Amphicarpoea monoica (Tribe 8).—The leaflets sink down vertically at

night, and the petioles likewise fall considerably. A petiole, which

was carefully observed, stood during the day 25° above the horizon and

at night 32° below it; it therefore fell 57°. A filament was fixed

transversely across the terminal leaflet of a fine young leaf (2 1/4

inches in length including the

petiole), and the movement of the whole leaf was traced on a vertical

glass. This was a bad plan in some respects, because the rotation of

the leaflet, independently of its rising or falling, raised and

depressed the filament; but it was the best plan for our special

purpose of observing whether the leaf moved much after it had gone to

sleep. The plant had twined closely round a thin stick, so that the

circumnutation of the stem was prevented. The movement of the leaf was

traced during 48 h., from 9 A.M. July 10th to 9 A.M. July 12th. In the

figure given (Fig. 151) we see how complicated its course was on both

days: during the second day it changed its course greatly 13 times. The

leaflets began to go to sleep a little after 6 P.M., and by 7.15 P.M.

hung vertically down and were completely asleep; but on both nights

they continued to move from 7.15 P.M. to 10.40 and 10.50 P.M., quite as

much as during the day; and this was the point which we wished to

ascertain. We see in the figure that the great sinking movement late in

the evening does not differ essentially from the circumnutation during

the day.

Fig. 151. Amphicarpoea monoica: circumnutation and nyctitropic movement

of leaf during 48 h.; its apex 9 inches from the vertical glass. Figure

reduced to one-third of original scale. Plant illuminated from above;

temp 17½°–18½° C.

Glycine hispida (Tribe 8).—The three leaflets sink vertically down at

night.

Erythrina (Tribe 8).—Five species were observed, and the leaflets of

all sank vertically down at night; with E. caffra and with a second

unnamed species, the petioles at the same time rose slightly. The

movements of the terminal leaflet of E. crista-galli (with the main

petiole secured to a stick) were traced from 6.40 A.M. June 8th, to 8

A.M. on the 10th. In order to observe the nyctitropic movements of this

plant, it is necessary that it should have grown in a warm greenhouse,

for out of doors in our climate it does not sleep. We see in the

tracing (Fig. 152) that the leaflet oscillated twice up and down

between early morning and noon; it then fell greatly, afterwards rising

till 3 P.M. At this latter hour the great nocturnal fall commenced. On

the second day (of which the tracing is not given) there was exactly

the same double oscillation before noon, but only a very small one in

the afternoon. On the third morning the leaflet moved laterally, which

was due to its beginning to assume an oblique position, as seems

invariably to occur with the leaflets of this species as they grow old.

On both nights after the leaflets were asleep and hung vertically down,

they continued to move a little both up and down, and from side to

side.

Erythrina caffra.—A filament was fixed transversely across

a terminal leaflet, as we wished to observe its movements when asleep.

The plant was placed in the morning of June 10th under a skylight,

where the light was not bright; and we do not know whether it was owing

to this cause or to the plant having been disturbed, but the leaflet

hung vertically down all day; nevertheless it circumnutated in this

position, describing a figure which represented two irregular ellipses.

On the next day it circumnutated in a greater degree, describing four

irregular ellipses, and by 3 P.M. had risen into a horizontal position.

By 7.15 P.M. it was asleep and vertically dependent, but continued to

circumnutate as long as observed, until 11 P.M.

Fig. 152. Erythrina crista-galli: circumnutation and nyctitropic

movement of terminal leaflet, 3 3/4 inches in length, traced during 25

h.; apex of leaf 3½ inches from the vertical glass. Figure reduced to

one-half of original scale. Plant illuminated from above; temp.

17½°–18½° C.

Erythrina corallodendron.—The movements of a terminal leaflet were

traced. During the second day it oscillated four times up and four

times down between 8 A.M. and 4 P.M., after which hour the great

nocturnal fall commenced. On the third day the movement was equally

great in amplitude, but was remarkably simple, for the leaflet rose in

an almost perfectly straight line from 6.50 A.M. to 3 P.M., and then

sank down in an equally straight line until vertically dependent and

asleep.

Apios tuberosa (Tribe 8).—The leaflets sink vertically down at night.

Phaseolus vulgaris (Tribe 8).—The leaflets likewise sink vertically

down at night. In the greenhouse the petiole of a young leaf rose 16°,

and that of an older leaf 10° at night. With plants growing out of

doors the leaflets apparently do not sleep until somewhat late in the

season, for on the nights of July 11th and 12th none of them were

asleep; whereas on the night of August 15th the same plants had most of

their leaflets vertically dependent and asleep. With Ph. caracalla and

Hernandesii, the primary unifoliate leaves and the leaflets of the

secondary trifoliate leaves sink vertically down at night. This holds

good with the secondary trifoliate leaves of Ph. Roxburghii, but it is

remarkable that the primary unifoliate leaves which are much elongated,

rise at night from about 20° to about 60° above the horizon. With older

seedlings, however, having the secondary leaves just developed, the

primary leaves stand in the middle of the day horizontally, or are

deflected a little beneath the horizon. In one such case the primary

leaves rose from 26° beneath the horizon at noon, to 20° above it at 10

P.M.; whilst at this same hour the leaflets of the secondary leaves

were vertically dependent. Here, then, we have the extraordinary case

of the primary and secondary leaves on the same plant moving at the

same time in opposite directions.

We have now seen that the leaflets in the six genera of Phaseoleae

observed by us (with the exception of the primary leaves of Phaseolus

Roxburghii) all sleep in the same manner, namely, by sinking vertically

down. The movements of the petioles were observed in only three of

these genera. They rose in Centrosema and Phaseolus, and sunk in

Amphicarpæa.

Sophora chrysophylla (Tribe 10).—The leaflets rise at night, and are at

the same time directed towards the apex of the leaf, as in Mimosa

pudica.

Caesalpinia, Hoematoxylon, Gleditschia, Poinciana.—The leaflets of two

species of Caesalpinia (Tribe 13) rose at night. With Haematoxylon

Campechianum (Tribe 13) the leaflets move forwards at night, so that

their midribs stand parallel to the petiole, and their now vertical

lower surfaces are turned outwards (Fig. 153). The petiole sinks a

little. In Gleditschia, if we understand correctly Duchartre’s

description, and in

Poinciana Gilliesii (both belonging to Tribe 13), the leaves behave in

the same manner.

Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B,

branch with leaves asleep, reduced to two-thirds of natural scale.

Cassia (Tribe 14).—The nyctitropic movements of the leaves in many

species in this genus are closely alike, and are highly complex. They

were first briefly described by Linnæus, and since by Duchartre. Our

observations were made chiefly on C. floribunda[18] and corymbosa, but

several other species were casually observed. The horizontally extended

leaflets sink down vertically at night; but not simply, as in so many

other genera, for each leaflet rotates on its own axis, so that its

lower surface faces outwards. The upper surfaces of the opposite

leaflets are thus brought into contact with one another beneath the

petiole, and are well protected (Fig. 154). The rotation and other

movements are effected by means of a well-developed pulvinus at the

base of each leaflet, as could be plainly seen when a straight narrow

black line had been painted along it during the day. The two terminal

leaflets in the daytime include rather less than a right angle; but

their divergence increases greatly whilst they

sink downwards and rotate, so that they stand laterally at night, as

may be seen in the figure. Moreover, they move somewhat backwards, so

as to point towards the base of the petiole. In one instance we found

that the midrib of a terminal leaflet formed at night an angle of 36°,

with a line dropped

perpendicularly from the end of the petiole. The second pair of

leaflets likewise moves a little backwards, but less than the terminal

pair; and the third pair moves vertically downwards, or even a little

forwards. Thus all the leaflets, in those species which bear only 3 or

4 pairs, tend to form a single packet, with their upper surfaces in

contact, and their lower surfaces turned outwards. Lastly, the main

petiole rises at night, but with leaves of different ages to very

different degrees, namely some rose through an angle of only 12°, and

others as much as 41°.

