 
basilaire de la feuille, et pouvant sou vent parcourir un arc do 
90 degres." As the lower surfaces of the leaves are white, 
whilst the upper are dark green, the tree presents a widely 
different appearance by day and night. The leaves on a small 
tree in a pot did not exhibit with us any nyctitropic move- 
ments. We have seen in a former chapter that the leaves of 
Pin/ is pinaster and Austriuca are continually circumuutating. 



MONOCOTYLEDONS. 

Thalia dealbata (Cannacese). The leaves of this plant sleep 
by turning vertically upwards ; they are furnished with a well- 
deyeloped pulvinus. It is the only instance known to us of 
a very large leaf sleeping. The blade of a young leaf, which 
was as yet only 13} inches in length and GJ 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 7.30A.M. and 10 P.M., was 
lOi- inches. The circumnutation of two young and dwarfed 
leaves, arising amongst the taller leaves at the base- of the plant, 
was traced on a vertical glass during two days. On the first day 
the apex of one, and on the second day the apex of the other leaf, 
described between 6.40 A.M. and 4 P M. two ellipses, the longer 
axes of which were extended in very different directions from the 
lines representing the great diurnal sinking and nocturnal rising 
movement. 

Maranta arundinacea (Cannacese). The blades of the leaves, 
which are furnished with a pulvinus, stand horizontally during 



* 'Comptes Renclus,' Jan. 1876, p. 171. 



390 MODIFIED CIRCUMNUTATION. CHAP. VII 

the day or between 10 and 20 above the horizon, and at night 
vertically upwards. They therefore rise between 70 and 90 at 
night. The plant was placed at noon in the dark in the hot- 
house, and on the following day the movements of the leaves 
were traced. Between 8.40 and 10 30 A.M. they rose, and then 
fell yreatly till 1.37 P.M. But by 3 P.M. they had again risen a 
little, and continued to rise during the rest of the afternoon ami 
night ; on the following morning they stood at the same level as 
on the previous day. Darkness, therefore, during a day and a 
half does not interfere with the periodicity of their movements. 
On a warm but stormy evening, the plant whilst being brought 
into the house, had its leaves violently shaken, and at night not 
one went to sleep. On" the next morning the plant was taken 
back to the hot-house, and again at night the leaves did not 
sleep ; but on the ensuing night they rose in the usual manner 
l>ctween 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 axtiquorum (Caladium esctihnlum, Hort.) (Aroidese). 
The leaves of this plant sleep by their blades sinking in the 
evening, so as to stand highly inclined, or even quite vertically 
with their tips pointing to the ground. They are not provided 
with a pulvinus. The blade of one stood at noon 1 beneath the 
horizon; at 4.20 P.M., O) ; at 6 P.M., 43 ; at 7.20 P.M., 69 ; and at 
8.30 P.M., 68 ; so it had now begun to rise ; at 10.15 P.M. it stood 
at 65, and on the following early morning at 11 beneath the 
horizon. Tbe 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 P M. or 9 P.M., when it 
rose a little and moved laterally. By an early hour on both 
mornings, it had assumed its diurnal position. The well-marked 
lateral movement for a short time in the early part of the night, 
was the only interesting fact which it presented, as this caused 
the ascending and descending lines not to coincide, in 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 havo 
Keen that in another genus of the Aroideae, namely, Pistia, the 



CHAP. VII. SLEEP OF LEAVES. 391 

leaves rise so much at night that they may almost be said to 
Bleep. 

Strephium floribundum* (Graminese). The oval leaves are 
provided with a pulvinus, and are extended horizontally or 
declined a little beneath the horizon during the day. Those 
on the upright culms simply rise up vertically at night, so 
that their tips are directed towards the zenith. (Fig. 16-1) 

Fig. 164. 




Sirephlum floribundum : culms with leaves during the day, and when 
at night. Figures reduced. 

Horizontally extended leaves arising from much inclined or 
almost horizontal culms, move at night so that their tips 
point towards the apex of the culm, with one lateral margin 
directed towards the zenith; and in order to assume this 
position the leaves have to twist on their own axes through an 
angle of nearly 90. Thus the surface of the blade always stands 
vertically, whatever may be the position of the midrib or of the 
leaf as a whole. 

The circumnutation of a young leaf (2 '3 inches in length) was 
traced during 48 h. (Fig. 165). The movement was remarkably 
simple; the leaf descended from before 6.40 A.M. until 2 or 
2.50 P.M., and then rose so as to stand vertically at about 6 P.M., 
descending again late in the night or in the very early morning. 



* A. Brongniart first observed la Soc. Bot. de Franco,' toin. vii 
th:it the leaves of this plant and 1860, p. 470. 
of Marsilea sleep : see ' Bull, de 
26 



392 



MODIFIED CIRCUMNUTATION. 



CHAP. VII 



On the second day ihe descending line zigzagged slightly. As 
Vie. 165. usual, the ascending and de- 

scending lines did not coincide. 
On another occasion, when the 
temperature was a little higher, 
viz. ; 2r-i26i C., a leaf was 
observed 17 times between 8.50 
A.M. and 12.10 P.M. ; it changed 
its course by as much as a 
rectangle six times in this in- 
terval of 3 h. 26 m., and de- 
scribed two irregular triangles 
and a half. The leaf, therefore, 
on this occasion circumnutated 
rapidly and in a complex 
manner. 



titrephium floribundum : circumnu- 
tation and nyctitropic movement 
of a leaf, traced from A.M. June 
26th to 8.45 A.M. 27th ; filament 
fixed along the midrib. Apex of 
leaf 8.J inches from the vertical 
glass ; plant illuminated from 
above. Temp. 23J-2-4 C. 



ACOTYLEDONS. 

JUarsika quadrifoliafa (Mar- 
sileacese). 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 



CHAP Vll 



SLEEP OF LEAVES. 



393 



during 24 h. (Fig. 167). The leaf fell from the early morning 
till 1.50 P.M., and then rose till 6 P.M., when it was asleep. A 

Fig. 166. 




Marsilea quadrifoliala: A, leaf during the day, seen from vertically above 
B, leaf beginning to go to sleep, seen laterally ; C, the same asleep. 
Figures reduced to one-half of natural scale. 

vertically dependent glass filament was now fixed to one of the 
terminal and inner leaflets ; and part of the tracing in Fig. 167, 
after 6 P.M., shows that it continued to sink, making one zigzag, 
until 10.40 P.M. At 6.45 A.M. on the following morning, the leaf 
was awaking, and the filament pointed above the vertical glass, 




itarsilca/iuadrifoliata: circumnutation and nyctitropic movement cf leaflet 
traced on vertical glass, during nearly 24 h. Figure reduced to two- 
thirds of original scale. Plant kept at rather too low a temperature. 

but by 8.25 A M. it occupied the position shown in the figure. 
The diagram differs greatly in appearance from most of those 
previously given; and this is due to the leaflet twisting and 
moving laterally as it approaches and comes into contact with 



304 MODIFIED CIRCUMNUTATION. CHAP. VH 

its fellow. The movement of another leaflet, when asleep, 
was traced between 6 P.M. and 10.35 P.M., and it clearly cir- 
cumnutated, for it continued for two hours to sink, then rose, 
and then sank still lower than it was at 6 P.M. It may lie 
seen in Ihe preceding figure (167) that the leaflet, when the 
plant was subjected to a rather low temperature in the house 
descended and ascended during the middle of the day in a 
somewhat zigzag line; but when kept in the hot-house from 
9 A.M. to 3 P.M. at a high but varying temperature (viz., between 
72 and 83 F.) a leaflet (with the petiole secured) circumnutated 
rapidly, for it made three large vertical ellipses in the course of 
the six hours. According to Brongniart, Marsilea pubescens sleeps 
like the present species. These plants are the sole cryptogamic 
ones known to sleep. 



Summary and Concluding Remarks on the Nyctitropio 
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 Melilotus likewise rotates, by which movement one 
of its lateral edges is directed upwards, and at the 
same time it moves either to the left or to the right, 
until its upper surface comes into contact with that ot 
the lateral leaflet on the same side, which has like- 
wise rotated on its own axis. With Arachis, all four 
leaflets form together during the night a single 
vertical packet; and to effect this the two anterior 
leaflets have to move upwards and the two posterior 
ones forwards, besides all twisting on their own axes. 
In the genus Sida the leaves of some species move at 
night through an angle of 90 upwards, and of others 



CHAP. VII. SUMMARY ON SLEEP OF LEAVES. 395 

through the same angle downwards. We have seen a 
similar difference in the nyctitropic movements of the 
cotyledons in the genus Oxalis. In Lupinus, again, 
the leaflets move either upwards or downwards ; and 
in some species, for instance L. luteus, those on one 
side of the star-shaped leaf move up, and those on the 
opposite side move down ; the intermediate ones rota- 
ting on their axes ; and by these varied movements, the 
whole leaf forms at night a vertical star instead of a 
horizontal one, as during the day. Some leaves and 
leaflets, besides moving either upwards or downwards, 
become more or less folded at night, as in Bauhinia 
and in some species of Oxalis. The positions, indeed, 
which leaves occupy when asleep are almost infinitely 
diversified ; they may point either vertically upwards 
or downwards, or, in the case of leaflets, towards the 
apex or towards the base of the leaf, or in any inter- 
mediate position. They often rotate at least as much 
as 90 on their own axes. The leaves which arise 
from upright and from horizontal or much inclined 
branches on the same plant, move in some few cases 
in a different manner, as with Porlieria and Strephium. 
The whole appearance of many plants is wonderfully 
changed at night, as may be seen with Oxalis, and 
still more plainly with Mimosa. A bush of Acacia 
Farnesiana appears at night as if covered with little 
dangling bits of string instead of leaves. Excluding 
a few genera not seen by ourselves, about which we 
are in doubt, and excluding a few others the leaflets of 
which rotate at night, and do not rise or sink much, 
there are 37 genera in which the leaves or leaflets rise, 
often moving at the same time towards the apex or 
towards the base of the leaf, and 32 genera in which 
they sink at night. 

The nyctitropic movements of leaves, leaflets, and 



3DG MODIFIED CIKCUMNUTATION. CHAP. VII 

petioles are effected in two different ways ; firstly, by 
alternately increased growth on their opposite sides, 
preceded by increased turgescence of the cells; and 
secondly by means of a pulvinus or aggregate of small 
cells, generally destitute of chlorophyll, which become 
alternately more turgescent on nearly opposite sides ; 
and this turgescence is not followed by growth except 
during the early age of the plant. A pulvinus seems 
to be formed (as formerly shown) by a group of cells 
ceasing to grow at a very early age, and therefore does 
not differ essentially from the surrounding tissues. 
The cotyledons of some species of Trifolium are pro- 
vided with a pulvinus, and others are destitute of one, 
and so it is with the leaves in the genus Sida. We 
see also in this same genus gradations in the state of 
the development of the pulvinus ; and in Nicotiana 
we have what may probably be considered as the 
commencing development of one. The nature of the 
movement is closely similar, whether a pulvinus is 
absent or present, as is evident from many of the 
diagrams given in this chapter. It deserves notice 
that when a pulvinus is present, the ascending and 
descending lines hardly ever coincide, so that ellipses 
are habitually described by the leaves thus provided, 
whether they are young or so old as to have quite 
ceased growing. This fact of ellipses being described, 
shows that the alternately increased turgescence of 
the cells does not occur on exactly opposite sides of the 
pulvinus, any more than the increased growth which 
causes the movements of leaves not furnished with 
pulvini. When a pulvinus is present, the nyctitropic 
movements are continued for a very much longer 
period than when such do not exist. This has been 
amply proved in the case of cotyledons, and Pfeffei 
has given observations to the same effect with resped 



CHAP. VII SUMMARY ON T SLEEP OF LEAVES. 397 

to leaves. We have seen that a leaf of Mimosa 
pudica continued to move in the ordinary manner, 
though somewhat more simply, until it withered and 
died. It may be added that some leaflets of Trifolium 
pratense were pinned open during 10 days, and on the 
first evening after being released they rose up and 
slept in the usual manner. Besides the long con- 
tinuance of the movements when effected by the aid 
of a pulvinus (and this appears to be the final cause 
of its development), a twisting movement at night, as 
Pfeffer has remarked, is almost confined to leaves thus 
provided. 

It is a very general rule that the first true leaf, 
though it may differ somewhat in shape from the 
leaves on the mature plant, yet sleeps like them ; and 
this occurs quite independently of the fact whether or 
not the cotyledons themselves sleep, or whether they 
sleep in the same manner. But with Phaseolus Eox- 
lurghii the first unifoliate leaves rise at night almost 
sufficiently to be said to sleep, whilst the leaflets of 
the secondary trifoliate leaves sink vertically at night. 
On young plants of Sida rhombaefolia, only a few 
inches in height, the leaves did not sleep, though on 
rather older plants they rose up vertically at night. 
On the other hand, the leaves on very young plants of 
Cijtisus fragrans slept in a conspicuous manner, whilst 
on old and vigorous bushes kept in the greenhouse, 
the leaves did not exhibit any plain nyctitropic move- 
ment. In the genus Lotus the basal stipule-like 
leaflets rise up vertically at night, and are provided 
with pulvini. 

As already remarked, when leaves or leaflets change 
their position greatly at night and by complicated 
movements, it can hardly be doubted that these must 
be in some manner beneficial to the plant. If so, we 



398 MODIFIED CIRCUMNUTATION CHAP. VTL 

must extend tKe same conclusion to a large number ol 
sleeping plants; for the most complicated and the 
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 
nyctitropio 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 



CHAI. VII. SUMMARY ON SLEEP OF LEAVES. 399 

always assume at night a vertical, or nearly vertical 
position, it is a point of complete indifference whether 
the apex, or the base, or one of the lateral edges, is 
directed to the zenith. It is a rule of wide generality, 
than whenever there is any difference in the degree of 
exposure to radiation between the upper and the lower 
surfaces of leaves and leaflets, it is the upper which is 
the least exposed, as may be seen in Lotus, Cytisus, 
Trifolium, and other genera. In several species of 
Lupinus the leaflets do not, and apparently from 
their structure cannot, place themselves vertically at 
night, and consequently their upper surfaces, though 
highly inclined, are more exposed than the lower ; and 
here we have an exception to our rule. But in other 
species of this genus the leaflets succeed in placing 
themselves vertically; this, however, is effected by a 
very unusual movement, namely, by the leaflets on 
the opposite sides of the same leaf moving in opposite 
directions. 

It is again a very common rule that when leaflets 
come into close contact with one another, they do so 
by their upper surfaces, which are thus best protected. 
In some cases this may be the direct result of their 
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 leaflets 
are pressed together, and are thus better protected 



400 MODIFIED CIRCUMNUTATION. CHAP. Vlt 

than the upper surfaces ; but this depends merely on 
each leaflet becoming folded at night so as to be able 
to sink vertically downwards. The torsion or rotation 
of leaves and leaflets, which occurs in so many cases, 
apparently always serves to bring their upper surfaces 
into close approximation with one another, or with 
other parts of the plant, for their mutual protection. 
We see this best in such cases as those of Arachis > 
Mimosa albida, and Marsilea, in which all the leaflets 
form together at night a single vertical packet. If 
with Mimosa pudica the opposite leaflets had merely 
moved upwards, their upper surfaces would have come 
into contact and been well protected; but as it is, 
they all successively move towards the apex of the 
leaf ; and thus not only their upper surfaces are 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 into 
close contact at night. Or the blade and the upper 
part of the petiole may both move. Moreover, the 
petiole as a whole commonly either rises or sinks at 
night. This movement is sometimes large : thus the 
petioles of Cassia pubescens stand only a little above 
the horizon during the day, and at night rise up 
almost, or quite, perpendicularly. The petioles of the 
younger 1 eaves of Desmodium gyrans also rise up ver- 
tically at night. On the other hand, with Amphi- 



CIIAP. VII. SUMMARY ON SLEEP OF LEAVES. 401 

carpaea, the petioles of some leaves sank down as 
much as 57 at night; with Arachis they sank 39, 
and then stood at right angles to the stem. Gene- 
rally, when the rising or sinking of several petioles on 
the same plant was measured, the amount differed 
greatly. This is largely determined by the age of the 
leaf: for instance, the petiole of a moderately old leaf 
of Desmodium gyrans rose only 46, whilst the young 
ones rose up vertically ; that of a young leaf of Cassia 
floribunda rose 41, whilst that of an older leaf rose 
only 12. It is a more singular fact that the age of 
the plant sometimes influences greatly the amount of 
movement ; thus with some young seedlings of a Bau- 
hinia the petioles rose at night 30 and 34, whereas 
those on these same plants, when grown to a height 
of 2 or 3 feet, hardly moved at all. The position of 
the leaves on the plant as determined by the light, 
seems also to influence the amount of movement 
of the petiole; for no other cause was apparent 
why the petioles of some leaves of Melilotus officinalis 
rose as much as 59, and others only 7 and 9 at 
night. 

In the case of many plants, the petioles move at 
night in one direction and the leaflets in a directly 
opposite one. Thus, in three genera of Phaseoleae the 
leaflets moved vertically downwards at night, and the 
petioles rose in two of them, whilst in the third they 
sank. Species in the same genus often differ widely 
in the movements of their petioles. Even on the same 
plant of Lupinus piibcscens some of the petioles rose 30 ? 
others only 6, and others sank 4 at night. The 
leaflets of Cassia Bardayana 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 ar parently indicate 'that the movements 



102 MODIFIED CI11CUMNUTATION. CIIAP. VII 

of the petioles are not performed for any special pur- 
pose ; though a conclusion of this kind is generally 
rash. When the leaflets sink vertically down at night 
and the petioles rise, as often occurs, it is certain that 
the upward movement of the latter does not aid the 
leaflets in placing themselves in their proper posi- 
tion at night, for they have to move through a 
greater angular space than would otherwise have been 
necessary. 

Notwithstanding what has just been said, it may be 
strongly suspected that in some cases the rising of 
the petioles, when considerable, does beneficially serve 
the plant by greatly reducing the surface exposed to 
radiation at night. If the reader will compare the 
two drawings (Fig. 155, .p. 371) of Cassia pubescens, 
copied from photographs, he will see that the dia- 
meter of the plant at night is about one-third of 
what it is by day, and therefore the surface exposed 
to radiation is nearly nine times less. A similar 
conclusion may be deduced from the drawings (Fig. 
149, p. 358) of a branch awake and asleep of Des- 
modium gyrans. So it was in a very striking manner 
with young plants of Bauhinia, and with Oxalis 
Ortegesii. 

We are led to an analogous conclusion with respect 
to the movements of the secondary petioles of certain 
pinnate leaves. The pinnae of Mimosa pudica con- 
verge at night; and thus the imbricated and closed 
leaflets on each separate pinna are all brought close 
together into a single bundle, and mutually protect 
one another, with a somewhat smaller surface exposed 
to radiation. With Albizzia lophantha the pinnae close 
together in the same manner. Although the pinnae 
of Acacia Farnesiana do not converge much, they 
sink downwards. Those of Neptunia oleracea likewise 



CHAP. VII. SUMMARY ON SLEEP OF LEAVES. 403 

move downwards, as well as backwards, towards the 
base of the leaf, whilst the main petiole rises. With 
Schrankia, again, the pinnae are depressed at night. 
Now in these three latter cases, though the pinnce 
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, Amphicarprca, 
two species of Ery thrina, 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 shoi t space ; so that with many plants a 



104 MODIFIED CIRCUMNUTATION. CHAP. 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 Thalia 
dealbata, it was strongly marked. In the case of Meli- 
lotus the ellipses described by the terminal leaflet 
during the day are laterally extended, instead of ver- 
tically, as is usual ; and this fact evidently stands in 
relation with the terminal leaflet moving laterally 
when it goes to sleep. With the majority of sleeping 
plants the leaves oscillate more than once up and 
down in the twenty-four hours ; so that frequently two 
ellipses, one of moderate size, and one of very large size 
which includes the nocturnal movement, are described 
within the twenty-four hours. For instance, a leaf 
which stands vertically up during the night will sink 
in the morning, then rise considerably, again sink in 
the afternoon, and in the evening reascend and assume 
its vertical nocturnal position. It will thus describe, 
in the course of the twenty-four hours, two ellipses of 
unequal sizes. Other plants describe within the same 
time, three, four, or five ellipses. Occasionally the 
longer axes of the several ellipses extend in different 
directions, of which Aeaeia Farnesiana offered a good 
instance. The following cases will give an idea of the 
rate of movement : Oxalis acetosella completed two 
ellipses at the rate of 1 h. 25 m. for each ; Marsilea 
quadrifoliata, at the rate of 2 h.; Trifolium subterraneum, 
one in 3 h. 30 m. ; and Aracliis liypogsea, in 4 h. 50 m. 
But the number of ellipses described within a given 
time depends largely on the state of the plant and 
on the conditions to which it is exposed. It often hap- 
pens that a single ellipse may be described during one 



CIIAP. VII. SUMMARY ON SLEEP OF LEAVES. 405 

day, and two on the next. Erytlirina corallodendron 
made four ellipses on the first day of observation 
and only a single one on the third, apparently owing 
to having been kept not sufficiently illuminated and 
perhaps not warm enough. But there seems likewise 
to be an innate tendency in different species of the 
same genus to make a different number of ellipses in 
the twenty-four hours : the leaflets of Trifolium repens 
made only one ; those of T. resupinatum two, and those 
of T. subterraneum three in this time. Again, the 
leaflets of Oxalis Plumierii made a single ellipse ; those 
of 0. bupleurifolia, two ; those of 0. Valdiviana, two or 
three; and those of 0. acetosella, at least five in the 
twenty-four hours. 

The line followed by the apex of a leaf or leaflet, 
whilst describing one or more ellipses during the day, 
is often zigzag, either throughout its whole course or 
only during the morning or evening : Robinia offered 
an instance of zigzagging confined to the morning, 
and a similar movement in the evening is shown in 
the diagram (Fig. 126) given under Sida. The amount 
of the zigzag movement depends largely on the plant 
being placed under highly favourable conditions. But 
even under such favourable conditions, if the dots which 
mark the position of the apex are made at consider- 
able intervals of time, and the dots are then joined, 
the course pursued will still appear comparatively 
simple, although the number of the ellipses will be 
increased; but if dots are made every two or three 
minutes and these are joined, the result often is that 
all the lines are strongly zigzag, many small loops, 
triangles, and other figures being also formed. This 
fact is shown in two parts of the diagram (Fig. 150) 
of the movements of Desmodium gyrans. Strephium 
floribundum, observed under a high temperature, 



t06 MODIFIED CIRCUMNUTATION. CHAP. VII 

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 ^ 5 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 Dionaea, as seen under the microscope, 
all probably come under this same head. We may 
suspect that we here see the energy which is freed 
during the incessant chemical changes in progress in 
the tissues, converted into motion. Finally, it should 
be noted that leaflets and probably some leaves, whilst 
describing their ellipses, often rotate slightly on their 
axes ; so. that the plane of the leaf is directed first to 
one and then to another side. This was plainly seen 
to be the case with the large terminal leaflets of Des- 
modium, Erythrina and Amphicarpaea, and is probably 
common to all leaflets provided with a pulvinus. 

With vespect to the periodicity of the movements of 
sleeping leaves, Pfeifer* has so clearly shown that 
this depends on the daily alternations of light and 
darkness, that nothing farther need be said on this 



Die Pcrio'lischcn Bewcgungen der Blattoigane,' 1875, p. 30, ei 



CHAP. VII. SUMMARY OX SLEEP OF LEAVES. 407 

head. But we may recall the behaviour of Mimosa 
in the North, where the sun does not set, and the 
complete inversion of the daily movements by artificial 
light and darkness. It has also been shown by us, 
that although leaves subjected to darkness for a mode- 
rately long time continue to circumnutate, yet the 
periodicity of their movements is soon greatly dis- 
turbed, or quite annulled. The presence of light or 
its absence cannot be supposed to be the direct cause 
of the movements, for these are wonderfully diversified 
even with the leaflets of the same leaf, although all 
have of course been similarly exposed. The move- 
ments depend on innate causes, and are of an adaptive 
nature. The alternations of light and darkness 
merely give notice to the leaves that the period has 
arrived for them to move in a certain manner. We 
may infer from the fact of several plants (Tropseolum, 
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," fr the after- 

ance ; he attribute^ (' Die Period. effects of light and dnrkn^ss. 