[18] I am informed by Mr. Dyer that Mr. Bentham believes that C.

floribunda (a common greenhouse bush) is a hybrid raised in France,

and that it comes very near to C. laevigata. It is no doubt the same

as the form described by Lindley (‘Bot. Reg.,’ Tab. 1422) as C.

Herbertiana.

Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at

night. Both figures copied from photographs.

Cassia calliantha.—The leaves bear a large number of leaflets, which

move at night in nearly the same manner as just described; but the

petioles apparently do not rise, and one which was carefully observed

certainly fell 3°. Cassia pubescens.—The chief difference in the

nyctitropic movements of this species, compared with those of the

former species, consists in the leaflets not rotating nearly so much;

therefore their lower surfaces face but little outwards at night. The

petioles, which during the day are inclined only a little above the

horizon, rise at night in a remarkable manner, and stand nearly or

quite vertically. This, together with the dependent position of the

leaflets, makes the whole plant wonderfully compact at night. In the

two foregoing figures, copied from photographs, the same plant is

represented awake and asleep (Fig. 155), and we see how different is

its appearance.

Fig. 155. Cassia pubescens: A, upper part of plant during the day; B,

same plant at night. Figures reduced from photographs.

Cassia mimosoides.—At night the numerous leaflets on each leaf rotate

on their axes, and their tips move towards the apex of the leaf; they

thus become imbricated with their lower surfaces directed upwards, and

with their midribs almost parallel to the petiole. Consequently, this

species differs from all the others seen by us, with the exception of

the following one, in the leaflets not sinking down at night. A

petiole, the movement of which was measured, rose 8° at night.

Cassia Barclayana.—The leaflets of this Australian species are

numerous, very narrow, and almost linear. At night they rise up a

little, and also move towards the apex of the leaf. For instance, two

opposite leaflets which diverged from one another during the day at an

angle of 104°, diverted at night only 72°; so that each had risen 16°

above its diurnal position. The petiole of a young leaf rose at night

34°, and that of an older leaf 19°. Owing to the slight movement of the

leaflets and the considerable movement of the petiole, the bush

presents a different appearance at night to what it does by day; yet

the leaves can hardly be said to sleep.

The circumnutating movements of the leaves of C. floribunda,

calliantha, and pubescens were observed, each during three or four

days; they were essentially alike, those of the last-named species

being the simplest. The petiole of C. floribunda was secured to a stick

at the base of the two terminal leaflets, and a filament was fixed

along the midrib of one of them. Its movements were traced from 1 P.M.

on August 13th to 8.30 A.M. 17th; but those during the last 2 h. are

alone given in Fig. 156. From 8 A.M. on each day (by which hour the

leaf had assumed its diurnal position) to 2 or 3 P.M., it either

zigzagged or circumnutated over nearly the same small space; at between

2 and 3 P.M. the great evening fall commenced. The lines representing

this fall and the early morning rise are oblique, owing to the peculiar

manner in which the leaflets sleep, as already described. After the

leaflet was asleep at 6 P.M., and whilst the glass filament hung

perpendicularly down, the movement of its apex was traced until 10.30

P.M.; and during this whole time it swayed from side to side,

completing more than one ellipse.

Fig 156. Cassia floribunda: circumnutation and nyctitropic movement of

a terminal leaflet (1 5/6 inch in length) traced from 8.30 A.M. to same

hour on following morning. Apex of leaflet 5½ inches from the vertical

glass. Main petiole 3 3/4 inches long. Temp. 16°–17½° C. Figure reduced

to one-half of the original scale.

Bauhinia (Tribe 15).—The nyctitropic movements of four species were

alike, and were highly peculiar. A plant raised from seed sent us from

South Brazil by Fritz Müller, was more especially observed. The leaves

are large and deeply notched at their ends. At night the two halves

rise up and close completely together, like the opposite leaflets of

many Leguminosae. With very young plants the petioles rise considerably

at the same time; one, which was inclined at noon 45° above the

horizon, at night stood at 75°; it thus rose 30°; another rose 34°.

Whilst the two halves of the leaf are closing, the midrib at first

sinks vertically downwards and afterwards bends backwards, so as to

pass close along one side of its own upwardly inclined petiole; the

midrib being thus directed towards the stem or axis of the plant. The

angle which the midrib formed with the horizon was measured in one case

at different hours: at noon it stood horizontally; late in the evening

it depended vertically; then rose to the opposite side, and at 10.15

P.M. stood at only 27° beneath the horizon, being directed towards the

stem. It had thus travelled through 153°.

Owing to this movement—to the leaves being folded—and to the petioles

rising, the whole plant is as much more compact at night than during

the day, as a fastigiate Lombardy poplar is compared with any other

species of poplar. It is remarkable that when our plants had grown a

little older, viz., to a height of 2 or 3 feet, the petioles did not

rise at night, and the midribs of the folded leaves were no longer bent

back along one side of the petiole. We have noticed in some other

genera that the petioles of very young plants rise much more at night

than do those of older plants.

Tamarindus Indica (Tribe 16).—The leaflets approach or meet each other

at night, and are all directed towards the apex of the leaf. They thus

become imbricated with their midribs parallel to the petiole. The

movement is closely similar to that of Haematoxylon (see Fig. 153), but

more striking from the greater number of the leaflets.

Adenanthera, Prosopis, and Neptunia (Tribe 20).—With Adenanthera

pavonia the leaflets turn edgeways and sink at night. In Prosopis they

turn upwards. With Neptunia oleracea the leaflets on the opposite sides

of the same pinna come into contact at night and are directed forwards.

The pinnae themselves move downwards, and at the same time backwards or

towards the stem of the plant. The main petiole rises.

Mimosa pudica (Tribe 20).—This plant has been the subject of

innumerable observations; but there are some points in relation to our

subject which have not been sufficiently attended to. At night, as is

well known, the opposite leaflets come into contact and point towards

the apex of the leaf; they thus become neatly imbricated with their

upper surfaces protected. The four pinnae also approach each other

closely, and the whole leaf is thus rendered very compact. The main

petiole sinks downwards during the day till late in the evening, and

rises until very early in the morning. The stem is continually

circumnutating at a rapid rate, though not to a wide extent. Some very

young plants, kept in darkness, were observed during two days, and

although subjected to a rather low temperature of 57°–59° F., the stem

of one described four small ellipses in the course of 12 h. We shall

immediately see that the main petiole is likewise continually

circumnutating, as is each separate pinna and each separate leaflet.

Therefore, if the movement of the apex of any one leaflet were to be

traced, the course described would be compounded of the movements of

four separate parts.

A filament had been fixed on the previous evening, longitudinally to

the main petiole of a nearly full-grown, highly-sensitive leaf (four

inches in length), the stem having been secured to a stick at its base;

and a tracing was made on a vertical glass in the hot-house under a

high temperature. In the figure given (Fig. 157), the first dot was

made at 8.30 A.M. August 2nd, and the last at 7 P.M. on the 3rd. During

12 h. on the first day the petiole moved thrice downwards and twice

upwards. Within the same length of time on the second day, it moved

five times downwards and four times upwards. As the ascending and

descending lines do not coincide, the petiole manifestly circumnutates;

the great evening fall and nocturnal rise being an exaggeration of one

of the circumnutations. It should, however, be observed that the

petiole fell much lower down in the evenings than could be seen on the

vertical glass or is represented in the diagram. After 7 P.M. on the

3rd (when the last dot in Fig. 157 was made) the pot was carried into a

bed-room, and the petiole was found at 12.50 A.M. (i.e. after midnight)

standing almost upright, and much more highly inclined than it was at

10.40 P.M. When observed again at 4 A.M. it had begun to fall, and

continued falling till 6.15 A.M., after which hour it zigzagged and

again circumnutated. Similar observations were made on another petiole,

with nearly the same result.