Bewt'gungen," pp. 30-ofi) the But we are unable to follow his 

periodicity when prolonged for train of reasoning. There does 

a day or two in darkness, to not seem to be anymore reason foi 

2T 



108 MODIFIED CIRCUMNUTATION. CHAP. VII 

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 
glept in this country at very different hours, which 
is not the case. Moreover, it has been observed that 
sleeping plants in their native homes change their 
times of sleep with the changing seasons. * 

We may now turn to the systematic list (p. 320). 
This contains the names of all the sleeping plants 
known to us, though the list undoubtedly is very 
imperfect. It may be premised that, as a general 
rule, all the species in the same genus sleep in 
nearly the same manner. But there are some ex- 
ceptions ; in several large genera including many 
sleeping species (for instance, Oxalis), some do not 
sleep. One species of Melilotus sleeps like a Tri- 
folium, and therefore very differently from its con- 
geners ; so does one species of Cassia. In the genus 
Sida, the leaves either rise or fall at night ; and with 
Lupinus they sleep in three different methods. He- 
turning to the list, the first point which strikes us, is 
that there are many more genera amongst the Legu- 
minosae (and in almost every one of the Leguminous 
tribes) than in all the other families put together; 
and we are tempted to connect this fact with the great 



attributing such movements to this effict must be produced on the 
cause than, for instance, the in- seeds by the long-contiuutd culti- 
hcrited habit of winter and vutiim of the parent-plants under 
summer wheat to grow best at different climates, but no one, pro- 
different seasons ; for this habit bably would call this the " Nacli- 
is lost after a few years, like the wirkung " of the climates. 
movements of leaves in darkness * Pfeffer, ibid., p. 46. 
after a few days. No doubt some 



CHAP. VII. SUMMARY ON SLEEP OF LEAVES. 40S 

mobility of the stems and leaves in this family, as 
shown by the large number of climbing species which 
it contains. Next to the Leguminosae come the Mal- 
vaceae, together with some closely allied families. But 
by far the most important point in the list, is that we 
meet with sleeping plants in 28 families, in all ihe 
great divisions of the Phanerogamic series, and in one 
Cryptogam. Now, although it is probable that with 
the Leguminosae the tendency to sleep may have been 
inherited from one or a few progenitors and possibly 
so. in the cohorts of the Mai vales 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 CIECUMNUTATION. CHAP. VII. 

case of a sleeping leaf, we see that it makes a single 
ellipse in the twenty-four hours, which resembles one 
described by a non-sleeping leaf in every respect, except 
that it is much larger. In both cases the course pursued 
is often zigzag. As all non-sleeping leaves are 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 circuinnutation ; 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 
ill length, so as to bring the leaf back again into its 
diurnal position, when it would again circumnntat.e 



CHAP. VII SUMMARY ON SLEEP OF LEAVES. 411 

until the evening. If the reader will lock, for in- 
stance, at the diagram (Fig. 142, p. 351), representing 
the nyctitropic movements of the terminal leaflet of 
Trifolium subterraneum, remembering that the curved 
broken lines at the top ought to be prolonged much 
higher up, he will see that the great rise in the evening 
and the great fall in the morning together form a 
large ellipse like one of those described during the 
daytime, differing only in size. Or, he may look at 
the diagram (Fig. 103, p. 236) of the 3 ellipses 
described in the course of 6 h. 35 m. by a leaf of 
Lupinus speciosus, which is one of the species in this 
genus that does not sleep ; and he will see that by 
merely prolonging upwards the line which was already 
rising late in the evening, and bringing it down 
again next morning, the diagram would represent the 
movements of a sleeping plant. 

With those sleeping plants which describe several 
ellipses in the daytime, and which travel in a strongly 
zigzag line, often making in their course minute loops, 
triangles, &c., if as soon as one of the ellipses begins 
in the evening to be greatly increased in size, dots are 
made every 2 or 3 minutes and these are joined, the 
line then described is almost strictly rectilinear, in 
strong contrast with the lines made during the day- 
time. This was observed with Desmodium gyrans and 
Mimosa pudica. With this latter plant, moreover, the 
pinna3 converge in the evening by a steady move- 
ment, whereas during the day they are continually, 
converging and diverging to a slight extent. In all 
such cases it was scarcely possible to observe the 
difference in the movement during the day and even- 
ing, without being convinced that in the evening the 
plant saves the expenditure of force by not moving 
laterally, and that its whole energy is now expended 



112 MODIFIED CIRCUMNUTATION. CHAP. VII 

in gaining quickly its proper nocturnal position by 
a direct course. In several other cases, for instance, 
when a leaf after describing during the day one or 
more fairly regular ellipses, zigzags much in the 
evening, it appears as if energy was being expended, 
so that the great evening rise or fall might coin- 
cide with the period of the day proper for this 
movement. 

The most complex of all the movements performed 
by sleeping plants, is that when leaves or leaflets, 
after describing in the daytime several vertically 
directed ellipses, rotate greatly on their axes in the 
evening, by which twisting movement they occupy 
a wholly different position at night to what they do 
during the day. For instance, the terminal leaflets 
of Cassia not only move vertically downwards in the 
evening, but twist round, so that their lower surfaces 
face outwards'. Such movements are wholly, or almost 
wholly, confined to leaflets provided with a pulvinus. 
But this torsion is not a new kind of movement 
introduced solely for the purpose of sleep; for it 
has been shown that some leaflets whilst describing 
their ordinary ellipses during the daytime rotate 
slightly, causing their blades to face first to one side 
and then to another. Although we can see how the 
slight periodical movements of leaves in a vertical 
plane could be easily converted into the greater yet 
simple 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 



CHAP. VII. MODIFIED CIRCUMNUTATION. 413 

and cotyledons, is merely a modification of their ordi- 
nary circumnutating movement, regulated in its period 
and amplitude by the alternations of light and dark- 
ness. The object gained is the protection of the upper 
surfaces of the leaves from radiation at night, often 
combined with the mutual protection of the several 
parts by their close approximation. In such cases as 
those of the leaflets of Cassia of the terminal leaflets 
of Melilotus of all the leaflets of Arachis, Marsilea, 
&c. we have ordinary circumnutation modified to the 
extreme extent known to us in any of the several great 
classes of modified circumnutation. On this view of 
the origin of nyctitropism we can understand how it 
is that a few plants, widely distributed throughout the 
Vascular series, have been able to acquire the habit of 
placing the blades of their leaves vertically at night, 
that is, of sleeping, a fact otherwise inexplicable. 

The leaves of some plants move during the day in 
a manner, which has improperly been called diurnal 
sleep ; for when the sun shines brightly on them, they 
direct their edges towards it. To such cases we shall 
recur in the following chapter on Heliotropism. It 
has been shown that the leaflets of one form of 
Porlieria liygrometrica keep closed during the day, as 
long as the plant; is scantily supplied with water, in 
the same manner as when asleep ; and this apparently 
serves to check evaporation. There is only one other 
analogous case known to us, namely, that of certain 
Gramineae, which fold inwards the sides of their narrow 
leaves, when these are exposed to the sun and to a 
dry atmosphere, as described by Duval-Jouve.* We 
have also observed the same phenomenon in Elymus 
arenareus. 

* Annal. des Rn. Nat. (Bot.),' 1875, torn. i. pp. 32C -329. 



11-4 STKUCTUEE OF CHAP. VII 

There is another movement, which since the time 
of Linnaeus has generally been called sleep, namely, 
that of the petals of the many flowers which close at 
night. These increments have been ably investigated 
by Pfeffer, who has shown (as was first observed by 
Hofmeister) that they are caused or regulated more 
by temperature than by the alternations of light and 
darkness. Although they cannot fail to protect the 
organs of reproduction from radiation at night, this 
does not seem to be their chief function, but rather 
the protection of the organs from cold winds, and 
especially from rain, during the day. The latter 
seems probable, as Kerner * has shown that a widely 
different kind of movement, namely, the bending down 
of the upper part of the peduncle, serves in many 
cases the same end. The closure of the flowers will 
also exclude nocturnal insects which may be ill-adapted 
for their fertilisation, and the well-adapted kinds at 
periods when the temperature is not favourable for 
fertilisation. Whether these movements of the petals 
consist, as is probable, of modified circumnutation we 
do not know. 

Embryology of Leaves. A few facts have been in- 
cidentally given in this chapter on what may be called 
the embryology of leaves. With most plants the 
first leaf which is developed after the cotyledons, 
resembles closely the leaves produced by the mature 
plant, but this is not always the case. The first 
leaves produced by some species of Drosera, for instance 
by D. Capensis, differ widely in shape from those 
borne by the mature plant, and resemble closely the 
oaves of D. rotuxdifolia, as was shown to us by Prof. 
Williamson of Manchester. The first true leaf of 



Die Schutzmittel dee Pollens,' 1873, pp. 30-39. 



CHAP. VII. FIRST-FORMED LEAVES. 415 

the gorse, or Ulex, is not narrow and spinose like the 
older leaves. On the other hand, with many Legumi- 
nous plants, for instance, Cassia, Acacia 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 Tn '.folium Pannonicum the first true leaf on 
some seedlings was unifoliate, and on others completely 
trifoliate ; and between these two extreme states there 
were all sorts of gradations, some seedlings bearing 
a single leaflet more or less deeply notched on one 
or both sides, and some bearing a single additional 
and perfect lateral leaflet. Here, then, we have the 
rare opportunity of seeing a structure proper to a more 
advanced age, in the act of gradually encroaching on 
and replacing an earlier or embryolo'gical condition. 

The genus Melilotus is closely allied to Trifolium, and 
the first leaf bears only a single leaflet, which at night 
rotates on its axis so as to present one lateral edge to 
the zenith. Hence it sleeps like the terminal leaflet 
of a mature plant, as was observed in 15 species, and 
wholly unlike the corresponding leaflet of Trifolium, 
which simply bends upwards. It is therefore a curious 
fact that in one of these 15 species, viz., M. Taurica (and 
in a lesser degree in two others), leaves arising from 
young shoots, produced on plants which had been cut 
down and kept in pots during the winter in the green- 
house, slept like the leaves of a Trifolium, whilst the 
leaves on the fully-grown branches on these same 
plants afterwards slept normally like tl ose of a Meli- 
lotus. If young shoots rising from the ground may 
be considered as new individuals, partaking to a certain 
extent of the nature of seedlings, then the peculiar 
manner in which their leaves slept may be considered 



4:16 STKUCTUKE OF CJIAP. VII. 

as an embryological habit, probably the result of Meli- 
lotus being descended from some form which slept like 
a Trifbliuni. This view is partially supported by the 
leaves on old and young branches of another species, 
M. Messanensis (not included in the above 15 species), 
always sleeping like those of a Trifolium. 

The first true leaf of Mimosa albida consists of a 
simple petiole, often bearing three pairs of leaflets, all 
of which are of nearly equal size and of the same 
shape : the second leaf differs widely from the first, 
and resembles that on a mature plant (see Fig. 159, 
p. 379), for it consists of two -pinna3, 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 gryrans the two lateral leaflets are 
very much smaller than the corresponding leaflets in 
most of the species in this large genus ; they vary 
also in position and size ; one or both are sometimes 
absent ; and they do not sleep like the fully-developed 
leaflets. They may therefore be considered as almost 
rudimentary ; and in accordance with the general prin- 
ciples of embryology, they ought to be more constantly 
and fully developed on very young than on old plants. 
But this is not the case, for they were quite absent 
on some young seedlings, and did not appear until 
from 10 to 20 leaves had been formed. This fact 
leads to the suspicion that D. gyrans is descended 
through a unifoliate form (of which some exist) from 
a trifoliate species ; and that the little lateral leaflets 
reappear through reversion. However this may be, 



C/IAP. 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 givcg 
as probably the proximate cause of their extraordinary 
power of gyration. 



MODIFIED CIRCUMNUTATIOX. CHAP. VI IL 



CHAPTER VIII. 

BICDIFIEU ClBCUMNCTATION : MOVEMENTS EXCITED BY LIGHT. 

Distinction between heliotropisra 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, Tropseolum, and Cassia 
Apheliotropic movements of tendrils of Bignonia Of flower- 
peduncles of Cyclamen Burying of the pods Heliotropism 
and apheliotropisin modified forms of circumnutation Stops by 
which one movement is converted into the other Transversal- 
heliotropismus or diaheliotropism, influenced by epinasty, the 
weight of the part and apogeotropism Apogeotropism overcome 
during the middle of the day by diahrliotropism Efl'ects of the 
weight of the blades of cotyledons So-called diurnal sleep Chloro- 
phyll injured by intense light Movements to avoid intense light. 

SACHS first clearly pointed out the important dif- 
ference between the action of light in modifying the 
periodic movements of leaves, and in causing them to 
bend towards its source.* The latter, or heliotropic 
movements are determined by the direction of the light, 
whilst periodic movements are affected by changes in 
its intensity and not by its direction. The periodicity 
of the circumnutating movement often continues for 
some time in darkness, as we have seen in the last 
chapter ; whilst heliotropic bending ceases very quickly 
when the light fails. Nevertheless, plants which have 
ceased through long-continued darkness to move pe- 
riodically, if re-exposed to the light are still, according 
to Sachs, heliotropic. 

Apheliotropism, or, as usually designated, negative 

Physiologic Veg' (French Translation), 1868, pp. 42, 517, &c. 



CHAP. VIII. MOVEMENTS EXCITED BY LIGHT. 419 

heliotropism, implies that a plant, when unequally 
illuminated on the two sides, bends from the light, 
instead of, as in the last sub-class of cases, towards it; 
but apheliotropism is comparatively rare, at least in a 
wpll-marked degree. There is a third and largo sub- 
class of cases, namely, those of " Transversal-Helio- 
tropismus " of Frank, which we will here call diahelio- 
tropism. Parts of plants, under this influence, place 
themselves more or less transversely to the direction 
whence the light proceeds, and are thus fully illumi- 
nated. There is a fourth sub-class, as far as the final 
cause of the movement is concerned ; for the leaves of 
some plants when exposed to an intense and injurious 
amount of light direct themselves, by rising or sinking 
or twisting, so as to be less intensely illuminated. 
Such movements have sometimes been called diurnal 
sleep. If thought advisable, they might be called 
paraheliotropic, and this term would correspond with 
our other terms. 

It will be shown in the present chapter that all the 
movements included in these four sub-classes, con- 
sist of modified circumnutation. We do not pretend to 
say that if a part of a plant, whilst still growing, did not 
circumnutate though such a supposition is most im- 
probable it could not bend towards the light ; but, as 
a matter of fact, heliotropism seems always to consist 
of modified circumnutation. Any kind of movement 
in relation to light will obviously be much facilitated 
by each part circumnutating or bending successively 
in all directions, so that an already existing movement 
has only to be increased in some one direction, and to 
be lessened or stopped in the other directions, in order 
that it should become heliotropic, apheliotropic, &c., 
as the case may be. In the next chapter some obser- 
vations on the sensitiveness of plants to light, their 



420 MODIFIED CIRCUMNUTATION. CHAP VIU 

rate of bending towards it, and the accuracy with 
which they point towards its source, &c., will be 
given. Afterwards it will be shown and this seems 
to us a point of much interest that sensitiveness to 
light is sometimes confined to a small part . of the 
plant ; and that this part when stimulated by light, 
transmits an influence to distant parts, exciting them 
to bend. 

Heliotropism. When a plant which is strongly 
heliotropic (and species differ much in this respect) 
is exposed to a bright lateral light, it bends quickly 
towards it, and the course pursued by the stem is 
quite or nearly straight. But if the light is much 
dimmed, or occasionally interrupted, or admitted in 
only a slightly oblique direction, 

. 168* , -j . - 

the course pursued is more or less 
zigzag ; and as we have seen and 
shall again see, such zigzag move- 
ment results from the elongation or 
drawing out of the ellipses, loops, 
&c., which the plant would have de- 
scribed, if it had been illuminated 
from above. On several occasions 
'-is: circumnu- we were much struck with this fact, 
tation of hypocotyl, de- whilst observing the circumnuta- 

flected by the light . 

being slightly lateral, tiou of highly sensitive seedlings, 




traced on a horizontal w hich were unintentionally illu- 

glass from 8.30 A.M. to -,-,, 

5.30P.M. Direction of the mmated rather obliquely, or only 

at successive intervals of time - 

For instance ' tv y ung seediings f 

dots. Figure reduced to Beta vulgaris were placed in the middle 

one-third of the original of a room with north-east windows, and 

were kept covered up, except during 

each observation which lasted for only a minute or two ; but the 
result was that their hypocotyls bowed themselves to the side, 
wheuco some light occasionally entered, in lines which were 



CHAP. VIII. 



HELIOTKOPISM. 



421 



Fig. 169. 



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. If we look at these diagrams, 
and suppose that the taper had been held 
more on one side, and that the hypocotyls, 
still circumnutating, had bent themselves 
within the same time much more towards 
the light, long zigzag lines would ob- 
viously have been the result. 

Again, two seedlings of Solanum lyco~ 
p^rsicum were illuminated from above, 
but accidentally a little more light entered 
on one than on any other side, and their 
hypocotyls became slightly bowed towards 
the brighter side; they moved in a zigzag 
line and described in their course two little 
triangles, as seen in Fig. 37 (p. 50), and 
in another tracing not given. The sheath- 
like cotyledons of Zea mays behaved, under 
nearly similar circumstances, in a nearly 
similar manner, as described in our first 
chapter (p. 64), for they bowed themselves 
during the whole day towards one side, 
making, however, in their course some 
conspicuous flexures. Before we .knew 
how greatly ordinary circumnutation was 
modified by a lateral light, some seedling oats, with rather old 
and therefore not highly sensitive cotyledons, were placed in 
front of a north-east window, towards which they bent all day in 
a strongly zigzag course. On the following day they continued 
to bend in the same direction (Fig. 169), but zigzagged much 
less. The sky, however, became between 12.40 and 2.35 P.BL 




nutation of sheath-like 
cotyledon (1| inch in 
height) traced on hori- 
zontal glass from 8 A.M. 
to 10.25 P.M. Oct. 16th. 



122 MODIFIED CIRCUMNUTATION. CHAP. VIIL 

overcast with extraordinarily dark thunder-clouds, and it was 
interesting to note how plainly the cotyledons circumnutated 
during tliis interval. 

The foregoing observations are of some 
Fig. 170. value, from having bten made when we were 

^-2 not attending to heliotropism ; and they led 
]j"S us to experiment on several kinds of seed- 
j>.| Ii n 8' s j by exposing them to a dim lateral light, 
g 2 so as to observe the gradations between 
,| 2 ordinary circumnutation and heliotropism. 
Q.J& Seedlings in pots were placed in front of, 
and about a yard from, a north-east window ; 

"25 on each side and over the pots black boards 
were placed ; in the rear the pots were open 
*^|L, to the diffused light of the room, which 
Sf r ^ had a second north-east and a north-west 
j5 ^ window. By hanging up one or more blinds 
a o before the window where the seedlings stood, 
^2 it was easy to dim the light, so that very 
*o little more entered on this side than on the 
*= a opposite one, which received the diffused 
t? light of the room. Late in the evening the 
~^n blinds were successively removed, and as the 

t^ plants had been subjected during the day to 
g a very obscure light, they continued to bend 
j?^ towards the window later in the evening than 
*S 3 woiild otherwise have occurred. Most of the 
g^ seedlings were selected because they were 
known to be highly sensitive to light, and 
I some because they were but little sensitive, 
* j or had become so from having grown old. 
'I'fl'S T lie movements were traced in the usual 
-|^-^ manner on a horizontal glass cover; a fine 
^ | .5 glass filament with little triangles of paper 
M **/ having been cemented in an upright position 
3 to the hypocotyls. Whenever the stem or 
"2 hypocotyl became much bowed towards the 
3 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. 
Apio* graveolens.The hypocotyl bends in a few hours vectan- 



CUAP. VIII. ITELIOTROPISM. 423 

gularly towards a bright lateral light. In order to ascertain 
Low straight a course it would pursue when fairly well illumi- 
nated on one side, seedlings were first placed before a south-west 
window on a cloudy and rainy morning ; and the movement of 
two hypocotyls were traced for 3 h., during which time they 
became greatly bowed towards the light. One of these tracings 
is given on p. 422 (Fig. 170), and the course may be seen to be 
almost straight. But the amount of light on this occasion was 
superfluous, for two seedlings were placed before a north-east 
window, protected by an ordinary linen and two muslin blinds, 
yet their hypocotyls moved towards this rather dim light in 
only slightly zigzag lines; but after 4 P.M., as the light waned, 
the lines became distinctly zigzag. One of these seedlings, 
moreover, described in the afternoon an ellipse of considerable 
size, with its longer axis directed towards the window. 

We now determined that the light should be made dim 
enough, so we began by exposing several seedlings before a 
north-east window, protected by one linen blind, three muslin 
blinds, and a towel. But so little light entered that a pencil 
cast no perceptible shadow on a white card, and the hypocotyls 
did not bend at all towards the window. During this time, 
from 8.15 to 10.50 A.M., the hypocotyls zigzagged or circum- 
uutated near the same spot, as may be seen at A, in Fig. 171. 
The towel, therefore, was removed at 10.50 A.M., and replaced 
by two muslin blinds, and now the light passed through 
one ordinary linen and four muslin blinds. When a pencil 
was held upright on a card close to the seedlings, it cast a 
shadow (pointing from the window; which could only just 
be distinguished. Yet this very slight excess of light on 
one side sufficed to cause the hypocotyls of all the seedlings 
immediately to begin bending in zigzag lines towards the 
window. The course of one is shown at A (Fig. 171) : after 
moving towards the window from 10.50 A.M. to 12.48 P.M. it 
bent from the window, and then returned in a nearly parallel 
line; that is, it almost completed between 12.48 and 2 P.M. 
a narrow ellipse. Late in the evening, as the light waned, 
the hypocotyl ceased to bend towards the window, and 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 
28 



424 



MODIFIED CIIiCUMNUTATION. CHAP. VIH 



similar to the previous one. In these two cases there could be 
no doubt that the ordinary circumnutating movement of the 
hypocotyl was modified and rendered heliotropic. 

Fig. 171. 




Apios tjrnreofens : heliotropic movement and circumnutation of the hypo- 
cotyls of two seedlings towards a dim lateral light, traced on a horizontal 
glass during the day. The broken lines show their return nocturnal 
courses. Height of hypocotyl of A - 5, and of B '55 inch. Figure reduced 
to one-half of original scale. 



_,_w derarea. The hypocotyl of the cabbage, when not 

disturbed by a lateral light, circumnutates in a complicated 



CHAP. VIII. 



HKLIOTBOI'ISM. 



manner over nearly the same space, and a figure formerly given 
is here reproduced (Fig. 172). If the hypocotyl is exposed to 
a moderately strong lateral light it moves quickly towards this 
side, travelling in a straight, or nearly straight, line. But when 
the lateral light is very dim its course is extremely tortuous, and 
evidently consists of modified circumnutation. Seedlings were 
placed before a north-east window, protected by a linen and 
muslin blind and by a towel. The sky was cloudy, and when- 
ever the clouds grew a little lighter an additional muslin blind 
was temporarily suspended. The light from the window was 




Drusicn olcracea ordinary ciroumnntntino; movement of tha hypceotyl of 

a seedling jilunt. 

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 
th<> case, as was shown by a very faint shadow cast by a pencil 
on a card. Nevertheless, this extremely small excess of light 
on one side caused the hypocotyls, which in the morning had 
stood upright, to bend at right angles towards the window, 
so that in the evening (after 4.23 V.M.) their course had to l>e 
traced on a vertical glass parallel to the window. It should bo 
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. CHAP. VIIL 



blinds being left suspended. In Fig. 173 the course pursued, 
between 8.9 A.M. and 7.10 P.M., by one ci' the kypocotyls thua 




31: 



^8 25' 

Brasska oleracea : heliotropic movement and circumnutation of a byporotyl 
towards a very dim lateral light, traced during 11 hours, on a horizontal 
glnss in the morning, and on a vertical glass in the evening. Figure 



glnss in the morning, 

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, 



CHAP. VIII. 



HELIOTEOPISM. 



127 



8JJP./M 



no doubt, was due to its then circumnutating in this direction, 
Similar cases were repeatedly observed, and a dim light rarely 
or never produced any effect until from a quarter to three- 
quarters of an hour had elapsed. After 5.15 P.M., by which 
time the light had become 

obscure, the hypocotyl Fl - 174 ' o 

began to circumnutate 
about the same spot. The 
contrast between the two 
figures (172 and 173) 
would have been more 
striking, if they had been 
originally drawn on the 
same scale, and had been 
equally reduced. But the 
movements shown in Fig. 
1 72 were at first more mag- 
nified, and have been re- 
duced to only one-half of 
the original scale; whereas 
those in Fig. 173 were at 
first less magnified, and 
have been reduced to a 
one-third scale. A tracing 
made at the same time 
with the last of the 
movements of a second 
hypocotyl, presented a 
Closely analogous appear- 
ance ; but it did not bend 
quite so much towards the 
light, and it circumnu- 
tated rather more plainly 
Phalaris Canariensis. 
The sheath-like cotyledons 
of this monocotyledonous 
plant were selected for 
trial, because they are very sensitive to light and circumnutate 
well, as formerly shown (see Fig. 49, p. 63). Although we felt 
no doubt about the result, some seedlings were first placed 
before a south-west window on a moderately bright morning, and 
the movements of one were traced. As is so common, it moved 




'anariensis : heliotropic movement 
and cimfmnutation of a rather old coty- 
ledon, towards a dull lateral light, traced 
on a horizontal glass from 8.15 A.M. Sept. 
16th to 7.45 A.M. 17th. Figure reduced 
to one-third of original scale. 



i-28 MODIFIED CIECUMNUTATION. CHAI-. VIT1 

for the first 45 m. in a zigzag line; it then felt the full influence 
of the light, and travelled towards it for the next '2 h. 30 m. in an 
almost straight line. The tracing has not been given, as it was 
almost identical with that of Apios under similar circum- 
stances (Fig. 170). By noon it had bowed itself to its full 
extent ; it then circumnutated about the same spot and described 
two ellipses; by 5 P.M. it had retreated considerably from tlio 
light, through the action of apogeotropism. After some pre- 
liminary trials for ascertaining the right degree of obscurity, 
gome 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 tent towards the light. From 
11 A.M., when the sky became rather duller, until 6.30 P.M., the- 
zigzagging was conspicuous, and evidently consisted of drawn- 
out ellipses. After 6.30 P.M. and during the night, it retreated 
in a crooked line from the window. Another and younger seed- 
ling moved during the same time much more quickly and to a 
much greater distance, in an only slightly zigzag line towards 
the light ; by 11 A.M. it was bent almost rectangularly in this 
direction, and now circumnutated about the same place. 

Tropceolum majus. Some very young seedlings, hearing 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 junction 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. 



CHAI-. 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 9.13 A.M. two additional muslin 
blinds were suspended, so that the seedlings received very little 
more light from the window than from the interior of the room 
The sky varied in brightness, and the seedlings occasionally 

Fig. 175. 




Tropccolum mnji.s: heliotropic movement and cirrumnutation of the.cpicotyt 
of a young seedling towards a dull lateral lisjht, -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 
ono of the blinds was temporarily removed. In the evening 
tho blinds were taken away, one by one. The course pursued 
by an epicotyl under these circumstances is shown in Fig. 175. 
During the whole day, until 6.45 p M., it plainly bowed itself 
towards the light ; and the tip moved over a considerable space. 
After 6.45 P.M. it moved backwards, or from the window, tilJ 



MODIFIED CIRCUMNUTATION. CHAP. VIII 



10.40 P.M., when the last dot was made. Here, then, we have 
u distinct heliotropic movement, effected by means of six 
elongated figures (which if dots had been made every few 
minutes would have been more or less elliptic) directed towards 
the light, with the apex of each suc- 
cessive ellipse nearer to the window 
than the previous one. Now, if the 
light had been only a little brighter, 
tlie epieotyl would have bowed itself 
more to the light, as we may safely 
conclude from the previous trials ; 
there would also have been less 
lateral movement, and the ellipses or 
other figures would have been drawn 
out into a strongly marked zigzag 
line, with probably one or two small 
loops still formed. If the light had 
been much brighter, we should have 
had a slightly zigzag line, or one 
quite straight, for there would have 
been more movement in the direc- 
tion of the light, and much less from 
side to side. 

Sachs states that the older inter- 
nodes of this Tropaeolum are aphe- 
liotopic; we therefore placed a 
plant, lit inches high, in a box, 
blackened within, but open on one 
side in front of a north-east window 
without any blind. A filament was 
fixed to the third internode from 
the summit on one plant, and to 
the fcrarth internode of another. 
These internodes were either not 
old enough, or the light was not suf- 
ficiently bright, to induce aphelio- 
tropism, for both plants ben 1 ; 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 ?oe 
that it either circumnutated on a small scale, or travelled in a 
zigzag line towards the light. We have thought this case of 
feeble heliotropism in one of the older internodes of a plant, 




Tropceolum majus : heliotropic 
movement and circumnuta- 
tion of an old internode to- 
wards a lateral light, traced 
on a horizontal glass from 8 
A.M. Nov. 2nd to 10 20 A.M. 
Nor. 4th. Broken lines show 
the nocturnal course. 



CHAP. V1IL 



HELIOTEOPISM. 



which, whilst young, is so extremely sensitive to light, woitb 
giving. 

Cassia tora. The cotyledons of this plant are extremely 
sensitive to light, w'hilst 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 
olind was suspended. After 
4 P.M. one blind and then the 
other was removed. The 
seedlings were protected on 
each side and above, but were 
open to the diffused light 
of the room in the rear. Up- 
right filaments were fixed to 
the hypocotyls of two seed- 
lings, which stood vertically 
in the morning. The accom- 
panying figure (Fig. 177) 
shows the course pursued by 
one of them during two days ; 
but it should be particularly 
noticed that during the 
second day the seedlings were 8hm. 
kept in darkness, and they 
then circunmutated round 
nearly the same small space. 
On the first day (Oct. 7th) 
the hypocotyl moved from 
8 A.M. to 12.23 P.M., toward 
the light in a zigzag line, then turned abruptly to the left 
and afterwards described a small ellipse. Another irregular 




icvement ana 
circumntitation of a hypocotyl (1J 
inch in height) traced on a horizontal 
glass from 8 A.M. to 10.10 P.M. Oct. 
7th. Also its circumnutation in 
darkness from 7 A.M. Oct. 8th to 7.45 
A.M Oct. 9th. 



MODIFIED CIRCUMXUTATLON. CIIAP. ViJi 



ellipse was completed between 3 P.M. and about 5.30 P.M., 
the hypocotyl still bending towards the light. The hypocotyi 



fig. 171 




Bicjnonia caprcolata : aphe- 
liotropic movement of a 
tendril, traced on a hori- 



was straight and upright in the morn- 
ing, but by 6 P.M'. its upper half was 
bowed towards the light, so that the 
chord of the arc thus formed stood at 
an angle of 20 with the perpendicular. 
After 6 P.M. its course was reversed 
through the action of apogeotropism, 
. and it continued to bend from the 
window during the night, as shown by 
the broken line, On the next day it 
was kept in the dark (excepting when 
each observation was made by the aid 
of a taper), and the course followed 
from 7 A.M. on the 8th to 7.45 A.M. on 
the 9th is here likewise shown. The 
difference between the two parts of the 
figure (177), namely, that described 
during the daytime on the 7th, when 
exposed to a rather dim lateral light, 
and that on the 8th in darkness, is 
striking. The difference consists in the 
lines during the first day having been 
drawn out in the direction of the light. 
The movements of the other seedling, 
traced under the same circumstances, 
were closely sirnilar. 

Aplteliotropism. We succeeded in 
observing only two cases of aphelio- 
tropism, for these are somewhat rare ; 
and the movements are generally so 
slow that they would have been very 
troublesome to trace. 

JBignonia capreolata. No organ of 



zontal glass from 6.45 any plant, as far as we have seen, bends 
uly 19th to 10 A.M. 



A.M. J 

20th. Movements as 

originally traced, little 
magnified, here reduced 



away so quickly from the light as do 
the tendrils of this Bignonia. They 
are also remarkable from drcum- 



to two-thirds of the nutating much less regularly than 



original scale. 



most other tendrils, often remaining 



stationary ; they depend on apheliotropism for coining into 



CHAP. Vlil. APHELIOTROPI^r. > 43'J 

contact with the trunks of trees." The stein of a young plant 
was tied to a stick at the base of a pair of line tendrils, which 
projected almost vertically upwards; and it was placed in 
front of a north-east window, being protected on all other sides 
from the light. The first dot was made at 6.45 A.M., and by 
7.35 A.M. both tendrils felt the full influence of the light, for 
they moved straight away from it until 9.20 A.M., when they 
circumnutated for a time, still moving, but only a little, from 
the light (see Fig. 178 of the left-hand tendril). After 3 P.M. 
they again moved rapidly away from the light in zigzag lines. 
By a late hour in the evening both had moved so far, that 
they pointed in a direct line from the light. During the night 
they returned a little in a nearly opposite direction. On the 
following morning they again moved from the light and con- 
verged, so that by the evening they had become interlocked, 
still pointing from the light. The right-hand tendril, whilst 
converging, zigzagged much more than the one figured. Both 
tracings showed that the apheliotropic movement was a modi- 
lied form of circumnutation. 

Cyclamen Persicum, Whilst this plant is in flower the peduncles 
stand upright, but their uppermost part is hooked so that the 
flower itself hangs downwards. As soon as the pods begin to 
swell, the peduncles increase much in length and slowly curve 
downwards, but the short, upper, hooked part straightens itself. 
Ultimately the pods reach the ground, and if this is covered 
with moss or dead leaves, they bury themselves. We have often 
seen saucer-like depressions formed by the pods in damp sand 
or sawdxist; and one pod ('3 of inch in diameter) buried itself 
in sawdust for three-quarters of its lecgth.f We shall htive 
occasion hereafter to consider the object gained by this burying 
process. The peduncles can change the direction of their cur- 
vature, for if a pot, with plants having their peduncles already 
bowed downwards, be placed horizontally, they slowly bend 
at right angles to their former direction towards the centre e 
the earth. We therefore at first attributed the movement to 
geotropism ; but a pot which had lain horizontally with the pods 



* 'The Movements and Habits tame Garden,' Canto., iii. p. 126), 

of Climbing Plants,' 1875, p. 97. the pods forcibly penetrate the 

t The peduncles of several earth. See also Grenier mid 

other species of Cyclamen twist Godron, 'Fl. ire de France,' toui. ii 

themselves into a spire, and ac- p. 4fi9. 
curding to Erasmus Darwin (/ Bo- 



434 MODIFIED CIRCUMNUTATION. CHAP. VIII. 

all pointing to the ground, was reversed, being still kept hori- 
zontal, so that the pods now pointed directly upwards ; it was 
then placed in a dark cupboard, but the pods still pointed up- 
wards after four days and nights. The pot, in the same position, 
was next brought back into the light, and after two days there 
was some bending downwards of the peduncles, and on the fourth 
day two of them pointed to the centre of the earth, as did the 
others after an additional day or two. Another plant, in a pot 
which had always stood upright, was left in the dark cupboard 
for six days ; it bore 3 peduncles, and only one became within this 

Fig. 179. 




Cyclamen Pi-rsfritm: downward apheliotropic movement of a flower-peduncle, 
.erctttly magnified (about 47 times ?), traced on a horizontal glass from 
1 P.M. Feb. 18th to 8 A.M. 21st. 

time at all bowed downwards, and that doubtfully. The weight, 
therefore, of the pods is not the cause of the bending down. 
This pot was then brought back into the light, and after three 
days the peduncles were considerably bowed downwards. We 
are thus led to infer that the downward curvature is due to 
apheliotropism ; though more trials ought to have been made. 

In order to observe the nature of this movement, a peduncle 
bearing a large pod which had reached and rested on the 
ground, was lifted a little up and secured to a stick. A filament 
was fixed across the pod with a mark beneath, and its move- 



CIIAP. VIII. APHELIOTROPISM. 436 

naent, greatly magnified, was traced on a horizontal glass during 
67 h. The plant was illuminated during the day from above. A 
copy of the tracing is given on p. 434 (Fig. 179) ; and there can 
be no doubt that the descending movement is one of modified 
circumnutation, but on an extremely small scale. The observa- 
tion was repeated on another pod, which, had partially buried 
itself in sawdust, and which was lifted up a quarter of an inch 
above the surface ; it described three very small circles in 24 h. 
Considering the great length and thinness of the peduncles 
and the lightness of the pods, we may conclude that they 
would not be able to excavate saucer-like depressions in sand 
or sawdust, or bury themselves in rnoss, &c., unless they were 
aided by their continued rocking or circumnutating move- 
ment. 

Relation "uetiveen Circumnutation and Heliotropism. 
Any one who will look at the foregoing diagrams, 
showing the movements of the stems of various plants 
towards a lateral and more or less dimmed light, will 
be forced to admit that ordinary circumnutation and 
heliotropisrn 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 Troprcoliun 
(Fig. 175) the stem or epicotyl obviously ciicumnu- 
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 a dim lateral light and 
to darkness, is very instructive ; as is that between 
the ordinary circumnutating movement of a seedling 
Brassica (Figs. 172, 173), or that of Phalaris (Figs. 
49, 174), and their heliotropic movement towards a 
window protected by blinds. In both these cases 



13tJ RELATION BETWEEN CHAP. VIII 

and in many others, it was interesting to notice Low 
gradually the sterns began to circurnnutate 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, hcliotropio 
course. 

A plant, when exposed to a lateral light, though 
this may be bright, commonly moves at first in a 
zigzag line, or even directly from the light ; and 
this no doubt is due to its circumnutating at the 
time in a direction either opposite to the source of 
the light, or more or less transversely to it. As soon, 
however, as the direction of the circumnutating move- 
ment nearly coincides with that of the entering light, 
the plant bends in a straight course towards the light, 
if this is bright. The course appears to be rendered 
more and more rapid and rectilinear, in accordance witli 
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 
t3 its direction, than with one in its own direction.* 



* In his paper, ' Ucber ortlio- tl cile' (' Avbeiten des Bot. {11*1. 
trope uud plugiotrope Ptlauzcn- in Wurzburg,' Band ii. Heft iii 



CHAP. VIII. CIRCUMNUTATION AND HELIOTROPIHM. -131 

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 circumimtating movement rectilinear and rapid in 
one direction alone, namely, towards the light. 

The common view seems to be that heliotropisrn is 
a quite distinct kind of movement from circuinnuta- 
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 circurnnutate, 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 arc therefore fully justified, as it seems to us, in 
beli(3ving that whenever light enters laterally, it is the 



1879), Ssichs hns discusser! the the organs of plants stand with 

manner in which jeotropisin find respect to the direction of th 

heliotropism are affected by dif- incident force, 
ierences in Ihe angles ut \\Lich 



i68 MODIFIED CIRCUMNUTATION. CHAP. VIII 

movement of circumrmtation which gives rise to, or ia 
converted into, heliotropism and apheliotropism. On 
this view we need not assume against all analogy that 
a lateral light entirely stops circumnutation ; it merely 
excites the plant to modify its movement for a time 
in a beneficial manner. The existence of every pos- 
sible gradation, between a straight course towards a 
lateral light and a course consisting of a series of loops 
or ellipses, becomes perfectly intelligible. Finally, 
the conversion of circumnutation into heliotropism or 
apheliotropism, is closely analogous to what takes place 
with sleeping plants, which during the daytime 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, corning 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 si des of the plant. 

Transvcrsal-lieliotropism.us (of Frank *) or Dialielio- 
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 natiirlic-ho Wugerechte Frige iiber Transver.-al Geo-uud 

QiehtBDg von Pnanzetitheileti,' HvUntropiaraos," ' Bot. Zoitung, 

18''0 See also s'nue interesting 1 187i>, p. 17 et seq. 
at tides by the same author, " Zur 



CHAP. VIII. DIAHELIOTROPISM. 439 

of diaheliotropism than that offered by many seed- 
lings, the cotyledons of which are extended hori- 
zontally. When they first burst from their seed-coats 
they are in contact and stand in various positions, 
often vertically upwards ; they soon diverge, and this 
is effected by epinasty, which, as we have seen, is a 
modified form of circumnutation. After they have 
diverged to their full extent, they retain nearly tho 
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 



110 MODIFIED CIBCUMNUTATION. CHAP, VIE 

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 bo 
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 



* 'ArbeiUm des Bot. Institute in Wurzburg.' Heft. ii. 1872, pp. 
JJ23-277. 



CHAP. VIII. DIAHELIOTBOI'ISM. 4-41 

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 light bends in an opposed direc- 
tion or downwards, and both place themselves as nearly 
as possible at right angles to the light. The move- 
ment, therefore, deserves a distinct name. As coty- 
ledons and leaves are continually oscillating up and 
down, and yet retain all day long their proper position 
with their upper surfaces directed transversely to the 
light, and if displaced reassume this position, dia- 
heliotropism must be considered as a modified form of 
circumnutation. This was often evident when the 
movements of cotyledons standing in front of a window 
were traced. We see something analogous in the case 
of sleeping leaves or cotyledons, which after oscillating 
up and down during the whole day, rise into a vertical 
position late in the evening, and on the following 
morning sink 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 
Tnovenients 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 subject. 



142 MODIFIED CmCUMNUTATION. CH,H>. VTIt 

than can the nyctitropic movements of cotyledons 
and leaves. In the latter case they place themselves 
so that their upper surfaces may radiate at night 
as little as possible into open space, with the upper 
surfaces of the opposite leaflets often in contact. These 
movements, which are sometimes extremely complex, 
are regulated, though not directly caused, by the alter- 
nations of light and darkness. In the case of diahelio- 
tropism, cotyledons and leaves place themselves so 
that their upper surfaces may be exposed to the light, 
and this movement is regulated, though not directly 
caused, by the direction whence the light proceeds. In 
both cases the movement consists of circumnutation 
modified by innate or constitutional causes, in the 
same manner as with climbing plants, the circumnu- 
tation of which is increased in amplitude and rendered 
more circular, or again with very young cotyledons 
and leaves which are thus brought down into a hori- 
zontal position by epinasty. 

We have hitherto referred only to those leaves and 
cotyledons which occupy a permanently horizontal 
position ; but many stand more or less obliquely, arid 
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 



Ciur. VIII. DIAHELIOTKOPISM. 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 light and darkness. De Vries states in 
the paper before referred to, that most petioles and 
midribs are apogeotropic ;* and apogeotropism would 
account for the above rising movement, which is com- 
mon to so many widely distinct species, if we suppose it 
to be conquered by diaheliotropism during the middle 
of the day, as long as it is of importance to the plant 
that its cotyledons and leaves should be fully exposed 
to the light. The exact hour in the afternoon at which 
they begin to bend slightly upwards, and the extent of 
the movement, will depend on their degree of sen- 
sitiveness to gravitation and on their power of resist- 
ing its action during the middle of the day, as well as 
on the amplitude of their ordinary circumnutating 
movements ; and as these qualities differ much in dif- 
ferent species, we might expect that the hour in the 
afternoon at which they begin to rise would differ 
much in different species, as is the case. Some other 
agency, however, besides apogeotropism, must come 
into play, either directly or indirectly,- in this upward 
movement. Thus a young bean (Vicia faba), growing 
in a small pot, was placed in front of a window in a 
klinostat ; and at night the leaves rose a little, although 



* According to Frank ('Die organs have been long kept in tho 
nat. Wagerechte Richtung YOU d>irk, the amount of water and of 
Ptlanzentheilen.' 1870, p. 46) the mineral matter which they con- 
root- leaves of many plants, kept tain is so much altered, and their 
in darkness, rise up and even be- regular growth is so much dis- 
come vertical ; and so it is in some turbed, thut it is perhaps rash to 
cases with shoots. (See Kauwen- infer from their movements what 
hoff, 'Archives Nc-erlandaises,' would occur under normal con- 
torn, xii. p. 32.) These movements clitions. (Sec Godlewski, ' Bot 
indicate apogeotropism ; but when Zeitung,' Feu. 14th, 1879.) 



144 MODIFIED CIECUMNUTATION. CHAP. VIII. 

the action of apogeotropisrn was ijuite eliminated. 
Nevertheless, they did not rise nearly so much at 
night, as when subjected to apogeotropism. Is it 
not possible, or even probable, that leaves and coty- 
ledons, which have moved upwards in the evening 
through the action of apogeotropism during countless 
generations, may inherit a tendency to this movement ? 
We have seen that the hypocotyls of several Legu- 
minous plants have from a remote period inherited a 
tendency to arch themselves ; and we know that the 
sleep-movements of leaves are to a certain extent 
inherited, independently of the alternations of light 
and darkness. 

In our observations on the circumnutation of those 
cotyledons and leaves which do not sleep at night, we 
met with hardly any distinct cases of their sinking 
a little in the evening, and rising again in the morn- 
ing, that is, of movements the reverse of those just 
discussed. We have no doubt that such cases occur, 
inasmuch as the leaves of many plants sleep by 
sinking vertically downwards. How to account for the 
few cases which were observed must be left doubtful. 
The young leaves of Cannabis sativa sink at night 
between 30 and 40 beneath the horizon ; and Kraus 
attributes this to epinasty in conjunction with the 
absorption of water. Whenever epinastic growth is 
vigorous, it might conquer diaheliotropism in the 
evening, at which time it would be of no import- 
ance to the plant to keep its leaves horizontal. 
The cotyledons of Anoda Wriglitii, of one variety of 
Gossypium, 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 large and heavy sink so much that 
they come under our definition of sleep. In the case of 



CHAP. VIII. TAEAHELIOTKOriSM. 445 

the Anoda and of some species of Ipomcea, it was proved 
that the downward movement did not depend on the 
weight of the cotyledons ; but from the fact of the move- 
ment being so much more strongly pronounced after 
the cotyledons have grown large and heavy, we may 
suspect that their weight aboriginally played some part 
in determining that the modification of the circum- 
nutating movement should be in a downward direction. 
The so-called Diurnal Sleep of Leaves, or Paralielio- 
tropism. This is another class of movements, dependent 
on the action of light, which supports to some extent 
the belief that the movements above described are 
only indirectly due to its action. We refer to the 
movements of leaves and cotyledons which when 
moderately illuminated are 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 Bobinia, they rise up and present their edges 
1o 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 Amphicarp&a monoica turned their edges to the sun; 
and an analogous movement of the little almost rudimentary 
basal leaflets of Mimosa afbida was on one occasion so rapid that 
it could be distinctly seen through a lens. The elongated, uiii- 
foiiate, first leaves of Phaseolus Boxluryhii stood at 7 A.M. at 20 
above the horizon, and no doubt they afterwards sank a littlo 
lower. At noon, after having been exposed for about 2h. to 

* Pfoffer ft-ives tho names and dates of several ancient writers iu his 
'Die r-criodischtn Bewegungen,' 1875, p. (52. 



446 MODIFIED CIKCUMNUTATIOX. CHAP. VIII. 

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 30m. they had fallen 4.0, fee 
they now stood at only 16 above the horizon. Some young 
plants of Phaseotus flernandesii 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 Is h. ; and now all the leaves and leaflets had re- 
assumed their ordinary sub-horizontal positions. The copper- 
coloured cotyledons of some seedlings of Cassia mimosoides were 
horizontal in the morning, but after the sun had shone on 
them, each had risen 45^ above the horizon. The movement 
in these several cases must not be confounded with the sudden 
closing of the leaflets of Mimosa pudica, which may sometimes 
be noticed when a plant which has been kept in an obscure 
place is suddenly exposed to the sun ; for in this case the light 
seems to act, as if it were a touch. 

From Prof. Wiesner's interesting observations, it is probable 
that the above movements have been acquired for a special 
purpose. The chlorophyll in leaves is often injured by too 
intense a light, and Prof. "Wiesner* believes that it is protected 
by the most diversified means, such as the presence of hairs, 
colouring matter, &c., and amongst other means by the leaves 
presenting their edges to the sun, so that the blades then 
receive much less light. He experimented on the young leaflets 
of Eobinia, by fixing them in such a position that they could 
not escape being intensely illuminated, whilst others were 
allowed to place themselves obliquely; and the former began to 
suffer from the light in the course of two days. 

In the cases above given, the leaflets move either up war da 



* ' Die Naturlichen Einrich- the action of concentrated liyht 

tun gen zum Schutze des Cliloro- from the sun, in the presence of 

phUls,' &c., 1876. Pringsheim oxygen. Seo, also, Stahl on the 

lias recently observed under the protection of chlorophyll from 

microscope the destruction of intense light, in *Bot. Zeitung, 

chlorophyll in a lew minutes by 18SO. 



CHAP. VIII. PARAHELIOTROriSM. 447 

or twist laterally, so as to place their edges in the direction of the 
sun's light; but Cohn long ago observed that the leaflets of 
Oxalis bend downwards when fully exposed to the sun. We 
witnessed a striking instance of this movement in the very 
large leaflets of O. Ortegesii. A similar movement may fre- 
quently be observed with the leaflets of Averrhoa UlimU (a 
member of the Oxalidse) ; and a leaf is here represented (Fig. 
180) on which the sun had shone. A diagram (Fig. 134) was 
given in the last chapter, representing the oscillations by which 
a leaflet rapidly descended under these circumstances ; and the 
movement may be ueeu closely to resemble that (Fig. 133) bv 




Avcrrhoa bilhnhi : leaf with ler.flets depressed after exposure to simshinp- 
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 acetost-Ua may be daily exposed to the sun during many 
weeks, and they do not suffer if they are allowed to depress 
themselves ; but if this be prevented, they lose their colour and 
wither in two or three days. Yet the duration of a leaf is about 
two months, when subjected only to diffused light; and in this 
case the leaflets never sink downwards during the day 

As the upward movements of the leaflets of 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 pm-pose 
of avoiding too intense an illumination. As it would 
Lave been very troublesome in all the above cases to 



143 MODIFIED CIRCUMNUTATION. CHAP. Via 

have watched for a fitting opportunity and to have 
traced the movement of the leaves whilst they were 
fully exposed to the sunshine, we did not ascertain 
whether paraheliotropism always consisted of modi- 
fied circumnutation ; but this certainly was the case 
with the Averrhoa, and probably with the other species, 
63 their leaves were continually circumrwtatiTig 1 . 



CHAT IX. SENSITIVENESS TO LIGUT. 



CHAPTEE IX. 

SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS. 

Uses of heliotropism Insectivorous and climbing plants not heliotropic 
Same organ heliotropic at one age and not at another Extra- 
ordinary sensitiveness of some plants to light The effects of light do 
not correspond with its intensity Effects of previous illumination 
Time required for the action of light After-effects of light 
Apogeotropism acts as soon as light fails Accuracy with which 
plants bend to the light This dependent on the illumination of 
one whole side of the part Localised sensitiveness to light and its 
transmitted effects Cotyledons of I halaris, manner of bending 
Results of the exclusion of light from their tips Effects trans- 
mitted beneatli 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 
iipper part Similar results with the hypocotyls of Brassica and 
Beta Radicles of Siiwpis npheliotropic, due to the sensitiveness of 
their tips Concluding remarks and summary of chapter Means 
by which circumnutation has been converted into heliotropism or 
apheliotropism. 

No one can look at the plants growing on a bank or 
on the borders of a thick wood, and -doubt that the 
young sterns 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 Gramineae, 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 viev 



450 SENSITIVENESS TO LIGHT. CHAP. IX. 

is strengthened by the fact that with Phalaris and 
A vena the first true leaf, which is bright green and no 
doubt decomposes carbonic acid, exhibits hardly a 
trace of heliotropism. The heliotropic movements oi 
many other seedlings probably aid them in liko 
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 heliotropism. Nor 
could we see any in Dionaea, though the plants were 
not so carefully observed. Sir J. Hooker exposed the 
pitchers of Sarracenia for some time to a lateral light, 
but they did not bend towards it.* 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 hero 
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 an? 
apheliotropic, for instance those of Bignonia capreulata 



* According to F. Kurtz (' Ver- tonia Calif ornif-a are strongly 

haiulL di-s Bot. Vereins dcr Pro- apheliotropic. We failed to detect 

vinz Brandenburg/ Bd. xx. 1)S78) this movement in a plant which 

the leaves or pitchers of Darliny- we possessed for a short time. 



CHAP. IX. 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 apheliot.ropic, and they thus find 
a support. The leaves, on the other hand, of most 
climbing plants are heliotropic ; but we could detect 
no signs of any such movement in those of Mutisia 
clematis. 