Fig. 157 Mimosa pudica: circumnutation and nyctitropic movement of main

petiole, traced during 34 h. 30 m.

On two other occasions the movement of the main petiole

was observed every two or three minutes, the plants being kept at a

rather high temperature, viz., on the first occasion at 77°–81° F., and

the filament then described 2½ ellipses in 69 m. On the second

occasion, when the temperature was 81°–86° F., it made rather more than

3 ellipses in 67 m. therefore, Fig. 157, though now sufficiently

complex, would have been incomparably more so, if dots had been made on

the glass every 2 or 3 minutes, instead of every hour or half-hour.

Although the main petiole is continually and rapidly describing small

ellipses during the day, yet after the great nocturnal rising movement

has commenced, if dots are made every 2 or 3 minutes, as was done for

an hour between 9.30 and 10.30 P.M. (temp. 84° F.), and the dots are

then joined, an almost absolutely straight line is the result.

To show that the movement of the petiole is in all probability due to

the varying turgescence of the pulvinus, and not to growth (in

accordance with the conclusions of Pfeffer), a very old leaf, with some

of its leaflets yellowish and hardly at all sensitive, was selected for

observation, and the plant was kept at the highly favourable temp. of

80° F. The petiole fell from 8 A.M. till 10.15 A.M., it then rose a

little in a somewhat zigzag line, often remaining stationary, till 5

P.M., when the great evening fall commenced, which was continued till

at least 10 P.M. By 7 A.M. on the following morning it had risen to the

same level as on the previous morning, and then descended in a zigzag

line. But from 10.30 A.M. till 4.15 P.M. it remained almost motionless,

all power of movement being now lost. The petiole, therefore, of this

very old leaf, which must have long ceased growing, moved periodically;

but instead of circumnutating several times during the day, it moved

only twice down and twice up in the course of 24 h., with the ascending

and descending lines not coincident.

It has already been stated that the pinnae move independently of the

main petiole. The petiole of a leaf was fixed to a cork support, close

to the point whence the four pinnae diverge, with a short fine filament

cemented longitudinally to one of the two terminal pinnae, and a

graduated semicircle was placed close beneath it. By looking vertically

down, its angular or lateral movements could be measured with accuracy.

Between noon and 4.15 P.M. the pinna changed its position to one side

by only 7°; but not continuously in the same direction, as it moved

four times to one side, and three times to the opposite side,

in one instance to the extent of 16°. This pinna, therefore

circumnutated. Later in the evening the four pinnae approach each

other, and the one which was observed moved inwards 59° between noon

and 6.45 P.M. Ten observations were made in the course of 2 h. 20 m.

(at average intervals of 14 m.), between 4.25 and 6.45 P.M.; and there

was now, when the leaf was going to sleep, no swaying from side to

side, but a steady inward movement. Here therefore there is in the

evening the same conversion of a circumnutating into a steady movement

in one direction, as in the case of the main petiole.

It has also been stated that each separate leaflet circumnutates. A

pinna was cemented with shellac on the summit of a little stick driven

firmly into the ground, immediately beneath a pair of leaflets, to the

midribs of both of which excessively fine glass filaments were

attached. This treatment did not injure the leaflets, for they went to

sleep in the usual manner, and long retained their sensitiveness. the

movements of one of them were traced during 49 h., as shown in Fig.

158. On the first day the leaflet sank down till 11.30 A.M., and then

rose till late in the evening in a zigzag line, indicating

circumnutation. On the second day, when more accustomed to its new

state, it oscillated twice up and twice down during the 24 h. This

plant was subjected to a rather low temperature, viz., 62°–64° F.; had

it been kept warmer, no doubt the movements of the leaflet would have

been much more rapid and complicated. It may be seen in the diagram

that the ascending and descending lines do not coincide; but the large

amount of lateral movement in the evening is the result of the leaflets

bending towards the apex of the leaf when going to sleep. Another

leaflet was casually observed, and found to be continually

circumnutating during the same length of time.

The circumnutation of the leaves is not destroyed by their being

subjected to moderately long continued darkness; but the proper

periodicity of their movements is lost. Some very young seedlings were

kept during two days in the dark (temp. 57°–59° F.) except when the

circumnutation of their stems was occasionally observed; and on the

evening of the second day the leaflets did not fully and properly go to

sleep. The pot was then placed for three days in a dark cupboard, under

nearly the same temperature, and at the close of this period the

leaflets showed no signs of sleeping, and were only slightly sensitive

to a touch. On the following day the stem was cemented to a

stick, and the movements of two leaves were traced on a vertical glass

during 72 h. The plants were still kept in the dark, excepting that at

each observation, which lasted 3 or 4 minutes, they were illuminated by

two candles. On the third day the leaflets still exhibited a vestige of

sensitiveness when forcibly pressed, but in the evening they showed no

signs of sleep. Nevertheless, their petioles continued to circumnutate

distinctly,

although the proper order of their movements in relation to the day and

night was wholly lost. Thus, one leaf descended during the first two

nights (i.e. between 10 P.M. and 7 A.M. next morning) instead of

ascending, and on the third night it moved chiefly in a lateral

direction. The second leaf behaved in an equally abnormal manner,

moving laterally during the first night, descending greatly during the

second, and ascending to an unusual height during the third night.

Fig 158. Mimosa pudica: circumnutation and nyctitropic movement of a

leaflet (with pinna secured), traced on a vertical glass, from 8 A.M.

Sept. 14th to 9 A.M. 16th.

With plants kept at a high temperature and exposed to the light, the

most rapid circumnutating movement of the apex of a leaf which was

observed, amounted to 1/500 of an inch in one second; and this would

have equalled 1/8 of an inch in a minute, had not the leaf occasionally

stood still. The actual distance travelled by the apex (as ascertained

by a measure placed close to the leaf) was on one occasion nearly 3/4

of an inch in a vertical direction in 15 m.; and on another occasion

5/8 of an inch in 60 m.; but there was also some lateral movement.

Mimosa albida.[19]—The leaves of this plant, one of which is here

figured (Fig. 159) reduced to 2/3 of the natural size, present some

interesting peculiarities. It consists of a long petiole bearing only

two pinnae (here represented as rather more divergent than is usual),

each with two pairs of leaflets. But the inner

basal leaflets are greatly reduced in size, owing probably to the want

of space for their full development, so that they may be considered as

almost rudimentary. They vary somewhat in size, and both occasionally

disappear, or only one. Nevertheless, they are not in the least

rudimentary in function, for they are sensitive, extremely heliotropic,

circumnutate at nearly the same rate as the fully developed leaflets,

and assume when asleep exactly the same position. With M. pudica the

inner leaflets at the base and between the pinnae are likewise much

shortened and obliquely truncated; this fact was well seen in some

seedlings of M. pudica, in which the third leaf above the cotyledons

bore only two pinnae, each with only 3 or 4 pairs of leaflets, of which

the inner basal one was less than half as long as its fellow; so that

the whole leaf resembled pretty closely that of M. albida. In this

latter species the main petiole terminates in a little point, and on

each side of this there is a pair of minute, flattened, lancet-shaped

projections, hairy on their margins, which drop off and disappear soon

after the leaf is fully developed. There can hardly be a doubt that

these little projections are the last and fugacious representatives of

an additional pair of leaflets to each pinna; for the outer one is

twice as broad as the inner one, and a little longer, viz. 7/100 of an

inch, whilst the inner one is only 5/100–6/100 long. Now if the basal

pair of leaflets of the existing leaves were to become rudimentary, we

should expect that the rudiments would still exhibit some trace of

their present great inequality of size. The conclusion that the pinnae

of the parent-form of M. albida possessed at least three pairs of

leaflets, instead of, as at present, only two, is supported by the

structure of the first true leaf; for this consists of a simple

petiole, often bearing three pairs of leaflets. This latter fact, as

well as the presence of the rudiments, both lead to the conclusion that

M. albida is descended from a form the leaves of which bore more than

two pairs of leaflets. The second leaf above the cotyledons resembles

in all respects the leaves on fully developed plants.