As heliotropism is so widely prevalent, and as 
twining plants are distributed throughout the whole 
vascular series, the apparent absence of any tendency 
in their stems to bend towards the light, seemed to 
us so remarkable a fact as to deserve further 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- 
cumnutatirig 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 Ipomcea 
cverulea and four of I. purpurea, growing in separate 
pots, were placed on a bright day before a north-east 
window in a room otherwise darkened, with the tips 
of their revolving stems fronting the window. When 
the tip of each plant pointed directly from the window, 
and when again towards it, the times were recorded. 
This was continued from 6.45 A.M. till a little after 
'2 r.M. on June 17th. After a few observations we 
concluded that we could safely estimate the time 



* Some erroneous statements number of observations, for \ve did 
are unfortunately given on tbis not then know at how unrqual 
subject, in 'The Movements and a rate the steins and tendrils of 
Habits of Climbing Plants,' 1875, climbing plants rometimes travel 
pp. 28, 32, 40, and 53. Conclusions in different parts of the same re- 
were drawn from an insufficient volution. 



152 SENSITIVENESS TO LIGHT. CHAP. IX 

taken by each semicircle, within a limit of error of at 
most 5 minutes. Although the rate of movement in 
different parts of the same revolution varied greatly, 
yet 22 semicircles to the light were completed, each 
on an average in 73'95 minutes ; and 22 semicircles 
from the light each in 73*5 minutes. It may, there- 
fore, be said that they travelled to and from the light 
at exactly the same average rate ; though probably 
the accuracy of the result was in part accidental. In 
the evening the stems were not in the least deflected 
towards the window. Nevertheless, there appears to 
exist a vestige of heliotropism, for with 6 out of the 
7 plants, the first semicircle from the light, described 
in the early morning after they had been subjected to 
darkness during the night and thus probably rendered 
more sensitive, required rather more time, and the first 
semicircle to the light considerably less time, than the 
average. Thus with all 7 plants, taken together, the 
mean time of the first semicircle in the morning from 
the light, was 76'8 minutes, instead of 73*5 minutes, 
which is the mean of all the semicircles during the 
day from the light ; and the mean of the first semi- 
circle to the light was only 63-1, instead of 73-95 
minutes, which was the mean of all the semicircles 
during the day to the light. 

Similar observations were made on Wistaria Sinensis, 
and the mean of 9 semicircles from the light was 
117 minutes, and of 7 semicircles to the light 122 
minutes, and this difference does not exceed the pro- 
bable limit of error. During the three days of expo- 
sure, the shoot did not become at all bent towards the 
window before which it stood. In this case the first 
eemisircle from the light in the early morning of each 
day, required rather less time for its performance that 
did the first semicircle to the light; and this result, 



Cif\p. IX. SENSITIVENESS 1O LIGHT. 453 

if not accidental, appears to indicate that the shoots 
retain a trace of an original apheliotropic tendency. 
With Lonicera ~bracliypoda 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 
cgsrulca and purpurea are extremely heliotropic, whilst 
the sterns 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 sterns 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 Tropteolum majus are highly heliotropic, whilst 
those of older plants, accordiug to Sachs, are slightly 
apheliotropic. In all these cases the heliotropism of 
the very young stems serves to expose the cotyledons, 
or when the cotyledons are hypogean the first true 
leaves, fully to the light ; and the loss of this power 
by the older stems, or their becoming apheliotropic, 
is connected with their habit of climbing. 

Most seedling plants are strongly heliotropic, and 



i54 SENSITIVENESS TO LIGHT. CHAP. IX 

it is no doubt a great advantage to them in then 
struggle for life to expose their cotyledons to the 
light as quickly and as fully as possible, for the sake 
of obtaining carbon. It has been shown in the first 
chapter that the greater number of seedlings circum - 
nutate largely and rapidly ; and as heliotropism con- 
sists of modified circumnutation, we are tempted to 
look at the high development of these two powers in 
seedlings as intimately connected. Whether there are 
any plants which circumnutate slowly and to a small 
extent, and' yet are highly heliotropic, we do not 
know ; but there are several, and there is nothing 
surprising in this fact, which circumnutate largely and 
are not at all, or only slightly, heliotropic. Of such 
cases Drosera rotundifolia offers an excellent instance, 
The stolons of the strawberry circumnutate almost 
like the stems of climbing plants, and they are not at 
all affected by a moderate light; but when exposed 
late in the summer to a somewhat brighter light they 
were slightly heliotropic; in sunlight, according to 
De Vries, they are apheliotropic. Climbing plants 
circumnutate much more widely than any other plants, 
yet they are not at all heliotropic. 

Although the stems of most seedling plants are 
strongly heliotropic, some few are but slightly 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 



CHAT. IX. SENSITIVENESS TO LIGHT. 4f>5 

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 light, and after 7 h. 40 m. from 
the first exposure, they were all plainly, though 
slightly, curved towards the lamp. Now, at this dis- 
tance of 12 feet, the light was so obscure that we could 
not see the seedlings themselves, nor read the large 
lioman figures on the white face of a watch, nor see a 
pencil line on paper, but could just distinguish a line 
made with Indian ink. It is a more surprising fact 
that no visible shadow was cast by a pencil held 
upright on a white card; the seedlings, therefore, 
were acted on by a difference in the illumination of 
their two sides, which the human eye could not dis- 
tinguish. On another occasion even a less degree of 
light acted, for some cotyledons of Phalaris became 
slightly curved towards the same lamp at a distance 
of 20 feet ; at this distance we could" not see a cir- 
cular dot 2-29 mm. ('09 inch) in diameter made with 
Indian ink on white paper, though we could just see a 
dot 3-56 mm. (-14 inch) in diameter; yet a dot of 
the former size appears large when seen in the light.* 

We next tried how small a beam of light would act ; 
us this bears on light serving as a guide to seedlings 
whilst they emerge through fissured or encumbered 
ground. A pot with seedlings of Phalaris was cove-ed 

* Straslmrger says (' Wirluing Hicmatococcus moved to a light 
de* Lichtes auf Sch \varmsporcn,' which only just sufficed to allovr 
187S, p. 52), that the spores of middle-sized type to be read. 

CO 



156 SENSITIVENESS TO LIGHT. CHAP. IX 

by a tin-vessel, having on one side a circular hole 
1'23 mm. in diameter (i.e. a little less than the ~ 7 ih of 
an inch) ; and the box was placed in front of a paraffin 
lamp and on another occasion in front of a window ; 
and both times the seedlings were manifestly bent 
after a few hours towards the little hole. 

A more severe trial was now made ; little tubes of 
very thin glass, closed at their upper ends and coated 
with black v-arnish, were slipped over the cotyledons 
of Phalaris (which had germinated in darkness) and 
just fitted them. Narrow stripes of the varnish had 
been previously scraped off one side, through which 
alone light could enter ; and their dimensions were 
afterwards measured under the microscope. As a 
control experiment, similar unvarnished and trans- 
parent tubes were tried, and they did not prevent the 
cotyledons bending towards the light. Two cotyledons 
were placed before a south-west window, one of which 
was illuminated by a stripe in the varnish, only '004 
inch (O'l mm.) in breadth and '016 inch (O4 mm.) in 
length ; and the other by a stripe '008 inch in breadth 
and '06 inch in length. The seedlings were examined 
after an exposure of 7 h. 40 m., and were found to be 
manifestly bowed towards the light. Some other coty- 
ledons were at the same time treated similarly, ex- 
cepting that the little stripes were directed not to the 
sky, but in such a manner that they received only the 
diffused light from the room ; and these cotyledons did 
net become at all bowed. Seven other cotyledons were 
illuminated through narrow, but comparatively long, 
cleared stripes in the varnish namely, in breadth 
between '01 and '026 inch, and in length between '15 
and -3 inch ; and these all became bowed to the side, 
t>y which light entered through the stripes, whether 
these were directed towards the sky or to one side oi 



CHAP IX. SENSITIVENESS TO LIGHT. 45? 

the room. That light passing through a hole only 
004 inch in breadth by '016 in length, should induce 
curvature, seems to us a surprising fact. 

Before we knew how extremely sensitive the coty- 
ledons of Phalaris were to light, we endeavoured to 
trace their circurnnutation in darkness by the aid < f 
a small wax taper, held for a minute or two at each 
observation in nearly the same position, a little on the 
left side in front of the vertical glass on which the 
tracing was made. The seedlings were thus observed 
seventeen times in the course of the day, at intervals of 
from half to three-quarters of an hour ; and late in the 
evening we were surprised to find that all the 29 coty- 
ledons were greatly curved and pointed towards the 
vertical glass, a little to the left where the taper had 
been held. The tracings showed that they had tra- 
velled in zigzag lines. Thus, an exposure to a feeble 
light for a very short time at the above specified 
intervals, sufficed to induce well-marked heliotropism. 
An analogous case was observed with the hypocotyls 
of iSolanwn lycopersicum. We at first attributed this 
result to the after-effects of the light on each occasion ; 
but since reading. Wiesner's observations,* which will 
be referred to in the last chapter, we cannot doubt that 
an intermittent light is more efficacious than a con- 
tinuous one, as plants are especially sensitive to any 
contrast in its amount. 

The cotyledons of Phaluris bend much more slowly 
towards a very obscure light than towards a bright 
one. Thus, in the experiments with seedlings placed 
in a dark room at 12 feet from a very small lamp, they 
were just perceptibly and doubtfully curved towards it 
after 3 h., and only slightly, yet certainly, after 4 Ju 

* Bitz. dor k. Akiul. der Wissensch.' (Vienna), Jan. 1880, p. 12. 



158 SENSITIVENESS TO LIGHT. CHAP. IS 

After 8 h. 40 m. the chords of their arcs were deflected 
from the perpendicular by an average angle of only 
16. Had the light been bright, they would have 
become much more curved in between 1 and 2 h. 
Several trials were made with seedlings placed at 
various distances from a small lamp in a dark room ; 
but we will give only one trial. Six pots were placed 
at distances of 2, 4, 8, 12, 16, and 20 feet from the 
lamp, before which they were left for 4 h. As light 
decreases in a geometrical ratio, the seedlings in the 
2nd pot received |th, those in the 3rd pot -r^th, 
those in the 4th -gV^hj those in the 5th ^ T th, and those 
in the 6th -j-^-oth of the light received by the seedlings in 
the first or nearest pot. Therefore it might have been 
expected that there would have been an immense differ- 
ence in the degree of their heliotropic curvature in the 
several pots ; and there was a well-marked difference 
between those which stood nearest and furthest from 
the lamp, but the difference in each successive pair of 
pots was extremely small. In order to avoid prejudice, 
we asked three persons, who knew nothing about the 
experiment, to arrange the pots in order according to 
the degree of curvature of the cotyledons. The first 
person arranged them in proper order, but doubted 
long between the 12 feet and 16 feet pots ; yet these 
two received light in the proportion of 36 to 64. The 
second person also arranged them properly, but 
doubted between the 8 feet and 12 feet pots, which 
received light in the proportion of 16 to 36. The 
third person arranged them in wrong order, and 
doubted about four of the pots. This evidence shows 
oonslusively how little the curvature of the seedlings 
differed in the successive pots, in comparison with tho 
great difference in the amount of light which they 
received ; and it should be noted that there svas no 



CHAP. IX. SENSITIVENESS TO LIGHT. 459 

excess of superfluous light, for the cotyledons became 
but little and slowly curved even in the nearest pot. 
Close to the 6th pot, at the distance of 20 feet from 
the lamp, the light allowed us just to distinguish 
a dot 3'56 mm. (-14 inch) in diameter, made with 
Indian ink on white paper, but not a dot 2*29 mm. 
(09 inch) in diameter. 

The degree of curvature of the cotyledons of Phalaris 
within a given time, depends not merely on the 
amount of lateral light which they may then receive, 
but on that which they have previously received from 
above and on all sides. Analogous facts have been 
given Avith respect to the nyctitropic and periodic 
movements of plants. Of two pots containing seedlings 
of Phalaris which had germinated in darkness, one was 
still kept in the dark, and the other was exposed (Sept. 
26th) to the light in a greenhouse during a cloudy day 
and on the following bright morning. On this morn- 
ing (27th), at 10.3U A.M., both pots were placed in a 
box, blackened within and open in front, before a 
north-east window, protected by a linen and muslin 
blind and by a towel, so that but little light was 
admitted, though the sky was bright. Whenever the 
pots were looked at, this was done as quickly as pos- 
sible, and the cotyledons were then held transversely 
with respect to the light, so that their curvature could 
not have been thus increased or diminished. After 
50 m. the seedlings which' had previously been kept 
in darkness, were perhaps, and after 70 m. were cer- 
tainly, curved, though very slightly, towards the 
window. After 85 m. some of the seedlings, which 
had previously been illuminated, were perhaps a little 
affected, and after 100 m. some of the younger ones 
were certainly a little curved towards the light. At 
this time (i.e. after 100m.) there was a plain difference 



100 SENSITIVENESS TO LIGHT. CIIAI. IX. 

in the curvature of the seedlings in the two pots. 
After 2 h. 12 m. the chords of the arcs of four o1 
the most strongly curved seedlings in each pot wevt 
measured, and the mean angle from the perpendicular 
of those which had previously been kept in darknes* 
Avas 19, and of those which had previously been illu- 
minated only 7. Nor did this difference diminish 
during two additional hours. As a check, the seed- 
lings in both pots were then placed in complete dark- 
ness for two hours, in order that apogeotropism should 
act on them ; and those in the one pot which were 
little curved became in this time almost completely 
upright, whilst the more curved ones in the other pot 
still remained plainly curved. 

Two days afterwards the experiment was repeated, 
with the sole difference that even less light was 
admitted through the window, as it was protected by a 
linen and muslin blind and by two towels ; the sky, 
moreover, was somewhat less bright. The result was 
the same as before, excepting that everything occurred 
rather slower. The seedlings which had been pre- 
viously kept in darkness were not in the least curved 
after 54 m., but were so after 70 m. Those which had 
previously been illuminated were not at all affected 
until 130 m. had elapsed, and then only slightly. 
After 145 in. some of the seedlings in this latter pot 
were certainly curved towards the light ; and there 
was now a plain difference between the two pots. After 
3 h. 45 m. the chords of the arcs of 3 seedlings in 
each pot were measured, and the mean angle from the 
perpendicular was 16 for those in the pot w r hich had 
previously been kept in darkness, and only 5 for 
those which had previously been illuminated. 

The curvature of the cotyledons of Phalaris towards 
a lateral light is therefore certainly influenced by tho 



CHAP. IX. SENSITIVENESS TO LIGHT. 4(51 

degree to which they have been previously illu- 
minated. We shall presently see that the influence 
of light on their bending continues for a short time 
after the light has been extinguished. These facts, as 
well as that of the curvature not increasing or de- 
creasing in nearly the same ratio with that of the 
amount of light which they receive, as shown in the 
trials with the plants before the lamp, all indicate 
that light acts on them as a stimulus, in somewhat 
the same manner as on the nervous system of animals, 
and not in a direct manner on the cells or cell-walls 
which by their contraction or expansion cause the 
curvature. 

It has already been incidentally shown how slowly 
the cotyledons of Phalaris bend towards a very dim 
light ; but when they were placed before a bright 
paraffin lamp their tips were all curved rectangularly 
towards it in 2 h. 20 m. The hypocotyls of Solatium 
lij coper sicum had bent in the morning at right angles 
towards a north-east window. At 1 P.M. (Oct. 21st) the 
pot was turned round, so that the seedlings now pointed 
from the light, but by 5 P.M. they had reversed their 
curvature and again pointed to the light. They had 
thus passed through 180 in 4 h., "having in the 
morning previously passed through about 90. But the 
reversal of the first half of the curvature will have 
been aided by apogeotropism. Similar cases were 
observed with other seedlings, for instance, with those 
of Sinapis alba. 

We attempted to ascertain in how short a time 
light acted on the cotyledons of Phalaris, but this 
was difficult on account of their rapid circumnutating 
movement ; moreover, they differ much in sensibility, 
recording to age ; nevertheless, some of our observa- 
tions are worth giving. Pots with seedlings we 



162 SENSITIVENESS TO LIGHT. CHAP. IS. 

placed under a microscope provided with an eye-piece 
micrometer, of which each division equalled -!-o-th of an 
inch (0*051 mm.) ; and they were at first illuminated 
by light from a paraffin lamp passing through a solu- 
tion of bichromate of potassium, which does not induce 
heliotropism. Thus the direction in which the coty- 
ledons were circumnutating could be observed inde- 
pendently of any action from the light ; and they could 
be made, by turning round the pots, to circumnutate 
transversely to the line in which the light would strike 
them, as soon as the solution was removed. The fact 
that the direction of the circumnutating movement 
might change at any moment, and thus the plant 
might bend either towards or from the lamp indepen- 
dently of the action of the light, gave an element of 
uncertainty to the results. After the solution had 
been removed, five seedlings which were circumnutat- 
ing transversely to the line of light, began to move 
towards it, in 6, 4, 7, 6, and 9 minutes. In one of 
these cases, the apex of the cotyledon crossed five 
of the divisions of the micrometer (i.e. y^th of an 
inch, or 0*254 mm.) towards the light in 3 m. Of two 
seedlings which were moving directly from the light at 
the time when the solution was removed, one began to 
move towards it in 13 m., and the other in 15 m. 
This latter seedling was observed for more than an 
hour and continued to move towards the light; it 
crossed at one time 5 divisions of the micrometer 
(0-254 mm.) in 2 m. 30 s. In all. these cases, the 
movement towards the light was extremely unequal in 
rate, and the cotyledons often remained almost sta- 
tionary for some minutes, and two of them retrograded 
a little. Another seedling which was circumnutating 
transversely to the line of light, moved towards it in 
4 m. after the solution was removed ; it then remained 



CHAP. fX. SENSITIVENESS TO LIGHT. 463 

almost stationary for 10 m. ; then crossed 5 divisions 
of the micrometer in 6m.; and then 8 divisions in 
11 m. This unequal rate of movement, interrupted 
by pauses, and at first with occasional retrogressions, 
accords well with our conclusion that heliotropism 
consists of modified circumnutation. 

In order to observe how long the after-effects of 
light lasted, a pot with seedlings of Phalaris, which 
had germinated in darkness, was placed at 10 . 40 A.M. 
before a north-east window, being protected on all 
other sides from the light; and the movement of a 
cotyledon was traced on a horizontal glass. It cir- 
cumnutated about the same space for the first 24 m., 
and during the next 1 h. 33 m. moved rapidly towards 
the light. The light was now (i.e. after 1 h. 57 m.) 
completely excluded, but the cotyledon continued 
bending in the same direction as before, certainly for 
more than 15 m., probably for about 27 m. The doubt 
arose from the necessity of not looking at the seed- 
lings often, and thus exposing them, though momen- 
tarily, to the light. This same seedling was now kept 
in the dark, until 2.18 P.M., by which time it had 
reacquired through apogeotropism its original upright 
position, when it was again exposed to the light from 
a clouded sky. By 3 P.M. it had moved a very short 
distance towards the light, but during the next 45 m. 
travelled quickly towards it. After this exposure of 
1 h 27 m. to a rather dull' sky, the light was again 
completely excluded, but the cotyledon continued to 
bend in the same direction as before for 14 m. within 
a very small limit of error. It was then placed in 
the dark, and it now moved backwards, so that after 
1 h. 7 m. it stood close to where it had started from at 
2 . 18 P.M. These observations show that the coty- 
ledons of Phalaris, after being exposed to a lateral 



164 SEXSniVENESS TO LIGHT. CI:A?. IX. 

light, continue to bend in the same direction for 
between a quarter and half an hour. 

In the two experiments just given, the cotyledons 
moved backwards or from the window shortly after 
being subjected to darkness ; and whilst tracing the 
circumnutation of various kinds of seedlings exposed 
to a lateral light, we repeatedly observed that late in 
the evening, as the light waned, they moved from it. 
This fact is shown in some of the diagrams given in 
the last chapter. We wished therefore to learn whether 
this was wholly due to apogeotropism, or whether an 
organ after bending towards the light tended from 
any other cause to bend from it, as soon as the light 
failed. Accordingly, two pots of seedling Phalaris 
and one pot of seedling Brassica were exposed for 8 h. 
before a paraffin lamp, by which time the cotyledons 
of the former and the hypocotyls of the latter were bent 
rectangularly towards the light. The pots were now 
quickly laid horizontally, so that the upper parts of 
the cotyledons and of the hypocotyls of 9 seedlings 
projected vertically upwards, as proved by a plumb-line. 
In this position they could not be acted on by apo- 
geotropism, and if they possessed any tendency to 
straighten themselves or to bend in opposition to their 
former heliotropic curvature, this would be exhibited, 
for it would be opposed at first very slightly by apogeo- 
tropism. They were kept in the dark for 4 h., during 
which time they were twice looked at ; but no uniform 
bending in opposition to their former heliotropic 
curvature could be detected. We have said uniform 
bending, because they circumnutated in their new 
position, and after 2 h. were inclined in different 
directions (between 4 and 11) from the perpendicular. 
Their directions were also changed after two additional 
hours, and again on the following morning. We may 



CHAP. IX. SENSITIVENESS TO LIGHT. 465 

therefore conclude that the bending back of plants 
from a light, when this becomes obscure or is extin- 
guished, is wholly due to apogeotropism.* 

In our various experiments we were often struck 
with the accuracy with which seedlings pointed to a 
light although of small size. To test this, many seed- 
lings of Phalaris, which had germinated in darkness in 
a very narrow box several feet in length, were placed 
in a darkened room near to and in front of a lamp 
having a small cylindrical wick. The cotyledons at 
the two ends and in the central part of the box, would 
therefore have to bend in widely different directions 
in order to point to the light. After they had become 
rectangularly bent, a long white thread was stretched 
by two persons, close over and parallel, first to one and 
then to another cotyledon ; and the thread was found 
in almost every case actually to intersect the small 
circular wick of the now extinguished lamp. The 
deviation from accuracy never exceeded, as far as we 
could judge, a degree or two. This extreme accuracy 
seems at first surprising, but is not really so, for an 
upright cylindrical stem, whatever its position may 
be with respect to the light, would have exactly half 
its circumference illuminated and half in shadow; and 
as the difference in illumination of the two sides is 
the exciting cause of heliotropism, a cylinder would 
naturally bend with much accuracy towards the light. 
The cotyledons, however, of- Phalaris are not cylin- 
drical, but oval in section; and the longer axis was 
to the shorter axis (in the one which was measured) 
as 100 to 70. Nevertheless, no difference could be 



* It appears from a reference heliotropically is at the same time 

in Wiesuer (' Die Uudulirende striving, through apogeotropisin, 

Nutation der Ink -rnodien,' p. 7), to raise itself into a vertical poai- 

that H. Miillor of Thurgau found tion. 
that a stein which is binding 



Ififi SENSITIVENESS TO LIGHT. CHAV. 13. 

detected in the accuracy of their bending, whether 
they stood with their broad or narrow sides facing 
the light, or in any intermediate position ; and so it 
was with the cotyledons 01 Avena sativa, which are 
likewise oval in section. Now, a little reflection will 
show that in whatever position the cotyledons may 
stand, there will be a line of greatest illumination, 
exactly fronting the light, and on each side of this 
line an equal amount of light will be received ; but 
if the oval stands obliquely with respect to the light, 
this will be diffused over a wider surface on one side 
of the central line than on the other. We may there- 
fore infer that the same amount of light, whether 
diffused over a wider surface or concentrated on a 
smaller surface, produces exactly the same effect; for 
the cotyledons in the long narrow box stood in all 
sorts of positions with reference to the light, yet all 
pointed truly towards it. 

That the bending of the cotyledons to the light 
depends on the illumination of one whole side or on 
the obscuration of the whole opposite side, and not on 
a narrow longitudinal zone in the line of the light 
being affected, was shown by the effects of painting 
longitudinally with Indian ink one side of five coty- 
ledons of Phalaris. These were then placed on a table 
near to a south-west window, and the painted half was 
directed either to the right or left. The result was that 
instead of bending in a direct line towards the window, 
they were deflected from the window and towards the 
vmpainted side, by the following angles, 35, 83, 31, 
43, and 39. It should be remarked that it was hardly 
possible to paint one-half accurately, or to place all 
the seedlings which are oval in section in quite the 
same position relatively to the light ; and this will 
account for the differences in the angles. Five coty- 



CHAP. JX. SENSITIVENESS TO LIGHT. 467 

ledons of Avena were also painted in the same manner, 
but with greater care; and they were laterally de- 
flected from the line of the window, towards the 
unpainted side, by the following angles, 44, 44, 55, 
51, and 57. This deflection of the cotyledons from 
the window is intelligible, for the whole unpainted 
side must have received some light, whereas the oppo- 
site and painted side received none ; but a narrow 
zone on the unpainted side directly in front of the 
window will have received most light, and all the 
hinder parts (half an oval' in section) less and less light 
in varying degrees ; and we may conclude that the 
angle of deflection is the resultant of the action of the 
light over the whole of the unpainted side. 

It should have been premised that painting with 
Indian ink does not injure plants, at least within 
several hours ; and it could injure them only by stop- 
ping respiration. To ascertain whether injury was thus 
soon caused, the upper halves of 8 cotyledons of Avena 
were thickly coated with transparent matter, 4 with 
gum, and 4 with gelatine ; they were placed in the 
morning before a window, and by the evening they 
were normally bowed towards the light, although the 
coatings now consisted of dry crusts of gum and 
gelatine. Moreover, if the seedlings which were painted" 
longitudinally with Indian ink had been injured on 
the painted side, the opposite side would have gone 
on growing, and they would consequently have become 
bowed towards the painted side ; whereas the curvature 
was always, as we have seen, in the opposite direction, 
or towards the unpainted side which was exposed to 
the light. We witnessed the effects of injuring longi- 
tudinally one side of the cotyledons of Avena and 
Phalaris ; for before we knew that grease was highly 
injurious to them, several were painted down one side 



408 TRANSMITTED EFFECTS OF LIGHT. CHAP. IX 

with a mixture of oil and lamp-black, and were then 
exposed before a window ; others similarly treated were 
afterwards tried in darkness. These cotyledons soon 
became plainly bowed towards the blackened side, 
evidently owing to the grease on this side having' 
checked -their growth, whilst growth continued on the 
opposite side. But it deserves notice that the curva- 
ture differed from that caused by light, which ulti- 
mately becomes abrupt near the ground. These 
seedlings did not afterwards die, but were much injured 
and grew badly. 

LOCALISED SENSITIVENESS TO LIGHT, AND ITS 
TRANSMITTED EFFECTS. 

Phalaris Canariensis. Whilst observing the accu- 
racy with which the cotyledons of this plant became 
bent towards the light of a small lamp, we were 
impressed with the idea that the uppermost part deter- 
mined the direction of the curvature of the lower part. 
When the cotyledons are exposed to a lateral light, 
the upper part bends first, and afterwards the bending 
gradually extends down to the base, and, as we shall 
presently see, even a little beneath the ground. 
This holds good with cotyledons from less than 
1 inch (one was observed to act in this manner which 
was only '03 in height) to about '5 of an inch in 
height ; but when they have grown to nearly an inch 
in height, the basal part, for a length of '15 to '2 of 
an inch above the ground, ceases to bend. As with 
young cotyledons the lower part goes on bending, 
after the upper part has become well arched towards 
a lateral light, the apex would ultimately point to 
the ground instead of to the light, did not the upper 
part reverse its curvature and straighten itself, as 



CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 469 

soon as the upper convex surface of the bowed- 
down portion received more light than the lower 
concave surface. The position ultimately assumed by 
young and upright cotyledons, exposed to light enter- 
ing obliquely from above through a window, is shown 
in the accompanying figure (Fig. 181) ; and here it 
may be seen that the whole upper part has become 
very nearly straight. When the cotyledons were 
exposed before a bright lamp, standing on the same 
level with them, the upper part, which was at first 

Fig. 181. 




Ph tiaris Canariensis : cotyledons alter exposure in a box open on one side 
in front of a south-west window during 8 h. Curvature towards the 
light accurately traced. The short horizontal lines show the level of 
the ground. 

greatly arched towards the light, became straight and 
strictly parallel with the surface of the soil in the 
pots ; the basal part being now rectangularly bent. 
All this great amount of curvature, together with the 
subsequent straightening of the upper part, was often 
effected in a few hours. 

After the uppermost part has become bowed a little to tho 
light, its overhanging weight must tend to increase the curva- 
ture of the lower part; but any such effect was shown in several 
ways to be quite insignificant. 'When little caps of tin-foil 
(hereafter to be described) were placed on the summits of the 
cotyledons, though this must have added considerably to their 
weight, the rate or amount of bending was not thus increased. 
But the best evidence was afforded by placing pots with seedlings 
of Phalaris before a lamp in such a position, that tho cotyledons 
wore horizontally extended and projected at right angles to tho 
line of light. . In the course of 54 h. they were directed towards 
the light with their bases bent at right angles ; at 1 this al-rupl 



470 TRANSMITTED EFFECTS OF LIGHT. CHAP. IX. 

curvature could not have been aided in the least by the weight 
cf the upper part, which acted at right angles to the plane of 
curvature. 

It will be shown that when the upper halves of the coty- 
ledons of Phalaris and Avena were enclosed in little pipes of 
tin-foil or o*f blackened glass, in which case the upper part was 
mechanically prevented from bending, the lower and unenclosed 
part did not bend when exposed to a lateral light; and it 
occurred to us that this fact might be due, not to the exclusion 
of the light from the upper part, but to some necessity of the 
bending gradually travelling down the cotyledons, so that 
unless the upper part first became bent, the lower could not 
bend, however much it might be stimulated. It was necessary 
for our purpose to ascertain whether this notion was true, and it 
was proved false ; for the lower halves of several cotyledons 
became bowed to the light, although their upper halves were 
enclosed in little glass tubes (not blackened), which prevented, 
as far as we could judge, their bending. Nevertheless, as the 
part within the tube might possibly bend a .very little, fine rigid 
rods or flat splinters of thin glass were cemented with shellac to 
one side of the upper part of 15 cotyledons ; and in six cases 
they were in addition tied on with threads. They were thus 
forced to remain quite straight. The result was that the lower 
halves of all became bowed to the light, but generally not in so 
great a degree as the corresponding part of the free seedlings 
in the same pots; and this may perhaps be accounted for by 
some Alight degree of injury having been caused by a consider- 
able surface having been smeared with shellac. It may be 
added, that when the cotyledons of Phalaris and Avena are 
acted on by apogeotropism, it is the upper part which begins 
first to bend ; and when this part was rendered rigid in the 
manner just described, the upward curvature of the basal part 
was not thus prevented. 

To test our belief that the upper part of the cotyledons of 
Phalaris, when exposed to a lateral light, regulates the bending 
cf the lower part, many experiments were tried ; but most of our 
ihst attempts proved useless from various causes not worth 
specifying. Seven cotyledons had their tips cut off for lengths 
varying between -1 and '16 of an inch, and these, when left 
exposed all day to a lateral light, remained upright. In another 
set of 7 cotyledons, the tips were cut off for a length of only 
about -05 of au inch (1-27 mm.) and these became bowed towards 



CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 471 

a lateral Light, but not nearly so much as the many other seed- 
lings in the same pots. This latter case shows that cutting off 
the tips does not by itself injure the plants so seriously as to 
prevent hcliotropism ; but we thought at the time, that such 
injury might follow when a greater length was cut off, as in the 
first set of experiments. Therefore, no more trials of this kind 
tt-tre made, which we now regret ; as we afterwards found that 
when the tips of three cotyledons were cut off for a length of 
2 inch, and of four others for lengths of -14, -12, -1, and '07 
inch, and they were extended horizontally, the amputation did 
not interfere in the least with their binding vertically upwards, 
through the action oi apogeotropism, like unmutilated speci- 
mens. It is therefore extremely improbable that the amputation 
of the tips for lengths of from - 1 to -14 inch, could from the 
injury thus caused have prevented the lower part from bending 
towards the light. 

AYe next tried the effects of covering the upper part of the 
cotyledons of Phalaris with little caps which were impermeable 
to light ; the whole lower part being left fully exposed before a 
south-west window or a bright paraffin lamp. Some of the caps 
were made of extremely thin tin-foil blackened within; these 
had the disadvantage of occasionally, though rarely, being too 
heavy, especially when twice folded. The basal edges could be 
pressed into close contact with the cotyledons ; though this 
again recpuired care to prevent injuring them. Nevertheless, 
any injury thus caused could be detected by removing the caps, 
and trying whether the cotyledons were then sensitive to light. 
Other caps were, made of tubes of the thinnest glass, which 
when painted black served well, with the one great disadvantage 
that the lower ends could not be closed. But tubes were used 
which fitted the cotyledons almost closely, and black paper was 
placed on the soil round each, to check the upward reflection of 
light from the soil. Such tubes were in one respect far better 
than caps of tin-foil, as it was possible to cover at jthe same 
time some cotyledons with transparent and others with opaque 
tubes ; and thus our experiments could be controlled. It should 
be kept in mind that young cotyledons were selected for trial, 
and that these when not interfered with become bowed down 
to the ground towards the light. 

We will begin with the glass-tubes. The summits of nine 
cotyledons, differing somewhat in height, weru enclosed for 
rather less than half their lengths in uncoloured or transparent 
31 



472 TRANSMITTED EFFECTS OF LIGHT CHAT. IX 

tubes ; and these wp.re then exposed before a south-west window 
on a bright day for 8 h. All of them became strongly curved 
towards the light, in the same degree as the many other freo 
seedlings in the same pots; so that the glass-tubes certainly did 
not prevent the cotyledons from bending towards the light. 
Nineteen other cotyledons were, at the same time, similarly 
enclosed in tubes thickly painted with Indian ink. On five of 
them, the paint, to our surprise, contracted after exposure 
to the sunlight, and very narrow cracks were formed, through 
which a little light entered; and these five cases were rejected. 
Of the remaining 14 cotyledons, the lower halves of which had 
been fully exposed to the light for the whole time, 7 continued, 
quite straight and upright ; 1 was considerably bowed to the 
light, and 6 were slightly bowed, but with the exposed bases of 
most of them almost or quite straight. It is possible that some 
light may have been reflected upwards from the soil and entered 
the bases of these 7 tubes, as the sun shone brightly, though 
bits of blackened paper had been placed on the soil round 
them. Nevertheless, the 7 cotyledons which were slightly 
bowed, together with the 7 upright ones, presented a most re- 
markable contrast in appearance with the many other seedlings 
in the same pots to which nothing had been done. The 
blackened tubes were then removed from 10 of these seedlings, 
and they were now exposed before a lamp for 8 h. : 9 of them 
became greatly, and 1 moderately, curved towards the light, 
proving that the previous absence of any curvature in the 
basal part, or the presence of only a slight degree of curvature 
there, was due to the exclusion of light from the upper part. 

Similar observations were made on 12 younger cotyledons 
with their upper halves enclosed within glass-tubes coated with 
black varnish, and with their lower halves fully exposed to 
bright sunshine. In these younger seedlings the sensitive zone 
seems to extend rather lower down, as was observed on some 
other occasions, for two became almost as much curved towards 
the light as the free seedlings; and the .remaining ten weie 
slightly curved, although the basal part of several of them, 
which normally becomes more curved than any other part, 
exhibited hardly a trace of curvature. These 12 seedlings 
taken together differed greatly in their degree of curvature from 
all the many other seedlings in the same pots. 

Better evidence of the efficiency of the blackened tubes was 
incidentally afforded by some experiments hereafter to be given, 



CHAP. IX TRANSMITTED EFFECTS OF LIGHT. 473 

in which the upper halves of 14 cotyledons were enclosed ic 
tubes from which an extremely narrow stripe of the black 
varnish had been scraped off. These cleared stripes were 
not directed towards the window, but obliquely to one sido 
of the room, so that only a very little light could act on the 
upper halves of the cotyledons. These 14 seedlings remained 
during eight hours of exposure before a south-west window on 
a hazy day quite upright; whereas all the other many free 
seedlings in the same pots became greatly bowed towards tho 
light. 

We will now turn to the trials with caps made of very thin 
tin-foil. These were placed at different times on the summits of 
24 cotyledons, and they extended down for a length of between 
15 and - 2 of an inch. The seedlings were exposed to a lateral 
light for periods varying between 6 h. 30 m. and 7 h. 45 m., 
which sufficed to cause all the other seedlings in the same pots 
to become almost rectangularly beut towards the light. They 
varied in height from only '04 to 1'15 inch, but the greater 
number were about "75 inch. Oi the 24 cotyledons with their 
summits thus protected, 3 became much bent, but not in the 
direction of the light, and as they did not straighten themselves 
through apogeotropism during the following night, either the 
caps were too heavy or the plants themselves were in a weak 
condition; and these three cases may be excluded. There 
are left for consideration 21 cotyledons ; of these 17 remained 
all the time quite upright ; the other 4 became slightly inclined 
to the light, but not in a degree comparable with that of tho 
many free seedlings in the same pots. As the. glass- tubes, when 
unpainted, did not prevent the cotyledons from becoming 
greatly bowed, it cannot be supposed that the caps o f very 
thin tin- foil did so, except through the exclusion of the light. 
To prove that the plants had not been injured, the caps were 
removed from 6. of the upright seedlings, and these were exposed 
before a paraffin lamp for the same length of time as before 
and they now all became greatly curved towards the light. 

As caps between -15 and -2 of an inch in depth were thus 
proved to be highly efficient in preventing the cotyledons from 
tending towards the light, 8 other cotyledons were protected 
with caps between only '06 and '12 in depth. Of these, two 
remained vertical, one was considerably and five slightly curved 
towards the light, but far less so than the free seedlings in the 
same pots. 



174 TRANSMITTED EFFECTS OF LIGHT. CHAP. IX 

Another trial was made in a different manner, namely, bj 
trandaging with strips of tin-foil, about '2 in breadth, the upper 
part, but not the actual summit, of eight moderately youug 
seedlings a little over half an inch in height. The summits and 
the basal parts were thus left fully exposed to a lateral light 
during 8 h. ; an upper intermediate 70110 being protected, 
With four of these seedlings the summits were exposed for 
a length of '05- inch, and in two of them this part became 
curved towards the light, but the whole lower part remained 
quite upright; whereas the entire length of the other two 
seedlings became slightly curved towards the light. The 
summits of the four other seedlings were exposed for a length 
of -04 inch, and of these one remained almost upright, whilst 
the other three became considerably curved towards the light. 
The many free seedlings in the same pots were all greatly 
curved towards the light. 

From these several sets of experiments, including those with 
the glass-tubes, and those when the tips were cut off, we may 
infer that the exclusion of light from the upper part of the 
cotyledons of Phalaris prevents the lower part, though fully 
exposed to a lateral light, from becoming curved. The summit 
for a length of *04 or - 05 of an inch, though it is itself sensitive 
and curves towards the light, has only a slight power of causing 
the lower part to bend. Nor has the exclusion of light from the 
summit for a length of '1 of an inch a strong influence on the 
curvature of the lower part. On the other hand, an exclusion 
for a length of between '15 and '2 of an inch, or of the whole 
upper half, plainly prevents the lower and fully illuminated 
part from becoming curved in the manner (see Fig. 181) which 
invariably occurs when a free cotyledon is exposed to a lateral 
light. With very young seedlings the sensitive zone seems to 
extend rather lower down relatively to their height than in older 
seedlings. We must therefore conclude that when seedlings 
are freely exposed to a lateral light some influence is trans- 
mitted from the upper to the lower part, causing the latter to 
bend. 

This conclusion is supported by what may be seen to occur 
on a small scale, especially with young cotyledons, without any 
artificial exclusion of the light ; for they bend beneath the earth 
where no light can enter. Seeds of Phalaris were covered 
with a layer one-fourth of an inch in thickness of very fine 
sand, consisting of extremely minute grains of silex coated with 



CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 475 

oxide of iron. A layer of this sand, moistened to the saino 
degree as that over the seeds, was spread over a glass-plate ; and 
when the layer was '05 of an inch in thickness (carefully mea- 
sured) no light from a bright sky could be seen to pass through 
it, unless it was viewed through a long blackened tube, and 
then a trace of light could be detected, but probably much too 
little to affect any plant. A layer ! of an inch in thickness was 
quite impermeable to light, as judged by the eye aided by the tube. 
It may be worth adding that the layer, when dried, remained 
equally impermeable to light. This sand yielded to very slight 
pressure whilst kept inoist, and in this state did not contract 
or crack in the least. In a first trial, cotyledons which had 
grown to a moderate height were exposed for 8 h. before a paraffin 
iamp, and they became greatly bowed. At their bases on the 
shaded side opposite to the light, well-defined, crescentic, open 
furrows were formed, which (measured under a microscope with 
a micrometer) were from '02 to -03 of an inch in breadth, and 
these had evidently been left by the bending of the buried bases 
of the cotyledons towards the light. On the side of the light 
the cotyledons were iu close contact with the sand, which was a 
very little heaped up. By removing with a sharp knife the 
sand on one side of the cotyledons in the line of the light, the 
bent portion and the open furrows were found to extend down 
to a depth of about -1 of an inch, where no light could enter. 
The chords of the short buried arcs formed in four cases angles 
of 11, 13, 15, and 18, with the perpendicular. By the 
following morning these short bowed portions had straightened 
themselves through apogeo+ropism. 

In the next trial much younger cotyledons were similarly 
treated, but were exposed to a rather obscure lateral light. 
After some hours, a bowed cotyledon, '3 inch in height, had an 
open furrow on the shaded side '01 inch in breadth; another 
cotyledon, only '13 inch in height, had left a furrow '02 inch in 
breadth. But the most curious case was that of a cotyledon which 
had just protruded above the ground arid was only '03 inch in 
height, and this was found to be bowed in the direction of the 
light to a depth of '2 of an inch beneath the surface. From 
what we know of the impermeability of this sand to light, the 
upper illuminated part in these several cases must have deter- 
mined the curvature of the lower buried portions. But an 
apparent cause of doubt maybe suggested: as the cotyledons 
lire continually circumnutating, they tend to form a minute 



176 TRANSMITTED EFFECTS OF LIGHT. CJIAI-. IK 

crack or furrow all round their bases, which would admit a 
little light on all sides; but this would not happen when thej 
were illuminated laterally, for we know that they quickly bend 
towards a lateral light, and they then press so firmly against the 
sand on the illuminated side as to furrow it, and this would 
effectually exclude light on this side. Any light admitted on 
the opposite and shaded side, where an open furrow is formed, 
would tend to counteract the curvature towards the lamp or 
other source of the light. It may be added, that the use of fine 
moist sand, which yields easily to pressure, was indispensable 
in the above experiments ; for seedlings raised in common soil, 
not kept especially damp, and exposed for 9 h. 30 m. to a strong 
lateral light, did not form an open furrow at their bases on the 
shaded side, and were not bowed beneath the surface. 

Perhaps the most striking proof of the action of the upper 
on the lower part of the cotyledons of Phalaris, when laterally 
illuminated, was afforded by the blackened glass-tubes (before 
alluded to) with very narrow stripes of the varnish scraped 
off on one side, through which a little light was admitted. 
The breadth of these stripes or slits varied between '01 and 
02 inch ('25 and -51 mm.). Cotyledons with their upper 
halves enclosed in such tubes were placed before a south-west 
window, in such a position, that the scraped stripes did not 
directly face the window, but obliquely to one side. The seed- 
lings were left exposed for 8 h., before the close of which time 
the many free seedlings in the same pots had become greatly 
bowed towards the window. Under these circumstances, the 
whole lower halves of the cotyledons, which had their summits 
enclosed in the tubes, were fully exposed to the light of the 
sky, whilst their upper halves received exclusively or chiefly 
diffused light from the room, and this only through a very 
narrow slit on one side. Now, if the curvature of the lower 
part had been determined by the illumination of this part, all 
the cotyledons assuredly would have become curved towards 
the window; but this was far from being the case. Tubes 
of the kind just described were placed on several occasions 
over the upper halves of 27 cotyledons ; 14 of them remained 
all the time quite vertical; so that sufficient diffused light 
did not enter through the narrow slits to produce any effect 
whatever; and they behaved in the f-amc manner as if their 
upper halves had been enclosed in completely blackened tiil>es. 
The lower haJvf.s oi tho 13 other cotyledons became bowuJ 



UHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 477 

not directly in the line of 'the window, but obliquely towards 
it ; one pointed at an angle of only 18, but the remaining 12 
at angles varying between 45 and 62 from the line of the 
window. At the commencement of the experiment, pins had 
been laid on the earth in the direction towards which the slits in 
the varnish faced ; and in this direction alone a small amount 
of diffused light entered. At the close of the experiment, 7 ot 
the bowed cotyledons pointed exactly in the line of the pins, 
and 6 of them in a line between that of the pins and that of the 
window. This intermediate position is intelligible, for any light 
from the sky which entered obliquely through the slits would 
be much more efficient than the diffused light which entered 
directly through them. After the 8 h. exposure, the contrast 
in appearance between these 13 cotyledons and the many other 
seedlings in the same pots, which were all (excepting the above 
14 vertical ones) greatly bowed in straight and parallel lines 
towards the window, was extremely remarkable. It is therefore 
certain that a little weak light striking the upper halves of the 
cotyledons of Phalaris, is far more potent in determining the 
direction of the curvature of the lower halves, than the full 
illumination of the latter during the whole time of exposure. 

In confirmation of the above results, the effect of thickly 
painting with Indian ink one side of the upper part of three coty- 
ledons of Phalaris, for a length of '2 inch from their tips, may be 
worth giving. These were placed so that the unpainted surface 
was directed not towards the window, but a little to one side; 
and they all became bent towards the unpainted side, and from 
the line of the window by angles amounting to 81, 35, and 83. 
The curvature in this direction extended down to their bases, 
although the whole lower part was fully exposed to the light 
from the window. - 

Finally, although there can be no doubt that the illumination 
of the upper part of the cotyledons of Phalaris greatly affects 
the power and manner of bending of the lower part, jot some 
observations seemed to render it probable that the simultaneous 
stimulation of the lower part by light greatly favours, or is 
almost necessary, for its well-marked curvature ; but our experi- 
ments were not conclusive, owing to the difficulty of excluding 
light from the lower halves without mechanically preventing 
their curvature. 

Ace/i,i satitxi. The cotyledons of this plant become quickly 
oc wed towards a lateral light, exactly like those of Plialam 



t78 TRANSMITTED EFFECTS OF LIGHT. CHAP. IX 

Experiments similar to the foregoing ones were tried, and we 
will give the results as briefly as possible. They are somewhat 
less conclusive than in the case of Phalaris, and this may 
possibly be accounted for by the sensitive zone varying in exten- 
sion, in a species so long cultivated and varial le as the common 
Oat. Cotyledons a little under three-quarters of an inch in 
height were selected for trial : .six had their summits protected 
from light by tin-foil caps, '25 inch in depth, and two others by 
caps - 3 inch in depth. Of these 8 cotyledons, five remained 
upright during 8 hours of exposure, although their lower parts 
were fully exposed to the light all the time; two were very slightly, 
and one considerably, bowed towards it. Caps only 2 or 2:2 inch 
in depth were placed over 4 other cotyledons, and now only one 
remained upright, one was slightly, and two considerably bowed 
to the light. In this and the following cases all the free seedlings 
in the same pots became greatly bowed to the light. 

Our next trial was made with short lengths of thin and 
fairly transparent quills ; for glass-tubes of sufficient" diameter 
to go over the cotyledons would have been too heavy. Firstly, 
the summits of 13 cotyledons were enclosed in unpaintcd 
quills, and of these 11 became greatly and 2 slightly bowed 
to the light ; so that the mere act of enclosure did not prevent 
the lower part from becoming bowed. Secondly, the summits 
of 1 1 cotyledons were enclosed in quills '3 inch in length, painted 
so as to be impermeable to light; of these, 7 did not be- 
come at all inclined towards the light, but 3 of them were 
slightly bent more or less transversely with respect to the lino 
of light, and these might perhaps have been altogether ex- 
cluded; one alone was slightly bowed towards the light. 
Painted quills, *25 inch in length, were placed over the summits 
of 4 other cotyledons ; of these, one alone remained upright, a 
second was slightly bowed, and the two others as much bowed 
to the light as the free seedlings in the same pots. These two 
latter cases, considering that the caps were 25 in length, are 
inexplicable. 

Lastly, the summits of 8 cotyledons were coated with flexible 
and highly transparent gold-beaters' skin, and all became an 
much bowed to the light as the free seediings. The summits of 
9 other cotyledons were similarly coated with gold-beaters' skin, 
which was then painted to a depth of between '25 and '3 inch, 
so as to bo impermeable to light; of these 5 remained upright, 
and 1 were well bowed to the light, almost or quite as well aa 



CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 479 

the tree seedlings. These latter four cases, as well as the t\vo 
in the last paragraph, offer a strong exception to the rule that 
the illumination of the upper part determines the curvature of 
the lower part. Nevertheless, 5 of these 8 cotyledons remained 
quite upright, although their lower halves were fully illuminated 
all the time ; and it would almost be a prodigy to find five free 
seedlings standing vertically after an exposure for several hours 
to a lateral light. 

The cotyledons of Avena, like those of Phalaris, when growing 
in soft, damp, fine sand, leave an open crescentric furrow on the 
shaded side, after bending to a lateral light ; and they become 
bowed beneath the surface at a depth to which, as we know, 
light cannot penetrate. The arcs of the chords of the buried 
bowed portions formed in two cases angles of 20 and 21 with 
the perpendicular. The open furrows on the shaded side were, 
in four cases, -008, '016, '024, and -024: of on inch in breadth. 

llrassim ohracea (Common Eed). It will here be shown that 
the upper half of the hypocotyl of the cabbage, when illuminated 
by a lateral light, determines the curvature of the lower half. 
It is necessary to cxperimentise on young seedlings about half 
an inch or rather less in height, for when grown to an inch and 
upwards the basal part ceases to bend. We first tried painting 
ihe hypoeotyls with Indian ink, or_cutting off their summits for 
various lengths ; but these experiments are not worth giving, 
though they confirm, as far as they can be trusted, the results 
of the following ones. These were made by folding gold-beaters' 
skin once round the upper halves of young hypoeotyls, and 
painting it thickly with Indian ink or with black grease. As 
a control experiment, the same transparent skin, left unpainted, 
was folded round the upper halves of 12 hypoeotyls : and these 
all became greatly curved to the light, excepting one which was 
only moderately curved. Twenty other young hyponotyls had 
the skin round their upper halves painted, whilst their lower 
halves were left quite uncovered. These seedlings were then 
exposed, generally for between 7 and 8 h., in a box blackened 
within and open in front, either before a south-west window or 
a paraffin lamp. This exposure was amply sufficient, as was 
shown by the strongly-marked heliotropism of all the free seed- 
lings in the same pots; nevertheless, some were left exposed 
to the light for a much longer time. Of the 20 hynocotyls 
thus treated, 14 remained quite upright, and 6 l>ecame sUerhtly 
bowed to the light; but 2 of these hitter cases were not really 



i80 TRANSMITTED EFFECTS OF LIG1LT. CHAP. IX 

exceptions, for on removing the skin the paint was found im- 
perfect and was penetrated by many small transparent spaces 
on the side which faced the light, Moreover, in two other cases 
the painted skin did not extend quite halfway down the hypo- 
cotyl. Altogether there was a wonderful contrast in the several 
pots between these 20 hypocotyls and the other many free 
seedlings, which were all greatly bowed down to their bases m 
the direction of the light, some being almost prostrate on the 
ground. 