[19] Mr. Thiselton Dyer informs us that this Peruvian plant (which was

sent to us from Kew) is considered by Mr. Bentham (‘Trans. Linn.

Soc.,’ vol. xxx. p. 390) to be “the species or variety which most

commonly represents the M. sensitiva of our gardens.”

Fig. 159. Mimosa albida: leaf seen from vertically above.

When the leaves go to sleep, each leaflet twists half round, so as to

present its edge to the zenith, and comes into close contact with its

fellow. The pinnae also approach each other closely, so that the four

terminal leaflets come together. The large basal leaflets (with the

little rudimentary ones in contact with them) move inwards and

forwards, so as to embrace the outside of the united terminal leaflets,

and thus all eight leaflets

(the rudimentary ones included) form together a single vertical packet.

The two pinnae at the same time that they approach each other sink

downwards, and thus instead of extending horizontally in the same line

with the main petiole, as during the day, they depend at night at about

45°, or even at a greater angle, beneath the horizon. The movement of

the main petiole seems to be variable; we have seen it in the evening

27° lower than during the day; but sometimes in nearly the same

position. Nevertheless, a sinking movement in the evening and a rising

one during the night is probably the normal course, for this was

well-marked in the petiole of the first-formed true leaf.

The circumnutation of the main petiole of a young leaf was traced

during 2 3/4 days, and was considerable in extent, but less complex

than that of M. pudica. The movement was much more lateral than is

usual with circumnutating leaves, and this was the sole peculiarity

which it presented. The apex of one of the terminal leaflets was seen

under the microscope to travel 1/50 of an inch in 3 minutes.

Mimosa marginata.—The opposite leaflets rise up and approach each other

at night, but do not come into close contact, except in the case of

very young leaflets on vigorous shoots. Full-grown leaflets

circumnutate during the day slowly and on a small scale.

Schrankia uncinata (Tribe 20).—A leaf consists of two or three pairs of

pinnae, each bearing many small leaflets. These, when the plant is

asleep, are directed forwards and become imbricated. The angle between

the two terminal pinnae was diminished at night, in one case by 15°;

and they sank almost vertically downwards. The hinder pairs of pinnae

likewise sink downwards, but do not converge, that is, move towards the

apex of the leaf. The main petiole does not become depressed, at least

during the evening. In this latter respect, as well as in the sinking

of the pinnae, there is a marked difference between the nyctitropic

movements of the present plant and of Mimosa pudica. It should,

however, be added that our specimen was not in a very vigorous

condition. The pinnae of Schrankia aculeata also sink at night.

Acacia Farnesiana (Tribe 22).—The different appearance presented by a

bush of this plant when asleep and awake is wonderful. The same leaf in

the two states is shown in the following figure (Fig. 160). The

leaflets move towards the apex of the pinna and become imbricated, and

the pinnae then look like bits of dangling string. The following

remarks and measurements

do not fully apply to the small leaf here figured. The pinnae move

forwards and at the same time sink downwards, whilst the main petiole

rises considerably. With respect to the degree of movement: the two

terminal pinnae of one specimen formed together an angle of 100° during

the day, and at night of only 38°, so each had moved 31° forwards. The

penultimate pinnae during the day formed together an angle of 180°,

that is, they stood in a straight line opposite one another, and at

night each had moved 65° forwards. The basal pair of pinnae were

directed during the day, each about 21° backwards, and at night 38°

forwards, so each had moved 59° forwards. But the pinnae at the same

time sink greatly, and sometimes hang almost perpendicularly downwards.

The main petiole, on the other hand, rises much: by 8.30 P.M. one stood

34° higher than at noon, and by 6.40 A.M. on the following morning it

was still higher by 10°; shortly after this hour the diurnal sinking

movement commenced. The course of a nearly full-grown leaf was traced

during 14 h.; it was strongly zigzag, and apparently

represented five ellipses, with their longer axes differently directed.

Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf

at night.

Albizzia lophantha (Tribe 23).—The leaflets at night come into contact

with one another, and are directed towards the apex of the pinna. The

pinnae approach one another, but remain in the same plane as during the

day; and in this respect they differ much from those of the above

Schrankia and Acacia. The main petiole rises but little. The

first-formed leaf above the cotyledons bore 11 leaflets on each side,

and these slept like those on the subsequently formed leaves; but the

petiole of this first leaf was curved downwards during the day and at

night straightened itself, so that the chord of its arc then stood 16°

higher than in the day-time.

Melaleuca ericaefolia (Myrtaceae).—According to Bouché (‘Bot. Zeit.,’

1874, p. 359) the leaves sleep at night, in nearly the same manner as

those of certain species of Pimelia.

Œnothera mollissima (Onagrarieae).—According to Linnæus (‘Somnus

Plantarum’), the leaves rise up vertically at night.

Passiflora gracilis (Passifloracae).—The young leaves sleep by their

blades hanging vertically downwards, and the whole length of the

petiole then becomes somewhat curved downwards. Externally no trace of

a pulvinus can be seen. The petiole of the uppermost leaf on a young

shoot stood at 10.45 A.M. at 33° above the horizon; and at 10.30 P.M.,

when the blade was vertically dependent, at only 15°, so the petiole

had fallen 18°. That of the next older leaf fell only 7°. From some

unknown cause the leaves do not always sleep properly. The stem of a

plant, which had stood for some time before a north-east window, was

secured to a stick at the base of a young leaf, the blade of which was

inclined at 40° below the horizon. From its position the leaf had to be

viewed obliquely, consequently the vertically ascending and descending

movements appeared when traced oblique. On the first day (Oct. 12th)

the leaf descended in a zigzag line until late in the evening; and by

8.15 A.M. on the 13th had risen to nearly the same level as on the

previous morning. A new tracing was now begun (Fig. 161). The leaf

continued to rise until 8.50 A.M., then moved a little to the right,

and afterwards descended. Between 11 A.M. and 5 P.M. it circumnutated,

and after the latter hour the great nocturnal fall commenced. At 7.15

P.M. it depended vertically. The dotted line ought to have been

prolonged much lower down in the figure. By 6.50 A.M. on the following

morning (14th) the

leaf had risen greatly, and continued to rise till 7.50 A.M., after

which hour it redescended. It should be observed that the lines traced

on this second morning would have coincided with and confused those

previously traced, had not the pot been slided a very little to the

left. In the evening (14th) a mark was placed behind the filament

attached to the apex of the leaf, and its movement was carefully traced

from 5 P.M. to 10.15 P.M. Between 5 and 7.15 P.M. the leaf descended in

a straight line, and at the latter hour it appeared vertically

dependent. But between 7.15 and 10.15 P.M. the line consisted of a

succession of steps, the cause of which we could not understand; it

was, however, manifest that the movement was no longer a simple

descending one.

Fig. 161. Passiflora gracilis: circumnutation and nyctitropic movement

of leaf, traced on vertical glass, from 8.20 A.M. Oct. 13th to 10 A.M.

14th. Figure reduced to two-thirds of original scale.

Siegesbeckia orientalis (Compositæ).—Some seedlings were raised in the

middle of winter and kept in the hot-house; they flowered, but did not

grow well, and their leaves never showed any signs of sleep. The leaves

on other seedlings raised in May were horizontal at noon (June 22nd),

and depended at a

considerable angle beneath the horizon at 10 P.M. In the case of four

youngish leaves which were from 2 to 2½ inches in length, these angles

were found to be 50°, 56°, 60°, and 65°. At the end of August when the

plants had grown to a height of 10 to 11 inches, the younger leaves

were so much curved downwards at night that they might truly be said to

be asleep. This is one of the species which must be well illuminated

during the day in order to sleep, for on two occasions when plants were

kept all day in a room with north-east windows, the leaves did not

sleep at night. The same cause probably accounts for the leaves on our

seedlings raised in the dead of the winter not sleeping. Professor

Pfeffer informs us that the leaves of another species (S. Jorullensis

?) hang vertically down at night.

Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day,

and asleep at night. Figures copied from photographs, and reduced.

Ipomœa caerulea and purpurea (Convolvulaceae).—The leaves on very young

plants, a foot or two in height, are depressed at night to between 68°

and 80° beneath the horizon; and some hang quite vertically downwards.

On the following morning they again rise into a horizontal position.

The petioles become at night downwardly curved, either through their

entire length or in the upper part alone; and this apparently causes

the depression of the blade. It seems necessary that the leaves should

be well illuminated during the day in order to sleep, for those which

stood on the back of a plant before a north-east window did not sleep.

Nicotiana tabacum (var. Virginian) and glauca (Solaneae).—The young

leaves of both these species sleep by bending vertically upwards.

Figures of two shoots of N. glauca, awake and asleep (Fig. 162), are

given on p. 385: one of the shoots, from which the photographs were

taken, was accidentally bent to one side.

Fig. 163. Nicotiana tabacum: circumnutation and nyctitropic movement of

a leaf (5 inches in length), traced on a vertical glass, from 3 P.M.

July 10th to 8.10 A.M. 13th. Apex of leaf 4 inches from glass. Temp.

17½°–18½° C. Figure reduced to one-half original scale.

At the base of the petiole of N. tabacum, on the outside, there is a

mass of cells, which are rather smaller than elsewhere, and

have their longer axes differently directed from the cells of the

parenchyma, and may therefore be considered as forming a sort of

pulvinus. A young plant of N. tabacum was selected, and the

circumnutation of the fifth leaf above the cotyledons was observed

during three days. On the first morning (July 10th) the leaf fell from

9 to 10 A.M., which is its normal course, but rose during the remainder

of the day; and this no doubt was due to its being illuminated

exclusively from above; for properly the evening rise does not commence

until 3 or 4 P.M. In the figure as given on p. 386 (Fig. 163) the first

dot was made at 3 P.M.; and the tracing was continued for the following

65 h. When the leaf pointed to the dot next above that marked 3 P.M. it

stood horizontally. The tracing is remarkable only from its simplicity

and the straightness of the lines. The leaf each day described a single

great ellipse; for it should be observed that the ascending and

descending lines do not coincide. On the evening of the 11th the leaf

did not descend quite so low as usual, and it now zigzagged a little.

The diurnal sinking movement had already commenced each morning by 7

A.M. The broken lines at the top of the figure, representing the

nocturnal vertical position of the leaf, ought to be prolonged much

higher up.

Mirabilis longiflora and jalapa (Nyctagineae).—The first pair of leaves

above the cotyledons, produced by seedlings of both these species, were

considerably divergent during the day, and at night stood up vertically

in close contact with one another. The two upper leaves on an older

seedling were almost horizontal by day, and at night stood up

vertically, but were not in close contact, owing to the resistance

offered by the central bud.

Polygonum aviculare (Polygoneae).—Professor Batalin informs us that the

young leaves rise up vertically at night. This is likewise the case,

according to Linnæus, with several species of Amaranthus

(Amaranthaceae); and we observed a sleep movement of this kind in one

member of the genus. Again, with Chenopodium album (Chenopodieae), the

upper young leaves of some seedlings, about 4 inches in height, were

horizontal or sub-horizontal during the day, and at 10 P.M. on March

7th were quite, or almost quite, vertical. Other seedlings raised in

the greenhouse during the winter (Jan. 28th) were observed day and

night, and no difference could be perceived in the position of their

leaves. According to Bouché (‘Bot. Zeitung,’ 1874, p. 359) the leaves

of Pimelia linoides and spectabilis (Thymeleae) sleep at night.

Euphorbia jacquiniaeflora (Euphorbiaceae).—Mr. Lynch called our

attention to the fact that the young leaves of this plant sleep by

depending vertically. The third leaf from the summit (March 11th) was

inclined during the day 30° beneath the horizon, and at night hung

vertically down, as did some of the still younger leaves. It rose up to

its former level on the following morning. The fourth and fifth leaves

from the summit stood horizontally during the day, and sank down at

night only 38°. The sixth leaf did not sensibly alter its position. The

sinking movement is due to the downward curvature of the petiole, no

part of which exhibits any structure like that of a pulvinus. Early on

the morning of June 7th a filament was fixed longitudinally to a young

leaf (the third from the summit, and 2 5/8 inches in length), and its

movements were traced on a vertical glass during 72 h., the plant being

illuminated from above through a skylight. Each day the leaf fell in a

nearly straight line from 7 A.M. to 5 P.M., after which hour it was so

much inclined downwards that the movement could no longer be traced;

and during the latter part of each night, or early in the morning, the

leaf rose. It therefore circumnutated in a very simple manner, making a

single large ellipse every 24 h., for the ascending and descending

lines did not coincide. On each successive morning it stood at a less

height than on the previous one, and this was probably due partly to

the increasing age of the leaf, and partly to the illumination being

insufficient; for although the leaves are very slightly heliotropic,

yet, according to Mr. Lynch’s and our own observations, their

inclination during the day is determined by the intensity of the light.

On the third day, by which time the extent of the descending movement

had much decreased, the line traced was plainly much more zigzag than

on any previous day, and it appeared as if some of its powers of

movement were thus expended. At 10 P.M. on June 7th, when the leaf

depended vertically, its movements were observed by a mark being placed

behind it, and the end of the attached filament was seen to oscillate

slowly and slightly from side to side, as well as upwards and

downwards.

Phyllanthus Niruri (Euphorbiaceae).—The leaflets of this plant sleep,

as described by Pfeffer,[20] in a remarkable manner, apparently like

those of Cassia, for they sink downwards at night and twist round, so

that their lower surfaces are turned

outwards. They are furnished as might have been expected from this

complex kind of movement, with a pulvinus.

[20] ‘Die Period. Beweg.,’ p. 159.

GYMNOSPERMS.

Pinus Nordmanniana (Coniferæ).—M. Chatin states[21] that the leaves,

which are horizontal during the day, rise up at night, so as to assume

a position almost perpendicular to the branch from which they arise; we

presume that he here refers to a horizontal branch. He adds: “En même

temps, ce mouvement d’érection est accompangé d’un mouvement de torsion

imprimé à la partie basilaire de la feuille, et pouvant souvent

parcourir un arc de 90 degrés.” As the lower surfaces of the leaves are

white, whilst the upper are dark green, the tree presents a widely

different appearance by day and night. The leaves on a small tree in a

pot did not exhibit with us any nyctitropic movements. We have seen in

a former chapter that the leaves of Pinus pinaster and Austriaca are

continually circumnutating.

[21] ‘Comptes Rendus,’ Jan. 1876, p. 171.

MONOCOTYLEDONS.

Thalia dealbata (Cannaceae).—the leaves of this plant sleep by turning

vertically upwards; they are furnished with a well-developed pulvinus.

It is the only instance known to us of a very large leaf sleeping. The

blade of a young leaf, which was as yet only 13 1/4 inches in length

and 6½ in breadth, formed at noon an angle with its tall petiole of

121°, and at night stood vertically in a line with it, and so had risen

59°. The actual distance travelled by the apex (as measured by an

orthogonic tracing) of another large leaf, between 7.30 A.M. and 10

P.M., was 10½ inches. The circumnutation of two young and dwarfed

leaves, arising amongst the taller leaves at the base of the plant, was

traced on a vertical glass during two days. On the first day the apex

of one, and on the second day the apex of the other leaf, described

between 6.40 A.M. and 4 P.M. two ellipses, the longer axes of which

were extended in very different directions from the lines representing

the great diurnal sinking and nocturnal rising movement.

Maranta arundinacea (Cannaceae).—The blades of the leaves, which are

furnished with a pulvinus, stand horizontally during

the day or between 10° and 20° above the horizon, and at night

vertically upwards. They therefore rise between 70° and 90° at night.