The most successful trial on any one clay (included in the 
above results) is worth describing in detail. Six young s?ed- 
lings were selected, the hypocotyls of which were nearly *45 inch, 
excepting one, which was 6 inch in height, measured from the 
bases of their petioles to the ground. Their upper halves, 
judged as accurately as could be done by the eye, were folded 
once round with gold-beaters' skin, and this was painter 1 
thickly with Indian ink. They were exposed in an otherwise 
darkened room before a bright paraffin lamp, which stood on 
a level with the two pots containing the seedlings. They 
were first looked at after an interval of 5 h. 10 m., and tivo 
of the protected hypocotyls were found quite erect, the sixth 
being very slightly inclined to the light ; whereas all the many 
i-^ee seedlings in the same two pots were greatly bowed 
to the light. They were again examined after a continuous 
exposure to the light of 20 h. 35 m. ; and now the contrast 
between the two sets was wonderfully great ; for the free seed- 
lings had their hypocotyls extended almost horizontally in the 
direction of the light, and were curved down to the ground ; 
whilst those with the upper halves protected by the painted 
skin, but with their lower halves fully exposed to the light, still 
remained quite upright, with the exception of the one which 
retained the same slight inclination to the light which it had 
before. This latter seedling was found to have been rather 
badly painted, for on the side facing the light the red colour 
of the hypocotyl could be distinguished through the paint. 

We next tried nine older seedlings, the liypocotyls of which 
varied between 1 and 1'6 inch in height. The gold-beaters' 
skin round their upper parts was painted with black grease to 
a depth of only '3 inch, that is, from less than a third to a fourth 
or fifth of their total heights. They were exposed to the light 
for 7 h. 15 in.; and the result showed that the whole of thfl 
zone, which determines the curvature of the lowej 



CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 481 

part, was not protected from the fiction of the light; for all 9 
l>eeame curved towards it, 4 of them very slightly, 3 moderately, 
and 2 almost as much as the unprotected seedlings. Neverthe- 
less, the whole 9 taken together differed plainly in their degree 
of curvature from the many free seedlings, and from somo 
which were wrapped in un painted skin, growing in the samo 
two pots. 

Seeds were covered with about a quarter of an inch of the fine 
sand described under Phalaris ; and when the hypocotyls had 
grown to a height of between '4 and '55 inch, they wore exposed 
during 9 h. before a paraffin lamp, their bases being at first 
closely surrounded by the damp sand. They all became bowed 
down to the ground, so that their upper parts lay near to and 
almost parallel to the surface of the soil. On the side of the 
light their bases were in close contact with the sand, which was 
here a very little heaped up; on the opposite or shaded side 
there were open, crescendo cracks or furrows, rather above '01 
of an inch in width ; but they were not so sharp and regular 
as those made by Phalaris and A vena, and therefore could not 
be so easily measured under the microscope. The hypocotyls 
were found, when the sand was removed on one side, to bo 
curved to a depth beneath the surface in three cases of at least 
1 1 inch, in a fourth case of '11, and in a fifth of '15 inch. The 
chords of the arcs of the short, buried, bowed portions formed 
angles of between 11 and 15 with the perpendicular. From 
Avhat we have seen of the impermeability of this sand to light, 
the curvature of the hypocotyls certainly extended down to a 
depth where no light could enter; and the curvature must 
have been caused by an influence transmitted from the upper 
illuminated part. 

The lower halves of five young hypocotyls were surrounded by 
unpainted gold-beaters' skin, and these, after an exposure of 8 h. 
before a paraffin lamp, ail became as 'much bowed to the light 
as the free seedlings. The lower halves of 10 other young 
hypocotyls, similarly surrounded with the skin, were thickly 
painted with Indian ink; their upper and unprotected halves 
became well curved to the light, but their lower and protected 
halves remained vertical in all the cases excepting one, and on 
this the layer of paint was imperfect. This result seems to 
prove that the influence transmitted from the upper part is 
not sufficient to cause the lower part to Ixjnd, unless it l>e at 
the same time illuminated ; but there remains tho doubt, as iu 



482 TRANSMITTED EFFECTS OF LIGHT. CHAP. LH 

the case of Phalaris, whether the skin covered with a rathoi 
thick crust of dry Indian ink did not mechanically preveirt 
their curvature. 

Btta, vulgaris.K few analogous experiments wero tried on 
this plant, which is not very well adapted for the purpose, as the 
basal part of the hypocotyl, after it has grown to above half an 
inch in height, does not bend much on exposure to a lateral 
light. Four hypocotyls were surrounded close beneath their 
petioles with strips of thin tin-foil, *2 inch in breadth, and they 
remained upright all day before a paraffin lamp ; two others 
were surrounded with strips '15 inch in breadth, and one of 
these remained upright, the other becoming bowed ; the band- 
ages in two other cases were only '1 inch in breadth, and both 
of these hypocotyls became bowed, though one only slightly, 
towards the light. The free seedlings in the same pots were 
ail fairly well curved towards the light ; and during the follow- 
ing night became nearly upright. The pots were now turned 
round and placed before a window, so that the opposite sides- 
of the seedlings were exposed to the light, towards which all 
the unprotected hypocotyls became bent in the course of 7 h. 
Seven out of the 8 seedlings with bandages of tin-foil remained 
upright, but one which had a bandage only *1 inch in breadth, 
became curved to the light. On another occasion, the upper 
halves of 7 hypocotyls were surrounded with painted gold- 
beaters' skin ; of these 4 remained upright, and 3 became a littlo 
curved to the light: at the same time 4 other seedlings sur- 
rounded with unpainted skin, as well as the free ones in the 
BMme pots, all became bowed towards the lamp, before which 
they had been exposed during 22 hours. 

Ji'adtcles of Sin apis alba. The radicles of some plants are 
indifferent, as far as curvature is concerned, to the action uf 
light ; whilst others bend towards and others from it.* Whether 
these movements are of any service to the plant is very doubtful, 
at least in the case of subterranean roots ; they probably result 
from the radicles being sensitive to contact, moisture, and gravi- 
tation, and as a consequence to other irritants which are never 
naturally encountered. The radicles of Sinapis alba, when 
immersed in Avater and exposed to a lateral light, bend from it, 
or are apheliotropic. They become bent for a length of about 
4 mm. from their tips. To ascertain whether this movement 



Sachs, 'Physiologic V(?getale,' 1808, j. 4-L 



Cfcxr. IX TRANSMITTED EFFECTS OF LIGHT. 48^ 

generally occurred, 41 radicles, which had germinated in damp 
/sawdust, were immersed in water and exposed to a lateral light; 
and they all, with two doubtful exceptions, became curved from 
the light. At the same time the tips of 54 other radicles, 
similarly exposed, were just touched with nitrate of silver. 
They were blackened for a length of from "05 to '07 mm., and 
probably killed ; but it should be observed that this did not 
check materially, if at all, the growth of the upper part; for 
several, which were measured, increased in the course of only 
8-9 h. by 5 to 7 mm. in length. Of the 54 cauterised radicles 
one case was doubtful, 25 curved themselves from the light in 
the normal manner, and 28, or more than half, were not in the 
least apheliotropic. There was a considerable difference, which 
we cannot account for, in the results of the experiments tried 
towards the end of April and in the middle of September. 
Fifteen radicles (part of the above 54) were cauterised at the 
former period and were exposed to sunshine, of which 12 failed 
to be apheliotropic, 2 were still apheliotropic, and 1 was doubt- 
ful. In September, 39 cauterised radicles were exposed to a 
northern light, being kept at a proper temperature ; and now 
23 continued to be apheliotropic in the normal manner, and 
only 16 failed to bend from the light. Looking at the aggregate 
results at both periods, there can be no doubt that the de- 
struction of the tip for less than a millimeter in length destroyed 
in more than half the cases their power of moving from the 
light. It is probable that if the tips had been cauterised for 
the length of a whole millimeter, all signs of apheliotropism 
would have disappeared. It may be suggested that although 
the application of caustic does not stop growth, yet enough may 
be absorbed to destroy the power of movement in the upper 
part; but this suggestion must be rejected, for we have seen 
and shall again see, that cauterising one side of the tip of various 
kinds of radicles actually excites movement. The conclusion 
seems inevitable that sensitiveness to light resides in the tip 
of the radicle of Sinapis alba; and that the tip when thus 
stimulated transmits some influence to the upper part, causing 
it to bond. The case in this respect is parallel with that of 
the radicles of several plants, the tips of which are sensitive to 
contact and to other irritants, and, as will be shown in the 
eleventh chapter, to gravitation. 



i4 CONCLUDING REMARKS AND CHAP. IX. 

CONCLUDING REMARKS AND SUMMARY OF CHAPTER. 

We do not know whether it is a general rule with 
seedling plants that the illumination of the upper 
part determines the curvature of the lower part. But 
as this occurred in the four species examined by us, 
belonging to such distinct families as the Graminea?, 
CruciferaB, and Chenopodese, it is probably of common 
occurrence. It can hardly fail to be of service to seed- 
lings, by aiding them to find the shortest path from 
the buried seed to the light, on nearly the same 
principle that the eyes of most of the lower crawling 
animals are seated at the anterior ends of their bodies. 
It is extremely doubtful whether with fully developed 
plants the illumination of one part ever affects the 
curvature of another part. The summits of 5 young 
plants of Asparagus officinalis (varying in height be- 
tween 1-1 and 2-7 inches, and consisting of several 
short internodes) were covered with caps of tin-foil 
from O3 to 0*35 inch in depth ; and the lower un- 
covered parts became as much curved towards a lateral 
light, as were the free seedlings in the same pots. 
Other seedlings of the same plant had their summits 
painted with Indian ink with the same negative result. 
Pieces of blackened paper were gummed to the edges 
and over the blades of some leaves on young plants of 
Tropseolum majus and Ranunculus ficaria ; these were 
then placed in a box before a window, and the petioles 
of the protected leaves became curved towards the 
light, as much as those of the unprotected leaves. 

The foregoing cases with respect to seedling plants 
have been fully described, not only because the trans- 
mission of any effect from light is a new physiological 
fact, but because we think it tends to modify somewhat 
the current views on heliotropic movements. Until 



CHAP. IX. SUMMARY OF CHAPTER. 485 

lately such movements were believed to result simply 
from increased growth on the shaded side. At presenl 
it is commonly admitted * that diminished light in- 
creases the turgesceiice of the cells, or the extensibility 
of the cell-walls, or of both together, on the shaded 
side, and that this is followed by increased growth, 
]>ut Pfeffer has shown that a difference in the tur- 
gescence on the two sides of a pulvinus, that is, an 
aggregate of small cells which have ceased to grow at 
an early age, is excited by a difference in the amount 
of light received by the two sides ; and that move- 
ment is thus caused without being followed by in- 
creased growth on the more turgescent side.f All 
observers apparently believe that light acts directly 
on the part which bends, but we have seen with the 
above described seedlings that this is not the case. 
Their lower halves were brightly illuminated for hours, 
and yet did not bend in the least towards the light, 
though this is the part which under ordinary circum- 
stances bends the most. It is a still more striking 
fact, that the faint illumination of a narrow stripe on 
one side of the upper part of the cotyledons of Phalaris 
determined the direction of the curvature of the lower 
part ; so that this latter part did not bend towards the 
bright light by which it had been fully illuminated, 



* Emil Godlewski has given G3, 123, &c. Frank htis also 

('Bot. Z.-ifung,' 1879, Nos. G-'J) insisted ('Die J^atnrliche \vii- 

.TII excellent account (p. 120) of gcrechte Riclitnng von Pfl-i'i- 

the present state of the qm-stion. zentheik-n,' 1870, p. 53) on the 

See also Vines in ' Arbeiten ' des important part which the pulvini 

Hot. List, in Wurznurg,' l:s7-\ B. of'tlie leaflets of compound leaves 

U. pp. 114-147. Hugo de Vries play in placing the ,'eaflets in a 

JIM recently published a still proper po.-ition with respect to tho 

ir.ore important articlo on this light. This holds good, especially 

.-uhject : ' Bot. Zeitung,' Dec. 19th with the leave- of climbing plants, 

and 26th, 1879. which are carried into all sorts 

t ' Die Poriodischen Bewegun- of positions, ill-adapted for th 

gen der Blattoruaac,' J875, pp. 7, nctiun of the light. 



i86 CONCLUDING REMARKS AND CHAP. IX. 

but obliquely towards one side whore only a littla 
light entered. These results seem to imply the pre- 
sence of some matter in the upper part which is acted 
on by light, and which transmits its effects to the 
lower part. It has been shown that this transmission 
is independent of the bending of the upper sensitive 
part. We have an analogous case of transmission in 
Drosera, for when a gland is irritated, the basal and 
not the upper or intermediate part of the tentacle 
bends. The flexible and sensitive filament of Diomea 
likewise transmits a stimulus, without itself bending ; 
as does the stem of Mimosa. 

Light exerts a powerful influence on most vege- 
table tissues, and there can be no doubt that it 
generally tends to check their growth. But when the 
two sides of a plant are illuminated in a slightly 
different degree, it does not necessarily follow that 
tne bending towards the illuminated side is caused by 
changes in the tissues of the same nature as those 
which lead to increased growth in darkness. We 
know at least that a part may bend from the light, 
and yet its growth may not be favoured by light. 
This is the case with the radicles of Sincqns alba, wh ich 
are plainly apheliotropic ; nevertheless, they grow 
quicker in darkness than in light.* So it is with 
many aerial roots, according to Wiesner ;| but there 
are other opposed cases. It appears, therefore, that 
light does not determine the growth of apheliotropic 
parts in any uniform manner. 

We should bear in mind that the power of bending 
to the light is highly beneficial to most plants. There 



* Francis Darwin, 'Uber dns Heft iii., 1880, p. 521. 

Wachsthurn negativ heliutropi- f ' Sitzb. derk. Alcad. dor^Tis 

*cher Wurzeln ' : ' Arbeih-n des soiisch ' (Vienna), 1880, p. 12. 
Bet. lust, in Wiirzburg,' 15. ii 



OHAT. IX. SUMMARY OF CHAPTER. 487 

is therefore no improbability in this power having been 
specially acquired. In several respects light seems to 
act on plants in nearly the same manner as it does 
on animals by means of the nervous system.* With 
seedlings the effect, as we have just seen, is trans< 
mitted from one part to another. An animal may be 
excited to move by a very small amount of light ; and 
it has been shown that a difference in the illumination 
of the two sides of the cotyledons of Phalaris, which 
could not be distinguished by the human eye, sufficed 
to cause them to bend. It has also been shown that 
there is no close parallelism between the amount of 
light which acts on a plant and its degree of curva- 
ture; it was indeed hardly possible to perceive any 
difference in the curvature of some seedlings of Phalaris 
exposed to a light, which, though dim, was very much 
brighter than that to which others had been exposed. 
The retina, after being stimulated by a bright light, 
feels the effect for some time ; and Phalaris continued 
to bend for nearly half an hour towards the side whicli 
had been illuminated! The retina cannot perceive 
a dim light after it has been exposed to a bright one ; 
and plants which had been kept in the daylight 
during the previous day and morning, did not move 
so soon towards an obscure lateral light as did others 
which had been kept in complete darkness. 

Even if light does act in such a manner on the 
growing parts of plants as always to excite in them 
a tendency to bend towards the more illuminated 
side a supposition contradicted by the foregoing 
experiments on seedlings and by all apheliotropic 



* Sachs lias made some striking See his paper ' T7?ber orthotrppe 

remarks to the same effect with und plagiotrope Pflanzentheile, 

respect to the various stimuli ' Arb. ties. Bot. Inst in Wiirzburg 

which excite movement in plants. 187U B. ii. p. 282. 
32 



483 CONCLUDING HEMARKS AND CHAP. IX 

organs yot the tendency differs greatly in different 
species, and is variable in degree in the individuals of 
the same species, as may be seen in almost any pot 
of seedlings of a long cultivated plant.* There is 
therefore a basis for the modification cf this tendency 
to almost any beneficial extent. That it has been 
modified, we see in many cases : thus, it is of more 
importance for insectivorous plants to place their 
leaves in the best position for catching insects than 
to turn their leaves to the light, and they have 
no such power. If the stems of twining plants were 
to bend towards the light, they would often be drawn 
away from their supports ; and as we have seen they 
do not thus bend. As the stems of most other plants 
are heliotropic, we may feel almost sure that twining 
plants, which are distributed throughout the whole 
vascular series, have lost a power that their non- 
climbing progenitors possessed. Moreover, with Ipo- 
mosa, and probably all other twiners, the stem of the 
young plant, before it begins to twine, is highly helio- 
tropic, evidently in order to expose the cotyledons or 
the first true leaves fully to the light. With the Ivy the 
stems of seedlings are moderately heliotropic, whilst 
those of the same plants when grown a little older 



* Strasburger has shown in his the light. Some individuals, more- 
interesting work ('Wirkung des over, appear to be indifferent to 
Lichtes . . . anf Kchwarmsporen,' the Jiglit; and those of different 
1878), that the movement of the species behave very differently, 
swarm-spores of various lowly The brighter the light, the 
organised plants to a lateral light straighter is their course. They 
is influent eil by their stage of exhibit also for a thort time the 
development, by the temperature after-effects of light. In all tiieso 
to which they are subjected, by respects they re.-ernble the higher 
the degree of illumination under plants. See, also. Stahl, Ueber 
which they have been raised, and den einfluss der Lichts auf die 
by other unknown causes; so that Bewegungs - erscheinungen der 
the swarm-sports of the same Schwarmsporen ' Vcrh. d. phys.- 
species may move across the field med. Geselsshalft in Wiirzburg 
of tho micnwo^ie cither to or from 15. xii. 1878. 



. IX. SUMMARY OF CHAPTER. 489 

are apheliotropic. Some tendrils which consist of 
modified leaves organs in all ordinary cases strongly 
diaheliotropic have been rendered apheliotropic, and 
their tips crawl into any dark crevice. 

Even in the case of ordinary heliotropic movements, 
it is hardly credible that they result directly from 
the action of the light, without any special adaptation. 
We may illustrate what we mean by the hygroscopic 
movements of plants : if the tissues on one side of an 
organ permit of rapid evaporation, they will dry 
quickly and contract, causing the part to bend to this 
side. Now the wonderfully complex movements of 
the pollinia of Orchis pijramidalis, by which they clasp 
the proboscis of a moth and afterwards change their 
position for the sake of depositing the pollen-masses 
on the double stigma or again the twisting move- 
ments, by which certain seeds bury themselves in 
the ground* follow from the manner of drying of 
the parts in question ; yet no one will suppose that 
these results have been gained without special adapta- 
tion. Similarly, we are led to believe in adaptation 
when we see the hypocotyl of a seedling, which contains 
chlorophyll, bending to the light ; for although it thus 
receives less light, being now shaded by its own coty- 
ledons, it places them the more important organs in 
the best position to be fully illuminated. The hypo- 
cotyl may therefore be said to sacrifice itself for the 
good of the cotyledons, or rather of the whole plant. 
But if it be prevented from bending, as must some- 
times occur with seedlings springing up in an en- 
tangled mass of vegetation, the cotyledons themselves 
bend so as to face the light ; the one farthest off rising 



* Francis Darwin, ' On the Hy- actions Linn. Soc.,' scries ii. vol. i 
groscopic Mechanism,' &c., ' Trans- p. 149, 1876. 



190 CONCLUDING REMARKS AND Ciur. IX. 

up, and that nearest to the light sinking down, or 
both twisting laterally.* We may, also, suspect that 
the extreme sensitiveness to light of the upper part 
of the sheath-like cotyledons of the Graminese, and 
their power of transmitting its effects to the lower 
part, are specialised arrangements for finding the 
shortest path to the light. With plants growing on 
a bank, or thrown prostrate by the wind, the manner 
in which the leaves move, even rotating on their own 
axes, so that their upper surfaces may be again directed 
to the light, is a striking phenomenon. Such facts 
are rendered more striking when we remember that 
too intense a light injures the chlorophyll, and that 
the leaflets of several Leguminosoo when thus exposed 
bend upwards and present their edges to the sun, thus 
escaping injury. On the other hand, the leaflets of 
Averrhoa and Oxalis, when similarly exposed, bend 
downwards. 

It was shown in the last chapter that heliotropism 
is a modified form of circumnutation ; and as every 
growing part of every plant circuninutates more or less, 
we can understand how it is that the power of bending 
to the light has been acquired by such a multitude 
of plants throughout the vegetable kingdom. The 
manner in which a circumnutating movement that 
is, one consisting of a succession of irregular ellipses 
or loops is gradually converted into a rectilinear 
course towards the light, has been already explained. 
First, we have a succession of ellipses with their 
longer axes directed towards the light, each of which 



* Wiosncr has made remarks to tracted from B. Ixxvii. (1878) 

nearly the same effect with respect Sitb. der k. Akad, der WissenscU. 

to leaves : Die undulirende Nu- Wien. 
tatton der Internodien,' p. G, ex- 



CHAP. IX. SUMMARY OF CHAPTER. 491 

is described nearer and nearer to its source ; then the 
loops are drawn out into a strongly pronounced zigzag 
line, with here and there a small loop still formed. 
At the same time that the movement towards the light 
is increased in extent and accelerated, that in the 
opposite direction is lessened and retarded, and at last 
stopped. The zigzag movement to either side is 
likewise gradually lessened, so that finally the course 
becomes rectilinear. Thus under the stimulus of a 
fairly bright light there is no useless expenditure of 
force. 

As with plants every character is more or less 
variable, there seems to be no great difficulty in be- 
lieving that their circumnutating movements may 
have been increased or modified in any beneficial 
manner by the preservation of varying individuals. 
The inheritance of habitual movements is a necessary 
contingent for this process of selection, or the survival 
of the fittest ; and we have seen good reason to believe 
that habitual movements are inherited by plants. In 
the case of twining species the circumnutating move- 
ments have been increased in amplitude and rendered 
more circular ; the stimulus being here an internal 
or innate one. With sleeping plants the movements 
have been increased in amplitude and often changed 
in direction ; and here the stimulus is the alternation 
of light and darkness, aided, however, by inheritance. 
In the case of heliotropism, the stimulus is the unequal 
illumination of the two sides of the plant, and this 
determines, as in the foregoing cases, the modifica- 
tion of the circumnutating movement in such a manner 
that the organ bends to the light. A plant which 
has been rendered heliotropic by the above means, 
might readily lose this tendency, judging from the 
cases already given, as soon as it became useless 01 



92 CONCLUDING REMAKES. CHAP. IX. 

injurious. A species which has ceased to be helio- 
tropic might also be rendered apheliotropic by the 
preservation of the individuals which tended to cir- 
cumnutate (though the cause of this and most other 
variations is unknown) in a direction more or le<s 
opposed to that whence the light proceeded. In like 
manner a plant might be rendered d iaheliotropie. 



CHAP X MOVEMENTS EXCITED BY GRAVITATION. 493 



CHAPTER X. 

MODIFIED CIRCUMNUTATION : MOVEMENTS EXCITED BY GHAVITATIOI?. 

Means of observation - -Apogeotropism Cytisus Verbena Beta 
Gradual conversion of the movement of circumnutation into apogeo- 
tropifrTi in Rubus, Lilium, Phalaris, Avena, and Bra.-sica Apogeo- 
tropism retarded by heliotropism Effected by the aid of joints 
or pulvini Movements of flower-peduncles of Oxalis General 
remarks on apogeotropism Geotropism Movements of radicles 
Burying of seed-capsules Use of process Trifolium subterraneum 
Arachis Amphicarpaea Diageotropism Conclusion. 

OUR object in the present chapter is to show that 
geotropism, apogeotropism, and diageotropism are mo- 
dified forms of circumnutation. Extremely fine fila- 
ments of glass, bearing two minute triangles of paper, 
were fixed to the summits of young stems, frequently 
to the hypocotyls of seedlings, to flower-peduncles, 
radicles, &c., and the movements of the parts were 
then traced in the manner already described on 
vertical and horizontal glass-plates. It should be 
remembered that as the stems or other parts become 
more and more oblique with respect to the glasses, the 
figures traced on them necessarily become more and 
more magnified. The plants were protected from light, 
excepting whilst each observation was being made, and 
'1hen the light, which was always a dim one, was 
allowed to enter so as to interfere as little as possible 
with the movement in progress ; and we did not detect 
any evidence of such interference. 

When observing the gradations between circunmu- 



49-1 MODIFIED CIECUMNUTATION. CHAP X. 

tation and heliotropism, we had the great advantage of 
being able to lessen the light ; but with geotropism 
analogous experiments were of course impossible. 
We could, however, observe the movements of stems 
placed at first only a little from the perpendicular, in 
which case geotropism did not act with nearly so muck 
power, as when the stems were horizontal and at right 
angles to the force. Plants, also, were selected which 
were but feebly geotropic or apogeotropic, or had 
become so from having grown rather old. Another 
plan was to place the stems at first so that they pointed 
30 or 40 degrees beneath the horizon, and then apo- 
geotropism had a great amount of work to do before 
the stem was rendered upright ; and in this case 
ordinary circumnutation was often not wholly oblite- 
rated. Another plan was to observe in the evening 
plants which during the day had become greatly 
curved heliotropically ; for their stems under the gra- 
dually waning light very slowly became upright through 
the action of apogeotropism ; and in this case modified 
circumnutation was sometimes well displayed. 

Apoyeotropism. Plants were selected for observation almost 
by chance, excepting that they were taken from widely different 
families. If the stem of a plant which is even moderately 
sensitive to apogeotropism be placed horizontally, the upper 
growing part bends quickly upwards, so as to become perpen- 
dicular; and the line traced by joining the dots successively 
made on a glass-plate, is generally almost straight. Eor in- 
stance, a young Cytisusfragrans, 12 inches in height, was placed 
so that the stem projected 10 beneath the horizon, and its 
course was traced during 72 h. At first it bent a very little 
downwards (Fig. 182), owing no doubt to the weight of the 
stem, as this occurred with most of the other plants observed, 
though, as they were of course circumnutating, the short down- 
ward lines were often oblique. After three-quarters of an hour 
the stem began to curve upwards, quickly during the first two 
hours, but much more slowly during the afternoon and night, 



CHAP. X. 