The plant was placed at noon in the dark in the hot-house, and on the

following day the movements of the leaves were traced. Between 8.40 and

10.30 A.M. they rose, and then fell greatly till 1.37 P.M. But by 3

P.M. they had again risen a little, and continued to rise during the

rest of the afternoon and night; on the following morning they stood at

the same level as on the previous day. Darkness, therefore, during a

day and a half does not interfere with the periodicity of their

movements. On a warm but stormy evening, the plant whilst being brought

into the house, had its leaves violently shaken, and at night not one

went to sleep. On the next morning the plant was taken back to the

hot-house, and again at night the leaves did not sleep; but on the

ensuing night they rose in the usual manner between 70° and 80°. This

fact is analogous with what we have observed with climbing plants,

namely, that much agitation checks for a time their power of

circumnutation; but the effect in this instance was much more strongly

marked and prolonged.

Colocasia antiquorum (Caladium esculentum, Hort.) (Aroideae).—The

leaves of this plant sleep by their blades sinking in the evening, so

as to stand highly inclined, or even quite vertically with their tips

pointing to the ground. They are not provided with a pulvinus. The

blade of one stood at noon 1 degree beneath the horizon; at 4.20 P.M.,

20°; at 6 P.M. 43°; at 7.20 P.M., 69°; and at 8.30 P.M., 68°; so it had

now begun to rise; at 10.15 P.M. it stood at 65°, and on the following

early morning at 11° beneath the horizon. The circumnutation of another

young leaf (with its petiole only 3 1/4 inches, and the blade 4 inches

in length), was traced on a vertical glass during 48 h.; it was dimly

illuminated through a skylight, and this seemed to disturb the proper

periodicity of its movements. Nevertheless, the leaf fell greatly

during both afternoons, till either 7.10 P.M. or 9 P.M., when it rose a

little and moved laterally. By an early hour on both mornings, it had

assumed its diurnal position. The well-marked lateral movement for a

short time in the early part of the night, was the only interesting

fact which it presented, as this caused the ascending and descending

lines not to coincide, in accordance with the general rule with

circumnutating organs. The movements of the leaves of this plant are

thus of the most simple kind; and the tracing is not worth giving. We

have seen that in another genus of the Aroideae, namely, Pistia, the

leaves

rise so much at night that they may almost be said to sleep.

Strephium floribundum[22] (Gramineæ).—The oval leaves are provided with

a pulvinus, and are extended horizontally or declined a little beneath

the horizon during the day. Those on the upright culms simply rise up

vertically at night, so that their tips are directed towards the

zenith. (Fig. 164.) Horizontally extended leaves arising from much

inclined or almost horizontal culms, move at night so that their tips

point towards the apex of the culm, with one lateral margin directed

towards the zenith; and in order to assume this position the leaves

have to twist on their own axes through an angle of nearly 90°. Thus

the surface of the blade always stands vertically, whatever may be the

position of the midrib or of the leaf as a whole.

[22] A. Brongniart first observed that the leaves of this plant and of

Marsilea sleep: see ‘Bull. de la Soc. Bot. de France,’ tom. vii. 1860,

p. 470.

Fig. 164. Strephium floribundum: culms with leaves during the day, and

when asleep at night. Figures reduced.

The circumnutation of a young leaf (2.3 inches in length) was traced

during 48 h. (Fig. 165). The movement was remarkably simple; the leaf

descended from before 6.40 A.M. until 2 or 2.50 P.M., and then rose so

as to stand vertically at about 6 P.M., descending again late in the

night or in the very early morning.

On the second day the descending line zigzagged slightly. As usual, the

ascending and descending lines did not coincide. On another occasion,

when the temperature was a little higher, viz., 24°–26½° C., a leaf was

observed 17 times between 8.50 A.M. and 12.16 P.M.; it changed its

course by as much as a rectangle six times in this interval of 3 h. 26

m., and described two irregular triangles and a half. The leaf,

therefore, on this occasion circumnutated rapidly and in a complex

manner.

Fig. 165. Strephium floribundum: circumnutation and nyctitropic

movement of a leaf, traced from 9 A.M. June 26th to 8.45 A.M. 27th;

filament fixed along the midrib. Apex of leaf 8 1/4 inches from the

vertical glass; plant illuminated from above. Temp. 23½°–24½° C.

ACOTYLEDONS.

Marsilea quadrifoliata (Marsileaceae).—The shape of a leaf, expanded

horizontally during the day, is shown at A (Fig. 166). Each leaflet is

provided with a well-developed pulvinus. When the leaves sleep, the two

terminal leaflets rise up, twist half round and come into contact with

one another (B), and are afterwards embraced by the two lower leaflets

(C); so that the four leaflets with their lower surfaces turned

outwards form a vertical packet. The curvature of the summit of the

petiole of the leaf figured asleep, is merely accidental. The plant was

brought into a room, where the temperature was only a little above 60°

F., and the movement of one of the leaflets (the petiole having been

secured) was traced

during 24 h. (Fig. 167). The leaf fell from the early morning till 1.50

P.M., and then rose till 6 P.M., when it was asleep. A vertically

dependent glass filament was now fixed to one of the terminal and inner

leaflets; and part of the tracing in Fig. 167, after 6 P.M., shows that

it continued to sink, making one zigzag, until 10.40 P.M. At 6.45 A.M.

on the following morning, the leaf was awaking, and the filament

pointed above the vertical glass, but by 8.25 A.M. it occupied the

position shown in the figure. The diagram differs greatly in appearance

from most of those previously given; and this is due to the leaflet

twisting and moving laterally as it approaches and comes into contact

with

its fellow. The movement of another leaflet, when asleep, was traced

between 6 P.M. and 10.35 P.M., and it clearly circumnutated, for it

continued for two hours to sink, then rose, and then sank still lower

than it was at 6 P.M. It may be seen in the preceding figure (167) that

the leaflet, when the plant was subjected to a rather low temperature

in the house, descended and ascended during the middle of the day in a

somewhat zigzag line; but when kept in the hot-house from 9 A.M. to 3

P.M. at a high but varying temperature (viz., between 72° and 83° F.) a

leaflet (with the petiole secured) circumnutated rapidly, for it made

three large vertical ellipses in the course of the six hours. According

to Brongniart, Marsilea pubescens sleeps like the present species.

These plants are the sole cryptogamic ones known to sleep.

Fig. 166. Marsilea quadrifoliata: A, leaf during the day, seen from

vertically above; B, leaf beginning to go to sleep, seen laterally; C,

the same asleep. Figures reduced to one-half of natural scale.

Fig. 167. Marsilea quadrifoliata: circumnutation and nyctitropic

movement of leaflet traced on vertical glass, during nearly 24 h.

Figure reduced to two-thirds of original scale. Plant kept at rather

too low a temperature.

Summary and Concluding Remarks on the Nyctitropic or Sleep-movements of

Leaves.—That these movements are in some manner of high importance to

the plants which exhibit them, few will dispute who have observed how

complex they sometimes are. Thus with Cassia, the leaflets which are

horizontal during the day not only bend at night vertically downwards

with the terminal pair directed considerably backwards, but they also

rotate on their own axes, so that their lower surfaces are turned

outwards. The terminal leaflet of Melilotus likewise rotates, by which

movement one of its lateral edges is directed upwards, and at the same

time it moves either to the left or to the right, until its upper

surface comes into contact with that of the lateral leaflet on the same

side, which has likewise rotated on its own axis. With Arachis, all

four leaflets form together during the night a single vertical packet;

and to the effect this the two anterior leaflets have to move upwards

and the two posterior ones forwards, besides all twisting on their own

axes. In the genus Sida the leaves of some species move at night

through an angle of 90° upwards, and of others

through the same angle downwards. We have seen a similar difference in

the nyctitropic movements of the cotyledons in the genus Oxalis. In

Lupinus, again, the leaflets move either upwards or downwards; and in

some species, for instance L. luteus, those on one side of the

star-shaped leaf move up, and those on the opposite side move down; the

intermediate ones rotating on their axes; and by these varied

movements, the whole leaf forms at night a vertical star instead of a

horizontal one, as during the day. Some leaves and leaflets, besides

moving either upwards or downwards, become more or less folded at

night, as in Bauhinia and in some species of Oxalis. The positions,

indeed, which leaves occupy when asleep are almost infinitely

diversified; they may point either vertically upwards or downwards, or,

in the case of leaflets, towards the apex or towards the base of the

leaf, or in any intermediate position. They often rotate at least as

much as 90° on their own axes. The leaves which arise from upright and

from horizontal or much inclined branches on the same plant, move in

some few cases in a different manner, as with Porlieria and Strephium.