APOGEOTROPISM. 



495 




and on the following day. During the second night it foil 
a little, and circumnutated 
during the following day; but it 
also moved a short distance to 
the right, which was caused by 
a little light having been ac- 
cidentally admitted on this side. 
The stem was now inclined 
60 5 above the horizon, and had 
therefore risen 70. With time 
allowed it would probably have 
become upright, and no doubt 
would have continued circurn- 
nutating. The sole remarkable 
feature in the figure here given 
is the straightness of the course 
pursued. The stem, however, 
did not move upwards at an 
equable rate, and it sometimes 
stood almost or quite still. 
Such periods probably represent 
attempts to circumnutate in a 
direction opposite to apogeo- 
tropisni. 

The herbaceous stem of a 
Verbena melindres (?) laid hori- 
zontally, roso in 7 h. so much 
that it could no longer be 
observed on the vertical glass 
which stood in front of the plant. 
The long line which was traced 
was almost absolutely straight. 
After the 7 h. it still continued 
to Ac, but flow circunmut 

Slightly. On the following day 
it stood upright, and circum- 
, nutated regularly, as shown in 
Fig. 82, given in the fourth 
chapter. The stems of several 
other plants which were highly 
sensitive to apogeotropism rose 
up in almost straight lines, and 



60 above horizon, traced on ver 
tical glass, from 8.30 A.M. March ' 
12th to 10.30 P.M. 13th. The sub- 
sequent circnmnutating movement 
is likewise shown up to 6.45 A.JL 
on the 15th. Nocturnal course 
represented, as usual, by a broken 
line. Movement not greatly mag- 
nified, and tracing reduced to two- 
thirds ot' original scale. 



496 



MODIFIED CIRCUMNUTATION. 



CHAP. X 



then suddenly began to circumnutate. A partially etiolated 



Fig. 183. 



\ 



rgffcmjs? i 

Beta 



and somewhat old hypocotyl 
of a seedling cabbage (21 
inches in height) was so 
sensitive that when placed 
at an angle of only 23 from 
the perpendicular, it became 
vertical in 33 minutes. As 
it could not have been 
strongly acted upon by 
apogeotropism in the abovo 
slightly inclined position, 
we expected that it would 
have circumnutated, or at 
least have moved in a zig- 
zag course. Accordingly, 
dots were made every 3 
minutes; but, when these 
were joined, the line was 
nearly straight. After this 
hypocotyl had become up- 
right it still moved onwards 
for half an hour in the same 
general direction, but in a 
zigzag manner. During the 
succeeding 9 h. it circum- 
nutated regularly, and de- 
scribed 3 large ellipses. In 
this case apogeotropism, 
although acting at a very 
unfavourable angle, quite 
overcame the ordinary cir- 
cumnutating movement. 

The hypocotyls of Beta 
vulgaris are highly sensitive- 
to apogeotropism. One was 



>ta t/r;am: apogeotropic movement j d f project 19 

of hypocotyl from 19 beneath horizon 

to a vertical position, with subsequent beneath the horizon ; it fell 

circumuutation, traced on a vertical at first a very little (see 

ttitiSttSttls. *Vfj' h n t f i ; bt f n? 

Figure reduced to one-third of original to lts we] 8 ht ? bnt as lfc was 

,cal . circumnutating the line was 



CHAP. X. APOGEOTROPISM. 497 

oblique During the next 3 h. 8 m. it rose in a nearly straight 
line, passing through an angle of 109, and then (at 12.3 P.M.) 
stood upright. It continued for 55 m. to move in the same 
general direction beyond the perpendicular, bat in a zigzag 
course. It returned also in a zigzag line, and then circumnu- 
tated regularly, describing three large ellipses during the 
remainder of the day. It should be observed that the ellipses 
in this figure are exaggerated in size, relatively to the length of 
the upward straight line, owing to the position of the vertical 
and horizontal glass-plates. Another and somewhat old hypo- 
cotyl was placed so as to stand at only 31 from the perpen- 
dicular, in which position apogeotropism acted on it with little 
force, and its course accordingly was slightly zigzag. 

The sheath-like cotyledons of Pliolnris Canariensis are ex- 
tremely sensitive to apogeotropism. One was placed so as to 
project 40 beneath the horizon. Although it was rather old 
and 1'3 inch in height, it became vertical in 4 h. 30 m., having 
passed through an angle of 130 in a nearly straight line. It then 
suddenly began to circumnutate in the ordinary manner. The 
cotyledons of this plant, after the first leaf has begun to pro- 
trude, are but slightly apogeotropic, though they still continue 
to circumnutate. One at this stage of development was placed 
horizontally, and did not become upright even after 13 h., and its 
course was slightly zigzag. So, again, a rather old hypocotyl 
of Cassia tora (li inch in height) required 28 h. to become up- 
right, and its course was distinctly zigzag ; whilst younger hypo- 
cotyls moved much more quickly and in a nearly straight line. 

When a horizontally placed stem or other organ rises in a 
zigzag line, we may infer from the many cases given in our 
previous chapters, that we have a modified form of circumnu- 
tatiou ; but when the course is straight, there is no evidence 
of circumnutation, and any one might maintain that this latter 
movement had been replaced by one of a wholly distinct kind. 
This view seems the more probable when (as sometimes 
occurred with the hypocotyls of Brassica and Beta, the stems of 
Cucurbita, and the cotyledons of Phalaris) the part in question, 
after bending up in a straight course, suddenly begins to circum- 
nutate to the full extent and in the usual manner. A fairly 
good instance of a sudden change of this kind that is, from a 
nearly straight upward movement to one of circumnutation 
is shown in Fig. 183 ; but more striking instances were occa- 
sionally observed with Beta, Brassica, and Phalaris. 

We will now describe a few cases in which it may ba 



498 MODIFIED CIRCUMNUTATION. CHAP. X 

seen how graduaJly circumnutation becomes changed into apogeo- 
tropism, under circumstances to be specified 
in each instance. 

Rubus idceus (hybrid). A young plant, 11 
\ , g 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 
-2 sensitive to apogeotropisin, or it was not 
jj capable of quick movement, for during the 

> a 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. 
*j When placed horizontally, it was evidently 
^ circumnutating, for it rose at first a little, 
^ notwithstanding the weight of the stem, and 
g then sank down ; so that it did not start on 
% its permanently upward course until 1 h. 
^g 25 m. had elapsed. On the second day, by 
g which time it had risen considerably, and 

1 when apogeotropism acted on it with somewhat 

2 less power, its course during 15 2 h. was clearly 
.SP zigzag, and the rate of the upward movement 

was not equable. During the third day, also 

% of 15| h., when apogeotropism acted on it 

^ with still less power, the stem plainly circum- 

*. nutated, for it moved during this day 3 times 

& up and 3 times down, 4 times to the left and 

3 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 
.S higher. Apogeotropism continued to act on 

* the fourth morning, as the stem was still 
o rising, though it now stood only 23 from the 
,d perpopdicular. In this diagram the several 
>"~ l stages may be followed by which an almost 
i | rectilinear, upward, apogeotropic course first 

f becomes zigzag, and then changes into a 

f J circumnutating movement, with most of the 

y fc$ successively formed, irregular ellipses directed 

upwards. 
Ltlium anratum. A plant 23 inches in height was placed 



CHAP. X 



APOGEOTEOPISM. 



499 



Fig. 185. 




horizontally, and the upper part of the stum 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 15i 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 Caiiariensis. A cotyledon 
of this plant (1'3 inch in height) has 
already been described as rising in 
4 h. 30 m. from 40 beneath the hori- 
zon into a vertical position, passing 
through an angle of 130 in a nearly 
straight line, and then abruptly be- 
ginning to circumnutate. Another 
somewhat old cotyledon of the same 
height (but from which a true leaf 
had not yet protruded), was similarly 
placed at 40 beneath the horizon. For 
the first 4 h. it rose in a nearly straight 
course (Fig. 186), so that by 1.10 P.M. 
it was highly inclined, and now apo- 
geotropism acted on it with much less 
power than before, and it began to 
zigzag. At 4.15 P.M. (i.e. in 7 h. from 
the commencement) it stood vertically, 
and afterwards continued to circum- 
nutate in the usual manner about the 
same spot. Here then we have a 
graduated change from a straight up- 
ward apogeotropic course into circum- Lilium nitration : apogt-> 
nutation, instead of an abrupt change, tropic movement of ste 
as in the former case. 

Avena saliva. 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 
straight. An oldish cotyledon, from which the first leaf began to 




traced on a vertical glass 
during 2 days and 2 
nights, from "l0.40 A.M. 
March 18th to 8 A.M. 
20th. Figure reduced to 
one-half of the original. 
scale. 



500 



MODIFIED CIRCUMNUTATION. 



CHAP. X 



Fig. 186. protrude whilst the fol- 

lowing observations were 
being made, was placed 
at 10 beneath the horizon, 
and it rose only 59 in 
24 h. It behaved rather 
differently from any other 
plant, observed by us, for 
during the first 4? h. it 
rose in a line not far from 
straight ; during the next 
6 h. it circumnutated, 
that is, it descended and 
again ascended in a 
strongly marked zigzag 
course; it then resumed 
its upward movement in 
a moderately straight line, 
and, with time allowed, 
no doubt would have be- 
come upright. In this 
case, after the first 4 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 2i inches, and 
was therefore rather old 
and not highly sensitive, 
was placed so that the 

ronffe: ipogeotropic move- hypocotyl projected at be- 
ment of cotyledon, traced on a vertical tween 30 and 40 beneath 
and horizontal glass, from 9. 10 A.M. Sept. tne horizon. The upper 
19th to 9 A.M. 20th. Figure here re- 
duced to one-fifth of original scale. P^ alone ^came CUrVed 



CHAP. X. 



APOGEOTROPISM. 



501 



Fig. 187. 



upwards, and rose during the first 3 h. 10 m. in a nearly straight 
line (Fig. 187); but it was not 
possible to trace the upward move- 
ment on the vertical glass for the 
first 1 h. 10 m., so that the nearly 
straight line in the diagram ought 
to have been much longer. During 
the next 11 h. the hypocotyl 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 Jlelio- 
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 Brassica oleracea: apogeotropic 




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 TropcKolum majus (see former 
Fig. 175) moved towards the dim 
evening light in a slightly zigzag 
line until 6.45 P.M., it then returned on 



movement, of hypocotyl, traced 
on vertical glass, from 9.20 
A.M. Sept. 12th to 8.30 A.M. 
13th. The upper part of the 
figure is more magnified than 
the lower part. If the whole 
course had been traced, the 
straight upright line wouM 
have been much longer. Figure 
here reduced to one-third oi 
the original scale. 



its course until 



502 MODIFIED CIRCUMNUTATION. CHAP. X 

10.40 P.M., during which time it zigzagged and described an 
ellipse of considerable size. The hypocotyl of Brussica, olcraceu 
(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 (if joints or pulvini. 
Movements of this kind are well known to occur in the 
Graminese, 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 thfe adjoining parts have ceased to grow. Wo 
therefore wished to ascertain whether this was the case with 
the Grarninese ; for if so, the upward curvature of their stems, 
when extended horizontally or laid prostrate, would be explained 
in accordance with our view namely, that apogeotropism 
results from modified circumnutation. After these joints have 
curved upwards, they are fixed in their new position by increased 
growth along their lower sides. 

Lolium perenne. A young stem, 7 inches in height, consist- 
ing of 3 intern odes, with the flower-head not yet protruded, 
was selected for observation. A long and very thin glass fila- 
ment was cemented horizontally to the stem close above the 
second joint, 3 inches above the ground. This joint was subse- 
quently proved to be in an active condition, as its lower side 
swelled much through the action of apogeotropism (in the 
manner described by De Vries) after the haulm had been 
fastened down for 4 h. in a horizontal position. The pot was 



* This structure has been re- die Aufrichtung des gelagertea 
cently described by De Vries in Getreides,' in ' Landwirtliscliaft- 
an interesting article, 'Ueber liche Jahrhiicher,' 1880, p. 473. 



CH4P. X. APOGEOTROPISM. 503 

BO placed that the end of the filament stood beneath the 2-inch 
object glass of a microscope with an eye-piece micrometer, each 
division of which equalled ^ f an incn - Tne 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- 
tueter (-^ inch) in 2 h. Occasionally it moved forwards by 
jerks, some of which were j^Vo ]ncn in length, and then slowly 
retreated a little, afterwards again jerking forwards. These 
oscillations were exactly like those described under Brassica 
and Dionaea, but they occurred only occasionally. We may 
therefore conclude that this moderately old joint was continually 
circumnutating on a small scale. 

Alopecurus pratensis.K 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 internocle, 
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 (-^ inch) in 3i h. ; but it sometimes moved 
at a quicker rate, for at one time it crossed 5 divisions in Is h. 
The pot had to be moved occasionally, as the end of the filament 
travelled beyond the field of vision; but as far as we could 
judge it followed during the daytime a semicircular course ; 
and it certainly travelled in two different directions at right 
angles to one another. It sometimes oscillated in the same 
manner as in the last species, some of the jerks forwards being 
as much as 10 * 00 of an inch. We may therefore conclude that 
the joints in this and the last species of grass long continue to 
circumnutate ; so that this movement would be ready to be 
converted into an apogeotropic movement, whenever the stem 
was placed in an inclined or horizontal position. 

Movements of the Flower-peduncles of Oxalis carnosa, due to 
apoytotropism and other forces. The movements of the main 
peduncle, and of the three or four sub-peduncles which each 
main peduncle of this plant bears, are extremely complex, and 
are determined by several distinct causes. Whilst the flowers 
are expanded, both kinds of peduncles circumnutate about the 
sairc spot, as we have seen (Fig 91) in the fourth chapter. 
But soon after the flowers have begun to wither the sul* 
33 



501 MODIFIED C1RCUMNUTATIOX. CHAP. X 

peduncles bend downwards, and this is due to epinasty; for 
on two occasions when pots were laid horizontally, the sub- 
peduncles assumed the same position relatively to the main 
peduncle, as would have been the case if they had remained 
upright; that is, each of them formed with it an angle of 
about 40. If they had been acted on by geotropism or aphelio- 
tropism (for the plant was illuminated from above), they would 
have directed themselves to the centre of the earth. A main 
peduncle was secured to a stick in an upright position, and one 
of the upright sub-peduncles which had been observed circum- 
nutating whilst the flower was expanded, continued to 'do so for 
at least 24 h. after it had withered. It then began to bend 
downwards, and after 36 h. pointed a little beneath the horizon. 
A new figure was now begun (A, Fig. 188), and the sub-peduncle 
was traced descending in a zigzag line from 7.20 P.M. on the 19th 
to 9 A.M. on the 22nd. It now pointed almost perpendicularly 
downwards, and the glass filament had to be removed and 
fastened transversely across the base of the young capsule. 
We expected that the sub-peduncle would have been motionless 
in its new position ; but it continued slowly to swing, like a 
pendulum, from side to side, that is, in a plane at right angles 
to that in which it had descended. This circumnutating move- 
ment was observed from 9 A.M. on 22nd to 9 A.M. 24th, as shown 
at B in the diagram. We were not able to observe this par- 
ticular sub-peduncle any longer; but it would certainly have 
gone on circumnutating until the capsule was nearly ripe (which 
requires only a short time), and it would then have moved 
upwards. 

The upward movement (C, Fig. 188) is effected in part by tha 
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 haA r e been caused 
by heliotropism or apogeotropism, but by hyponasty. Besides 
this movement at the joint, there is another of a very different 
kind, for the sub-peduncle becomes upwardly bent in the middle 
part. If the sub-peduncle happens at the time to be inclined 
much downwards, the upward curvature is so great that the 
whole forms a hook. The upper end bearing the capsule, thus 
always places itself upright, and as this cccurs in darkness, and 
in whatever position the main peduncle may have been secured, 



CHAV. X. 



APOGEOTROPISM. 



505 



t]ie upward curvature cannot be due to heliotropism or hypo- 
nasty, but to apogeotropism. 



Fig. 188. 




OxaKs cai-nosa : movements of flower-peduncle, traced on a vertical |iau 
A, pinastic downward movement; B, circumnutation whilst depend- 
ing vertically ; C, subsequent upward movement, due to apogeotropisra 
and hvpoimstv combined 



506 MODIFIED CIRCUMNUTATION. CHAP. X. 

la order to trace this upward movement, a filament was fixed 
to a sub-peduncle bearing a capsule nearly ripe, which waa 
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 
bursting of the capsule; the walls of which are so extremely 
thin, like silver paper, that they would easily be permeated by 
rain. But as soon as the petals wither, the sepals rise up and 
enclose the young capsule, forming a perfect roof over it as 
soon as the sub-peduncle has bent itself downwards. By its 
subsequent upward movement, the capsule stands when ripe 
at a greater height above the ground by twice the length of the 
sub-peduncle, than it did when dependent, and is thus able 
to scatter its seeds to a greater distance. The sepals, which 
enclose the ovarium whilst it is young, present an additional 
adaptation by expanding widely when the seeds are ripe, so as 
not to interfere with their dispersal. In the case of Oxalis 
acetosella, the capsules are said sometimes to bury themselves 
under loose leaves or moss on the ground, but this cannot occur 
with those of 0. carnosa, as the woody stem is too high. 

Oxalis ucetoeella. The peduncles are furnished with a joint in 




Ojoa'fg acetosella : course pursued by the upper part of a peduncle, whilst 
rising, traced from 11 A.M. June 1st to 9A.M. 3rd. Figure here rt~ 
duced to one-half of the original scale. 

the middle, so that the lower part answers to the main peduncle, 



CHAP. X. APOGEOTROPISM. 507 

and the upper part to one of the sub-peduncles of 0. cat nosa. 
The upper part bends downwards, after the flower has begun 
to wither, and the whole peduncle then forms a hook; that 
this bending is due to epinasty we may infer from the case of 
0. carnosa. When the pod is nearly ripe, the upper part 
straightens itself and becomes erect ; and this is due to hypo- 
nasty or apogeotropism, or both combined, and not to helio- 
tropism, for it occurred in darkness. The short, hooked part of 
the peduncle of a cleistogamic flower, bearing a pod nearly ripe, 
was observed in tbe 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, circnm- 
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 RemarJcs 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 thi? 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 CIEOUMNUTAT1ON. CHAP. X. 

tinually circumnutating, we may conclude that even 
a rectilinear course is merely an extremely modified 
form of circumnutation. It is remarkable that a stem 
or other organ which is highly sensitive to apogeo- 
tropism, and which has bowed itself rapidly upwaids 
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 circuinnutates. Two 
instances of this were observed with the hypocotyls of 
Beta vulgaris, one of which is shown in Fig. 183, and 
two other instances with the hypocotyls of Brassica. 
This momentum-like movement probably results from 
the accumulated effects of apogeotropism. For the 
sake of observing how long such after-effects lasted, 
a pot with seedlings of Beta was laid on its side in the 
dark, and the hypocotyls in 3 h. 15 in. 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 37m., 
perhaps for 48 in. ; 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 in. 

Different species and different parts of the same 
species are acted on by apogeotropism in very dif- 
ferent degrees. Young seedlings, most of which cir- 
cumnutate quickly and largely, bend upwards and 
become vertical in much less time than do any older 
plants observed by us; but whether this is due to 
their greater sensitiveness to apogeotropism, or merely 
to their greater flexibility we do not know. A hypo- 
cotyl of Beta traversed an angle of 109 in 3 h. 8 m., 
and a cotyledon of Phalaris an angle of 130 in 4 h. 
30 m. On the other hand, the stem of a herbaceous 



CHAP. X. APOGEOTKOPISM. 602 

Verbena rose 90 in about 24 h. ; that of Kubus 67, 
in 70 h. ; that of Cytisus 70, in 72 h. ; that of a young 
American Oak only 37, in 72 h. The stem of a 
young Cyperus alternifolius rose only 11 in 96 h. : 
the bending being confined to near its base. Though 
the sheath-like cotyledons of Phalaris are so extremely 
sensitive to apogeotropism, the first true leaves which 
protrude from them exhibited only a trace of this 
action. Two fronds of a fenij Nephrodium molle, both 
of them young and one with the tip still inwardly 
curled, were kept in a horizontal position for 46 h., 
and during this time they rose so little that it was 
doubtful whether there was any true apogeotropic 
movement. 

The most curious case known to us of a difference 
in sensitiveness to gravitation, and consequently of 
movement, in different parts of the same organ, is that 
offered by the petioles of the cotyledons of Ipomcea 
leptopliylla. 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 
Btood at 40 above the horizon ; it then began to eir- 
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 ('ark, 
In this position the tendril circumnutated and made 



510 MODIFIED CIECUMNUTATION. CHAP. X. 

several large ellipses during 14 h., as it likewise did 
on the following day ; but during this whole time it 
was not in the least affected by apogeotropism. On the 
other hand, when branches of another Cucurbitaceous 
plant, Echinocytis lobata, were fixed in the dark so that 
the tendrils depended beneath the horizon, these began 
immediately to bend upwards, and whilst thus moving 
they ceased to circumnutate in any plain manner; 
but as soon as they had become horizontal they re- 
commenced to revolve conspicuously.* The tendrils 
of 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 circum- 
nutated during the remaining 8 h. of observation. 

A part or organ which whilst young is extremely 
sensitive to apogeotropism ceases to be so as it grows 
old ; and it is 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 
12 h. ; that of an older seedling, 1^ inch in height, 



* For details see The Movements and Habits of Climbing Plants, 
1875, p. 131. 



CHAP. X. APOGEOTROPISM. 511 

became so in 28 h. ; and that of another still older 
one, 1^ inch in height, remained horizontal d iring 
two days, but distinctly circumnutated during this 
whole time. 

When the cotyledons of Phalaris or Avena are laid 
horizontally, the uppermost part first bends upwards, 
and then the lower part ; consequently, after the lower 
part has become much curved upwards, the upper part 
is compelled to curve backwards in an opposite direc- 
tion, in order to straighten itself and to stand ver- 
tically ; and this subsequent straightening process is 
likewise due to apogeotropism. The upper part of 
8 young cotyledons of Phalaris were made rigid by 
being cemented to thin glass rods, so that this part 
could not bend in the least ; nevertheless, the basal 
part was not prevented from curving upward. A stem 
or other organ which bends upwards through apogeo- 
tropism exerts considerable force ; its own weight, 
which has of course to be lifted, was sufficient in 
almost every instance to cause the part at first to bend 
a little downwards ; but the downward course was 
often rendered oblique by the simultaneous circum- 
nutating movement. The cotyledons of Avena placed 
horizontally, besides lifting their own weight, were 
able to furrow the soft sand above them, so as to leave 
little crescentic open spaces on the lower sides of their 
bases; and this is a remarkable proof of the force 
exerted. 

As the tips of the cotyledons of Phalaris and Avena 
bend upwards through the action of apogeotropism 
before the basal part, and as these same tips when 
excited by a lateral light transmit some influence to 
the lower part, causing it to bend, we thought that 
the same rule might hold good with apogeotropism. 
Consequently, the tips of 7 cotyledons of Phalaris were 



512 MODIFIED CIRCUMNUTATION. CHAT X 

cut off fcr a length in three cases of '2 inch and in 
the four other cases of '14, '12, !, and '07 inch. But 
these cotyledons, after being extended horizontally, 
bowed themselves upwards as effectually as the un- 
mutilated specimens in the same pots, showing that 
sensitiveness to gravitation is not confined to their tips. 

GEOTROPISM. 

This movement is directly the reverse of apogeo- 
tropism. Many organs bend downwards through epi- 
nasty or apheliotropism or from their own weight ; but 
we have met with very few cases of a downward move- 
ment in sub-aerial organs due to geotropism. We 
shall, however, give one good instance in the following 
section, in the case of Trifolium subterraneum, and 
probably in that of Aracliis hypogsea. 

On the other hand, all roots which penetrate the 
ground (including the modified root-like petioles of 
Megarrhiza and Ipomcea leptopliylla) are guided in their 
downward course by geotropism ; and so are many 
aerial roots, whilst others, as those of the Ivy, appear 
to be indifferent to its action. In our first chapter the 
movements of the radicles of several seedlings were 
described. We may there see (Fig. 1) how a radicle 
of the cabbage, when pointing vertically upwards so 
as to be very little acted on by geotropism, circum- 
nutated ; and how another (Fig. 2) which was at first 
placed in an inclined position bowed itself downwards 
in a zigzag line, sometimes remaining stationary for a 
time. Two other radicles of the cabbage travelled 
downwards in almost rectilinear courses. A radicle of 
the bean placed upright (Fig. 20) made a great sweep 
and zigzagged; but as it sank downwards and was 
more strongly acted on by geotropism, it moved in an 



CHAP. X. GEOTROPISM. 513 

almost straight course. A radicle of Cucurbita, directed 
upwards (Fig. 26), also zigzagged at first, and de- 
scribed small loops ; it then moved in a straight line. 
Nearly the same result was observed with the radicles 
of Zea mays. But the best evidence of the intimate 
connection between circumnutation and geotropism 
was afforded by the radicles of Phaseolus, Vicia, and 
Quercus, and in a less degree by those of Zea and 
^sculus (see Figs. 18, 19, 21, 41, and 52) ; for when 
these were compelled to grow and slide down highly 
inclined surfaces of smoked glass, they left distinctly 
serpentine tracks. 