The whole appearance of many plants is wonderfully changed at night, as

may be seen with Oxalis, and still more plainly with Mimosa. A bush of

Acacia Farnesiana appears at night as if covered with little dangling

bits of string instead of leaves. Excluding a few genera not seen by

ourselves, about which we are in doubt, and excluding a few others the

leaflets of which rotate at night, and do not rise or sink much, there

are 37 genera in which the leaves or leaflets rise, often moving at the

same time towards the apex or towards the base of the leaf, and 32

genera in which they sink at night.

The nyctitropic movements of leaves, leaflets, and

petioles are effected in two different ways; firstly, by alternately

increased growth on their opposite sides, preceded by increased

turgescence of the cells; and secondly by means of a pulvinus or

aggregate of small cells, generally destitute of chlorophyll, which

become alternately more turgescent on nearly opposite sides; and this

turgescence is not followed by growth except during the early age of

the plant. A pulvinus seems to be formed (as formerly shown) by a group

of cells ceasing to grow at a very early age, and therefore does not

differ essentially from the surrounding tissues. The cotyledons of some

species of Trifolium are provided with a pulvinus, and others are

destitute of one, and so it is with the leaves in the genus Sida. We

see also in this same genus gradations in the state of the development

of the pulvinus; and in Nicotiana we have what may probably be

considered as the commencing development of one. The nature of the

movement is closely similar, whether a pulvinus is absent or present,

as is evident from many of the diagrams given in this chapter. It

deserves notice that when a pulvinus is present, the ascending and

descending lines hardly ever coincide, so that ellipses are habitually

described by the leaves thus provided, whether they are young or so old

as to have quite ceased growing. This fact of ellipses being described,

shows that the alternately increased turgescence of the cells does not

occur on exactly opposite sides of the pulvinus, any more than the

increased growth which causes the movements of leaves not furnished

with pulvini. When a pulvinus is present, the nyctitropic movements are

continued for a very much longer period than when such do not exist.

This has been amply proved in the case of cotyledons, and Pfeffer has

given observations to the same effect with respect

to leaves. We have seen that a leaf of Mimosa pudica continued to move

in the ordinary manner, though somewhat more simply, until it withered

and died. It may be added that some leaflets of Trifolium pratense were

pinned open during 10 days, and on the first evening after being

released they rose up and slept in the usual manner. Besides the long

continuance of the movements when effected by the aid of a pulvinus

(and this appears to be the final cause of its development), a twisting

movement at night, as Pfeffer has remarked, is almost confined to

leaves thus provided.

It is a very general rule that the first true leaf, though it may

differ somewhat in shape from the leaves on the mature plant, yet

sleeps like them; and this occurs quite independently of the fact

whether or not the cotyledons themselves sleep, or whether they sleep

in the same manner. But with Phaseolus Roxburghii the first unifoliate

leaves rise at night almost sufficiently to be said to sleep, whilst

the leaflets of the secondary trifoliate leaves sink vertically at

night. On young plants of Sida rhombaefolia, only a few inches in

height, the leaves did not sleep, though on rather older plants they

rose up vertically at night. On the other hand, the leaves on very

young plants of Cytisus fragrans slept in a conspicuous manner, whilst

on old and vigorous bushes kept in the greenhouse, the leaves did not

exhibit any plain nyctitropic movement. In the genus Lotus the basal

stipule-like leaflets rise up vertically at night, and are provided

with pulvini.

As already remarked, when leaves or leaflets change their position

greatly at night and by complicated movements, it can hardly be doubted

that these must be in some manner beneficial to the plant. If so, we

must extend the same conclusion to a large number of sleeping plants;

for the most complicated and the simplest nyctitropic movements are

connected together by the finest gradations. But owing to the causes

specified in the beginning of this chapter, it is impossible in some

few cases to determine whether or not certain movements should be

called nyctitropic. Generally, the position which the leaves occupy at

night indicates with sufficient clearness, that the benefit thus

derived, is the protection of their upper surfaces from radiation into

the open sky, and in many cases the mutual protection of all the parts

from cold by their being brought into close approximation. It should be

remembered that it was proved in the last chapter, that leaves

compelled to remain extended horizontally at night, suffered much more

from radiation than those which were allowed to assume their normal

vertical position.

The fact of the leaves of several plants not sleeping unless they have

been well illuminated during the day, made us for a time doubt whether

the protection of their upper surfaces from radiation was in all cases

the final cause of their well-pronounced nyctitropic movements. But we

have no reason to suppose that the illumination from the open sky,

during even the most clouded day, is insufficient for this purpose; and

we should bear in mind that leaves which are shaded from being seated

low down on the plant, and which sometimes do not sleep, are likewise

protected at night from full radiation. Nevertheless, we do not wish to

deny that there may exist cases in which leaves change their position

considerably at night, without their deriving any benefit from such

movements.

Although with sleeping plants the blades almost

always assume at night a vertical, or nearly vertical position, it is a

point of complete indifference whether the apex, or the base, or one of

the lateral edges, is directed to the zenith. It is a rule of wide

generality, that whenever there is any difference in the degree of

exposure to radiation between the upper and the lower surfaces of

leaves and leaflets, it is the upper which is the least exposed, as may

be seen in Lotus, Cytisus, Trifolium, and other genera. In several

species of Lupinus the leaflets do not, and apparently from their

structure cannot, place themselves vertically at night, and

consequently their upper surfaces, though highly inclined, are more

exposed than the lower; and here we have an exception to our rule. But

in other species of this genus the leaflets succeed in placing

themselves vertically; this, however, is effected by a very unusual

movement, namely, by the leaflets on the opposite sides of the same

leaf moving in opposite directions.

It is again a very common rule that when leaflets come into close

contact with one another, they do so by their upper surfaces, which are

thus best protected. In some cases this may be the direct result of

their rising vertically; but it is obviously for the protection of the

upper surfaces that the leaflets of Cassia rotate in so wonderful a

manner whilst sinking downwards; and that the terminal leaflet of

Melilotus rotates and moves to one side until it meets the lateral

leaflet on the same side. When opposite leaves or leaflets sink

vertically down without any twisting, their lower surfaces approach

each other and sometimes come into contact; but this is the direct and

inevitable result of their position. With many species of Oxalis the

lower surfaces of the adjoining leaflets are pressed together, and are

thus better protected

than the upper surfaces; but this depends merely on each leaflet

becoming folded at night so as to be able to sink vertically downwards.

The torsion or rotation of leaves and leaflets, which occurs in so many

cases, apparently always serves to bring their upper surfaces into

close approximation with one another, or with other parts of the plant,

for their mutual protection. We see this best in such cases as those of

Arachis, Mimosa albida, and Marsilea, in which all the leaflets form

together at night a single vertical packet. If with Mimosa pudica the

opposite leaflets had merely moved upwards, their upper surfaces would

have come into contact and been well protected; but as it is, they all

successively move towards the apex of the leaf; and thus not only their

upper surfaces are protected, but the successive pairs become

imbricated and mutually protect one another as well as the petioles.

This imbrication of the leaflets of sleeping plants is a common

phenomenon.