Tl/e Burying of Seed-capsules : Trifolium subterraneum. The 
flower-heads of this plant are remarkable from producing only 
3 or 4 perfect flowers, which are situated exteriorly. All the 
other many flowers abort, and are modified into rigid points, 
with a bundle of vessels running up their centres. After a time 
5 long, elastic, claw-like projections, which represent the divi- 
sions of the calyx, are developed on their summits. As soon as 
the perfect flowers wither they bend downwards, supposing the 
peduncle to stand upright, and they then closely surround its 
upper part. This movement is due to epinasty, as is likewise 
the case with the flowers of T. repens. The imperfect central 
flowers ultimately follow, one after the other, the same course. 
Whilst the perfect flowers are thus bending down, the whole 
peduncle curves downwards and increases much in length, 
until the flower-head reaches the ground. Vaucher * says that 
when the plant is so placed that the heads cannot soon reach 
the ground, the peduncles grow to the extraordinary length of 
from 6 to 9 inches. In whatever position the branches may be 
placed, the upper part of the peduncle at first bends vertically 
upwards through heliotropism ; but as soon as the flowers 
begin to wither the downward curvature of the whole peduncle 
commences. As this latter movement occurred in complete 
darkness, and with peduncles arising from upright and from 
dependent branches, it cannot be due to apheliotropism or to 
epinasty, but must be attributed to geotropism. Nil* teen 



'Hist Phys. cles Pluntes d'Europe,' torn. ii. 18tl, p. 106 



514 MODIFIED CIKCUMNUTATION CHAP. X. 

upright flower-heads, arising from branches in all sorts of posi- 
tions, on plants growing in a warm greenhouse, were marked 
with thread, and after 24 h. six of them were vertically depen- 
dent ; these therefore had travelled through 180 in this time. 
Ten were extended sub-horizontally, and these had moved 
through about 90. Three very young peduncles had as yet 
moved only a little downwards, but after an additional 24 h. 
were greatly inclined. 

At the time when the flower-heads reach the ground, the 
younger imperfect flowers in the centre are still pressed closely 
together, and form a conical projection ; whereas the perfect and 
imperfect flowers on the outside are upturned and closely sur- 
round the peduncle. They are thus adapted to offer as lit'tle 
resistance, as the case admits of, in penetrating the ground, 
though the diameter of the flower-head is still considerable. 
The means by which this penetration is effected will presently 
be described. The flower-heads are able to bury themselves in 
common garden mould, and easily in sand or in fine sifted 
cinders packed rather closely. The depth to which they pene- 
trated, measured from the surface to the base of the head, was 
between i and d inch, but in one case rather above 0'6 inch. 
With a plant kept in the house, a head partly buried itself in 
sand in 6 h. : after 3 days only the tips of the reflexed calyces 
were visible, and after 6 days the whole had disappeared. But 
with plants growing out of doors we believe, from casual obser- 
vations, that they bury themselves in a much shorter time. 

After the heads have buried themselves, the central aborted 
flowers increase considerably in length and rigidity, and 
become bleached. They gradually curve, one after the other, 
upwards or towards the peduncle, in the same manner as 
did the perfect flowers at first. In thus moving, tlie long claws 
on their summits carry with them some earth. Hence a flower- 
head which has been buried for a sufficient time, forms a rather 
large ball, consisting of the aborted flowers, separated from one 
another by earth, and surrounding the little "pods (the product 
of the perfect flowers) which lie close round the upper part of 
the peduncle. The calyces of the perfect and imperfect flowers 
are clothed with fJinple and multicellular hairs, which have the 
power of absorption; for when placed in a weak solution of 
carbonate of ammonia (2 gr. to 1 oz. of water) their proto- 
plasmic contents immediately became aggregated and afterwards 
displayed the usual slow movements. This clover general!} 



CHAP. X. GEOTROriSM. 515 

grows in dry soil, Imt whether the power of absorption by the 
hairs on the buried flower-heads is of any importance to them 
we do not know. Only a few of the flower-heads, which from 
their position are not able to reach the ground and bury them- 
selves, yield seeds ; whereas the buried ones never failed, as far 
as we observed, to produce as many seeds as there had been 
perfect flowers. 
We will now consider the movements of the peduncle whilst 



Fig. 190. 




Trifolium sulterrancum : downwar;'. movement of peduncle from 19 beneath 
the horizon to a nearly vertically dependent position, traced from 
1 I A.M. July 22nd to the morning of 25th. Glass filament fixed 
transversely across peduncle, at base of flower-head. 

curving down to the ground. "\Ve have seen in Chap. IV., 
Fi^r. 92, p. 225. that an upright young flower-head circumnu- 
tated conspicuously; and that this movement continued after 
the peduncle had begun to bend downwards. The same 
peduncle was observed when inclined at an angle of 19 abc e 
the horizon, and it circumnutated duriug two days. Another 




516 MODIFIED CIRCUMNUTATION. CHAP. X. 

which was already cvirved 86 beneath the horizon, was observed 
from 11 A.M. July 22nd to the 27th, by which latter date it 
had become vertically dependent. Its course during the first 
12 h. is shown in Fig. 190, and its position on the three 
succeeding mornings until the 25th, 
Fig. 191. when it was nearly vertical. During 

the first day the peduncle clearly 
circumnutated, for it moved 4 times 
down and 3 times up; and on each 
succeeding day, as it sank downwards, 
the same movement continued, but 
was only occasionally observed and 
was less strongly marked. It should 
Trlfolium'ubterraneum: cir- be stated that these peduncles were 
cumnutating movement of observed under a double skylight in 
peduncle, whilst the flower- the house, and that they generally 
head was burying itself in d Downwards very much more 

sand, with the reflexed tips 

of the calyx still visible ; slowly than those on plants growing 
traced from 8 A.M. July out of doors or in the greenhouse. 
2i;th to 9A.M. on 27th. The movement of another vertically 
versel Across * peduncle" dependent peduncle with the flower- 
'near flower- head. head standing half an inch above the 

ground, was traced, and again when 

it first touched the ground; in both cases irregular ellipses 
were described every 4 or 5 h. A peduncle on a plant which 
had been brought into the house, 
Fig. 192. moved from an upright into a ver- 

tically dependent position in a 
single day; and here the course 
during the first 12 h. was nearly 
straight, but with a few well-mark*. d 
. subterraneum : move- zigzags which betrayed the essential 
ment of same peduncle, with nature of the movement. Lastly,, 
flower-head completely buried the circurnnutation of a peduncle 
was traced during 51 h. whilst in 
the act of burying itself obliquely 
in a little heap of sand. After it had buried itself to such a 
depth that the tips of the sepals were alone visible, the above 
figure (Fig. 191) was traced during 25 h. When the flower- 
head had completely disappeared beneath the. sand, another 
tracing was made during 11 h. 45m. (Fig. 192); and here again 
that the peduncle was circuranututing. 




CHAP. X. GEOTROPISM. . 617 

Any one who will observe a flower-head burying itself, will be 
convinced that the rocking rrovement, due to the continued 
circumnutation of the peduncle, plays an important part in the 
act. Considering that the flower-heads are very light, that tho 
peduncles are long, thin, and flexible, and that they arise from 
flexible branches, it is incredible that an object as blunt as one 
of these flower-heads could penetrate the ground by means of 
the growing force of the peduncle, unless it were aided by the 
locking movement. After a flower-head has penetrated the 
ground to a small depth, another and efficient agency comes into 
play ; the central rigid aborted flowers, each terminating in five 
long claws, curve up towards the peduncle; and in doing so 
can hardly fail to drag the head down to a greater depth, aided 
as this action is by the circumnutating movement, which con- 
tinues after the flower-head has completely buried itself. The 
aborted flowers thus act something like the hands of the mole, 
which force the earth backwards and the body forwards. 

It is well known that the seed-capsules of various widely 
distinct plants either bury themselves in the ground, or are 
produced from imperfect flowers developed beneath the surface. 
Besides the present case, two other well-marked instances will 
be immediately given. It is probable that one chief good thus 
gained is the protection of the seeds from animals which prey on 
them. In the case of T. subterraneum, the seeds are not only 
concealed by being buried, but are likewise protected by being 
closely surrounded by the rigid, aborted flowers. We may the 
more confidently infer that protection is here aimed at, because 
the seeds of several species in this same genus are protected in 
other ways ;* namely, by the swelling and closure of the calyx, 
or by the persistence and bending down of the standard-petal, &c. 
But the most curious instance is that of T. ylobosum, in which 
the upper flowers are sterile, as in T. subterraneum, but are here 
developed into large brushes of hairs which envelop and protect 
the seed-bearing flowers. Nevertheless, in all these cases tho 
capsules, with their seeds, may profit, as Mr. T. Thiselton Dyer 
has remarked,t by their being kept somewhat damr and the 
advantage of such dampness perhaps throws light on the pre- 
sence of the absorbent hairs on the buried flower-heads of T. sub- 
tenuueum. According to Mr. Bentham, as quoted by Mr. Bycr, 



* Vancher, 'Hist. Phys. (lea t See his interesting article ia 
Hantes d'Europe,' torn. ii. p. 1 10. ' Nature,' April 4lh, 1878, p. 440 



518 . MODIFIED CIRCUMNUTATION. CHAP. X 

the prostrate habit of Hclianthemum prostratum " brings tiie 
capsules in contact with the surface of the ground, postpones 
their maturity, and so favours the seeds attaining a larger size." 
The capsules of Cyclamen and of Oxalis ucetosdla are only occa- 
sionally buried, and this only beneath dead leaves or moss. If 
it be an advantage to a plant that its capsules should be kept 
damp and cool by. being laid on the ground, we have in these 
latter cases the first step, from which the power of penetrating 
the ground, with the aid of the always present movement of 
circumnutation, might afterwards have been gained. 

Arachis hypagcea. The flowers which bury themselves, rise 
from stiff branches a few inches above the ground, and stand 
upright. After they have fallen off, the gynophore, that is the 
part which supports the ovarium, grows to a great length, even 
to 3 or 4 inches, and bends perpendicularly downwards. It 
resembles closely a peduncle, but has a smooth and pointed 
apex, which contains the ovules, and is at first not in the least 
enlarged. The apex after reaching the ground penetrates it, in 
one case observed by us to a depth of 1 inch, and in another 
to 0'7 inch. It there becomes developed into a large pod. 
Flowers which are seated too high on the plant for the gyno- 
phore to reach the ground are said* never to produce pods. 

The movement of a young gynophore, rather under an inch 
in length and vertically dependent, was traced during 46 h. by 
means of a glass filament (with sights) fixed transversely a 
little above the apex. It plainly circumnutated (Fig. 193) 
whilst increasing in length and growing downwards. It was 
then raised up, eo as to be extended almost horizontally, and 
the terminal part curved itself downwards, following a nearly 
straight course during 12 h., but with one attempt to circum- 
nutate, as shown in Fig. 194. After 24 h. it had become nearly 
vertical. Whether the exciting cause of the downward move- 
ment is geotropism or apheliotropism was not ascertained ; but 
probably it is not apheliotropism, as all the gynophores grew 
straight down towards the ground, whilst -the light in the hot- 
house entered from one side as well as from above. Another 
and older gynophore, the apex of which had nearly reached the 
ground, was observed during 3 days in the same manner as the 
first-mentioned short one ; and it was found to be always circum- 
nutating. During the first 34 h. it described a figure which 



Card. Chronicle,' 1857, p. 5G6. 



GEOTKOPISM. 



represented four ellipses. Lastly, a long gynophore, the apex of 
which had buried itself to the depth of about half an inch, ww 

Fig. 194 



Kg. 193 




ArarJtis hupogcea : circum- 
nutation of vertically 
dependent young gyno- 
phore, traced on a Ver- 
tical glass from 10 A.M. 
July 31st to 8 A.M. Aug. 
2nd. 



Arachis hypo'/cea: down- 
ward movement of same 
young gynophore, after 
being extended horizon- 
tally; traced on a vertical 
glass from 8.30 A.M. to 
8.30 P.M. Aug. 2nd. 



pulled up and extended horizontally : it quickly began to curve 
downwards in a zigzag line ; but on the following day the ter- 
' 



520 MODIFIED CIRCUMNUTATION. CBAP. X. 

minal bleached portion was a little shrivelled. As the gyno- 
phores are rigid and arise from stiff branches, and as they 
terminate in sharp smooth points, it is probable that they could 
penetrate the ground by the mere force of growth. But thia 
action must be aided by the circumnutating movement, for fine 
sand, kept moist, was pressed close round the apex of a gyno- 
phore which had reached the ground, and after a few hours it 
was surrounded by a narrow open crack. After three weeks 
this gynophore was uncovered, and the apex was found at a 
depth of rather above half an inch developed into a small, white, 
oval pod. 

Amphicarpcea monoica. This plant produces long thin shoots, 
which twine round a support and of course circumnutate. 
Early in the summer shorter shoots are produced from the 
lower parts of the plant, which grow perpendicularly downwards 
and penetrate the ground. One of these, terminating in a 
minute bud, was observed to bury itself in sand to a depth of 
0'2 inch in 24 h. It was lifted up and fixed in an inclined 
position about 25 beneath the horizon, being feebly illuminated 
from above. In this position it described two vertical ellipses 
in 24 h. ; but on the following day, when brought into the house, 
it circumnutated only a very little round the same spot. Other 
branches were seen to penetrate the ground, and were after- 
wards found running like roots beneath the surface for a length 
of nearly two inches, and they had grown thick. One of these, 
after thus running, had emerged into the air. How far circum- 
nutation aids these delicate branches in entering the ground wo 
do not know ; but the reflexed hairs with which they are clothed 
will assist in the work. This plant produces pods in the air, 
and others beneath the ground ; which differ greatly in appear- 
ance. Asa Gray says* that it is the imperfect flowers on the 
creeping branches near the base of the plant which produce the 
subterranean pods; these flowers, therefore, must bury them- 
selves like those of Arachis. But it may be suspected that the 
branches which were seen by us to penetrate the ground also 
produce subterranean flowers and pods. 

DlAGEOTKOPISM. 

Besides geotropism and apogeotropisin, there is, 
according to Frank, an allied form of movement, 

Manual of the Botany of the Northern United States,' 1856, p. 106. 



CHAP. X DIAGEOTKOPISM. 521 

namely, " transverse-geotropism," or diageotroptsm, as 
we may call it for the sake of matching our other 
terms. Under the influence of gravitation certain 
parts are excited to place themselves more or less 
transversely to the line of its action.* We made no 
observations on this subject, and will here only re- 
mark that the position of the secondary radicles of 
various plants, which extend horizontally or are a 
little inclined downwards, would probably be con- 
sidered by Frank as due to transverse-geotropism. 
As it has been shown in Chap. I. that the secondary 
radicles of Cucurbita made serpentine tracks on a 
smoked glass-plate, they clearly circumnutated, 
and there can hardly be a doubt that this holds 
good with other secondary radicles. It seems there- 
fore highly probable that they place themselves in 
their diageotropic position by means of modified 
circumnutation. 

Finally, we may conclude that the three kinds of 
movement which have now been described and which 
are excited by gravitation, consist of modified circum- 
nutation. Different parts or organs on the same plant, 
and the same part in different species, are thus excited 
to act in a widely different manner. We can see no 
reason why the attraction of gravity should directly 
modify the state of turgescence and subsequent growth 
of one part on the upper side and of another part on 
the lower side. We are therefore led to infer that both 
geotropic, apogeotropic, and diageotropic movements, 
the purpose of which we can generally understand, 



* rifving has lately described excellent instance of such niove- 
''Arbiiten des Bot. Institnts in menls in the ihizomes of certain 
,' 13. u. ISfeO, p. -ii&j au plants. 



522 MODIFIED CIIICUMNUTATION. CHAP X 

have been acquired for the advantage of the plant by 
the modification of the ever-present movement of 
oircumnutatioiu This, however, implies that gravi- 
tation produces some effect on the young tissues 
sufficient to serve as a guide to the plant. 



CHAP. XL SENSITIVENESS TO GBAVITAT1ON. 



CHAPTER XL 

LOCALISED SENSITIVENESS TO GRAVITATION, AND rra TBANSJIITY:D 
EFFECTS. 

General considerations "Vioia faba, effects of amputating the tips of 
the radicles Regeneration of tlie tips Effects of a short exposure 
of the tips to geotropic action and their subsequent amputation 
Efft-cts of amputating the tips obliquely Effects of cauterising the 
tips Effects of grease on the tips Pisum sativum, tips of radicks 
cauterised transversely, and on their upper and lower sides 
Phaseotus, cauterisation and grease on the tips Gossypium 
Cucurbita, tips cauterised transversely, and on their upper and 
lower sides Zea, tips cauterised Concluding remarks and 
summary of chapter Advantages of the sensibility to geotropism 
being localised in the tips of the radicles. 

CIESIELSKI states * that when the roots of Pisum, 
Lens and Vicia were extended horizontally with their 
tips cut off, they were not acted on by geotropism ; 
but some days afterwards, when a new root-cap and 
vegetative point had been formed, they bent them- 
selves perpendicularly downwards. He further states 
that if the tips are cut off, after the roots have been 
left extended horizontally for some little time, but 
before they have begun to bend downwards, they may 
be placed in any position, and yet will bend as if still 
acted on by geotropism ; and this shows that some 
influence had been already transmitted to the bending 
part from the tip before it was amputated. Sachs 
repeated these experiments ; he cut off a length of 
between -05 and 1 mm. (measured from the apex of the 



* ' Abwartskriimmung der Wurzel,' Inaug. Dissert, Breslau. 1871, 
p. 29. 



524 SENSITIVENESS TO GRAVITATION. CHAP. XL 

vegetative point) of the tips of the radicles of the 
bean (Vicia faba), and placed them horizontally or 
vertically in damp air, earth, and water, with the 
result that they became bowed in all sorts of direc- 
tions.* He therefore disbelieved in Ciesielski's con- 
clusions. But as we have seen with several plants 
that the tip of the radicle is sensitive to contact and 
to other irritants, and that it transmits some influence 
to the upper growing part causing it to bend, there 
seemed to us to "be no a priori improbability in 
Ciesielski's statements. We therefore determined to 
repeat his experiments, and to try others on several 
species by different methods. 

Vicia faba,. Radicles of this plant were extended horizontally 
either over water or with their lower surfaces just touching it. 
Their tips had previously been cut off, in a direction as accu- 
rately transverse as could be done, to different lengths, measured 
from the apex of the root-cap, and which will be specified in 
each case. Light was always excluded. We had previously 
tried hundreds of unmutilated radicles under similar circum- 
stances, and found that every one that was healthy became 
plainly geotropic in under 12 h. In the case of four radicles 
which had their tips cut off for a length of 1*5 mm., new root- 
caps and new vegetative points were re-formed after an interval 
of 3 days 20 h. ; and these when placed horizontally were acted 
on by geotropism. On some other occasions this regeneration 
of the tips and reacquired sensitiveness occurred within a some- 
what shorter time. Therefore, radicles having their tips 
amputated should be observed in from 12 to 48 h. after the 
operation. 

Four radicles were extended horizontally with their lower 
surfaces touching the water, and with their tips cut off for a 
length of only 0'5 mm. : after 23 h. three of them were still 
horizontal ; after 47 h. one of the three became fairly geotropic; 
and after 70 h. the other two showed a trace of this action. The 
fourth radicle was vertically geotropic after 23 h. ; but by an 



' Arbeiten dcs Bot. Instituts in Wiirzburg,' Heft. iii. 1873, p. 432. 



CHAP. XI. TKANSMITTED EFFECTS : VICIA. 525 

accident the root-cap alone and not the vegetative point was 
found to have been amputated ; so that this case formed no real 
exception and might have been excluded. 

Five radicles were extended horizontally like the last, and 
had their tips cut off for a length of 1 mm. ; after 22-23 h., four 
of them were still horizontal, and one was slightly geotropic ; 
after 48 h. the latter had become vertical ; a second was also 
somewhat geotropic ; two remained approximately horizontal ; 
and the last or fifth had grown in a disordered manner, for it 
was inclined upwards at an angle of 65 above the horizon. 

Fourteen radicles were extended horizon tally at a little height 
over the water with their tips cut off for a length of 1'5 mm. ; 
after 12 h. all were horizontal, whilst five control or standard 
specimens in the same jar were all bent greatly downwards. 
After' 24 h. several of the amputated radicles remained hori- 
zontal, but some showed a trace of geotropism, and one was 
plainly geotropic, for it was inclined at 40 beneath the horizon. 

Seven horizontally extended radicles from which the tips had 
been cut off for the unusual length of 2 mm. unfortunately were 
not looked at until 35 h. had elapsed ; three were still horizontal, 
but, to our surprise, four were more or less plainly geotropic. 

The radicles in the foregoing cases were measured before their 
tips were amputated, and in the course of 24 h. they had all 
increased greatly in length; but the measurements are not 
worth giving. It is of more importance that Sachs found that 
the rate of growth of the different parts of radicles with 
amputated tips was the same as with unmutilated ones. Alto- 
gether twenty-nine radicles were operated on in the manner 
above described, and of these only a few showed any geotropic 
curvature within 24 h. ; whereas radicles with unmutilated tips 
always became, as already stated, much bent down in less than 
half of this time. The part of the radicle which bends most lies 
at the distance of from 3 to 6 mm. from the tip, and as the 
bending part continues to grow after the operation, there does 
not seem any reason why it should not have been acted on by 
geotropism, unless its curvature depended on some influence 
transmitted from the tip. And we have clear evidence of such 
transmission in Ciesielski's experiments, which we repeated and 
extended in the following manner. 

Bsans were embedded in friable peat with the hilum down- 
wards, and after their radicles had grown perpendicularly down 
for a length of from to 1 inch, sixteen were selected which 



526 SENSITIVENESS TO GRAVITATION. CHAP. XI. 

were perfectly straight, and these were placed horizontally on 
the peat, being covered by a thin layer of it. They "were thus 
left for an average period of 1 h. 37 m. The tips were then cut 
off transversely for a length of 1'5 mm., and immediately after- 
wards they were embedded vertically in the peat. In this position 
geotropism would not tend to induce any curvature, but if some 
influence had already been transmitted from the tip to the part 
which bends most, we might expect that this part would become 
curved in the direction in which geotropism had previously 
acted; for it should be noted that these radicles being now 
destitute of their sensitive tips, would not be prevented by 
geotropism from curving in any direction. The result was that 
of the sixteen vertically embedded radicles, four continued for 
several days to grow straight downwards, whilst twelve became 
more or less bowed laterally. In two of the twelve, a trace of 
curvature was perceptible in 3 h. 30 m., counting from the time 
when they had first been laid horizontally ; and all twelve were 
plainly bowed in 6 h., and still more plainly in 9 h. In every 
one of them the curvature was directed towards the side which 
had been downwards whilst the radicles remained horizontal. 
The curvature extended for a length of from 5 to, in one in- 
stance, 8 mm., measured from the cut-off end. Of the twelve 
bowed radicles five became permanently bent into a right angle ; 
the other seven were at first much less bent, and their curvature 
generally decreased after 24 h., but did not wholly disappear. 
This decrease of curvature would naturally follow, if an ex- 
posure of only 1 h. 37 m. to geotropism, served to modify the 
turgescence of the cells, but not their subsequent growth to 
the full extent. The five radicles which were rectangularly 
bent became fixed in this position, and they continued to grow 
out horizontally in the peat for a length of about 1 inch during 
from 4 to 6 days. By this time new tips had been formed ; and 
it should be remarked that this regeneration occurred slower in 
the peat than. in water, owing perhaps to the radicles being 
often looked at and thus disturbed. After the tips had been 
regenerated, geotropism was able to act on them, so that they 
now became bowed vertically downwards. An accurate draw- 
ing (Fig. 195) is given on the opposite page of one of these five 
radicles, reduced to half the natural size. 

We next tried whether a shorter exposure to geotropism 
would suffice to produce an after-effect. Seven radicles were 
extended horizontally for an hour, instead of 1 h. 37 m. as in th 



CHAP. XL TRANSMITTED EFFECTS : VICIA. 



527 



Fig. 195. 



former trial ; and after their tips (1-5 nim. in length) hail been 
amputated, they were placed vertically in damp peat Of these, 
three were not in the least affected and continued for days to 
grow straight downwards. Four showed after 8 h. 30 m. a mere 
trace of curvature in the direction in which they had been acted 
on by geotropism; and in this respect they differed much from 
those which had been exposed for 
1 h. 37 m., for many of the latter 
were plainly curved in b' h. The 
curvature of one of these four 
radicles almost disappeared after 
21 h. In the second, the cur- 
vature increased during two days 
and then decreased. The third 
radicle became permanently bent, 
so that its terminal part made an 
angle of about 45 with its original 
vertical direction. The fourth 
radicle became horizontal. These 
two latter radicles continued 
during two more days to grow 
in the peat in the same directions, 
that is. at an angle of 45 
neath the horizon and horizon- 
tally. By the fourth morning new 
tips had l>een re-formed, and now 
geotropism was able to act on 
them again, and they became 
bent perpendicularly downwards, 
exactly as in the case of the 
five radicles described in the 
last paragraph and as is shown in 
the figure (Fig. 195) here given. 

lastly, live other radicles were similarly treated, but were ex- 
posed to geotropism during only 45 m. After 8 h. 30 m. only 
one was doubtfully affected; after 'J-t h. two were just per- 
ceptibly curved towards the side which had been acted on by 
geotropism ; after -18 h. the one first mentioned had a radius of 
curvature of 60 mm. That this curvature was due to the action 
of geotropism during the horizontal ]>ositiou of the radicle, waa 
shown after 4 days, when a in w tip hail Iven reformed, for it 
then grew perpendicularly downwards. We Kara from this 




Vicia faba : radicle, rectangularly 
bent at A. after the amputation 
of the tip, due to the previous 
influence of geotropi>ra. L, si.Je 
of bean which lay on the peat, 
whilst geotropism acted on the 
radicle. A, point of chief cur- 
vature of the rad