The nyctitropic movement of the blade is generally effected by the

curvature of the uppermost part of the petiole, which has often been

modified into a pulvinus; or the whole petiole, when short, may be thus

modified. But the blade itself sometimes curves or moves, of which fact

Bauhinia offers a striking instance, as the two halves rise up and come

into close contact at night. Or the blade and the upper part of the

petiole may both move. Moreover, the petiole as a whole commonly either

rises or sinks at night. This movement is sometimes large: thus the

petioles of Cassia pubescens stand only a little above the horizon

during the day, and at night rise up almost, or quite, perpendicularly.

The petioles of the younger leaves of Desmodium gyrans also rise up

vertically at night. On the other hand, with

Amphicarpæa, the petioles of some leaves sank down as much as 57° at

night; with Arachis they sank 39°, and then stood at right angles to

the stem. Generally, when the rising or sinking of several petioles on

the same plant was measured, the amount differed greatly. This is

largely determined by the age of the leaf: for instance, the petiole of

a moderately old leaf of Desmodium gyrans rose only 46°, whilst the

young ones rose up vertically; that of a young leaf of Cassia

floribunda rose 41°, whilst that of an older leaf rose only 12°. It is

a more singular fact that the age of the plant sometimes influences

greatly the amount of movement; thus with some young seedlings of a

Bauhinia the petioles rose at night 30° and 34°, whereas those on these

same plants, when grown to a height of 2 or 3 feet, hardly moved at

all. The position of the leaves on the plant as determined by the

light, seems also to influence the amount of movement of the petiole;

for no other cause was apparent why the petioles of some leaves of

Melilotus officinalis rose as much as 59°, and others only 7° and 9° at

night.

In the case of many plants, the petioles move at night in one direction

and the leaflets in a directly opposite one. Thus, in three genera of

Phaseoleae the leaflets moved vertically downwards at night, and the

petioles rose in two of them, whilst in the third they sank. Species in

the same genus often differ widely in the movements of their petioles.

Even on the same plant of Lupinus pubescens some of the petioles rose

30°, others only 6°, and others sank 4° at night. The leaflets of

Cassia Barclayana moved so little at night that they could not be said

to sleep, yet the petioles of some young leaves rose as much as 34°.

These several facts apparently indicate that the movements

of the petioles are not performed for any special purpose; though a

conclusion of this kind is generally rash. When the leaflets sink

vertically down at night and the petioles rise, as often occurs, it is

certain that the upward movement of the latter does not aid the

leaflets in placing themselves in their proper position at night, for

they have to move through a greater angular space than would otherwise

have been necessary.

Notwithstanding what has just been said, it may be strongly suspected

that in some cases the rising of the petioles, when considerable, does

beneficially serve the plant by greatly reducing the surface exposed to

radiation at night. If the reader will compare the two drawings (Fig.

155, p. 371) of Cassia pubescens, copied from photographs, he will see

that the diameter of the plant at night is about one-third of what it

is by day, and therefore the surface exposed to radiation is nearly

nine times less. A similar conclusion may be deduced from the drawings

(Fig. 149, p. 358) of a branch awake and asleep of Desmodium gyrans. So

it was in a very striking manner with young plants of Bauhinia, and

with Oxalis Ortegesii.

We are led to an analogous conclusion with respect to the movements of

the secondary petioles of certain pinnate leaves. The pinnae of Mimosa

pudica converge at night; and thus the imbricated and closed leaflets

on each separate pinna are all brought close together into a single

bundle, and mutually protect one another, with a somewhat smaller

surface exposed to radiation. With Albizzia lophantha the pinnae close

together in the same manner. Although the pinnae of Acacia Farnesiana

do not converge much, they sink downwards. Those of Neptunia oleracea

likewise

move downwards, as well as backwards, towards the base of the leaf,

whilst the main petiole rises. With Schrankia, again, the pinnae are

depressed at night. Now in these three latter cases, though the pinnae

do not mutually protect one another at night, yet after having sunk

down they expose, as does a dependent sleeping leaf, much less surface

to the zenith and to radiation than if they had remained horizontal.

Any one who had never observed continuously a sleeping plant, would

naturally suppose that the leaves moved only in the evening when going

to sleep, and in the morning when awaking; but he would be quite

mistaken, for we have found no exception to the rule that leaves which

sleep continue to move during the whole twenty-four hours; they move,

however, more quickly when going to sleep and when awaking than at

other times. That they are not stationary during the day is shown by

all the diagrams given, and by the many more which were traced. It is

troublesome to observe the movements of leaves in the middle of the

night, but this was done in a few cases; and tracings were made during

the early part of the night of the movements in the case of Oxalis,

Amphicarpæa, two species of Erythrina, a Cassia, Passiflora, Euphorbia

and Marsilea; and the leaves after they had gone to sleep, were found

to be in constant movement. When, however, opposite leaflets come into

close contact with one another or with the stem at night, they are, as

we believe, mechanically prevented from moving, but this point was not

sufficiently investigated.

When the movements of sleeping leaves are traced during twenty-four

hours, the ascending and descending lines do not coincide, except

occasionally and by accident for a short space; so that with many

plants a

single large ellipse is described during each twenty-four hours. Such

ellipses are generally narrow and vertically directed, for the amount

of lateral movement is small. That there is some lateral movement is

shown by the ascending and descending lines not coinciding, and

occasionally, as with Desmodium gyrans and Thalia dealbata, it was

strongly marked. In the case of Melilotus the ellipses described by the

terminal leaflet during the day are laterally extended, instead of

vertically, as is usual; and this fact evidently stands in relation

with the terminal leaflet moving laterally when it goes to sleep. With

the majority of sleeping plants the leaves oscillate more than once up

and down in the twenty-four hours; so that frequently two ellipses, one

of moderate size, and one of very large size which includes the

nocturnal movement, are described within the twenty-four hours. For

instance, a leaf which stands vertically up during the night will sink

in the morning, then rise considerably, again sink in the afternoon,

and in the evening reascend and assume its vertical nocturnal position.

It will thus describe, in the course of the twenty-four hours, two

ellipses of unequal sizes. Other plants describe within the same time,

three, four, or five ellipses. Occasionally the longer axes of the

several ellipses extend in different directions, of which Acacia

Farnesiana offered a good instance. The following cases will give an

idea of the rate of movement: Oxalis acetosella completed two ellipses

at the rate of 1 h. 25 m. for each; Marsilea quadrifoliata, at the rate

of 2 h.; Trifolium subterraneum, one in 3 h. 30 m.; and Arachis

hypogaea, in 4 h. 50 m. But the number of ellipses described within a

given time depends largely on the state of the plant and on the

conditions to which it is exposed. It often happens that a single

ellipse may be described during one

day, and two on the next. Erythrina corallodendron made four ellipses

on the first day of observation and only a single one on the third,

apparently owing to having been kept not sufficiently illuminated and

perhaps not warm enough. But there seems likewise to be an innate

tendency in different species of the same genus to make a different

number of ellipses in the twenty-four hours: the leaflets of Trifolium

repens made only one; those of T. resupinatum two, and those of T.

subterraneum three in this time. Again, the leaflets of Oxalis

Plumierii made a single ellipse; those of O. bupleurifolia, two; those

of O. Valdiviana, two or three; and those of O. acetosella, at least

five in the twenty-four hours.

The line followed by the apex of a leaf or leaflet, whilst describing

one or more ellipses during the day, is often zigzag, either throughout

its whole course or only during the morning or evening: Robinia offered

an instance of zigzagging confined to the morning, and a similar

movement in the evening is shown in the diagram (Fig. 126) given under

Sida. The amount of the zigzag movement depends largely on the plant

being placed under highly favourable conditions. But even under such

favourable conditions, if the dots which mark the position of the apex

are made at considerable intervals of time, and the dots are then

joined, the course pursued will still appear comparatively simple,

although the number of the ellipses will be increased; but if dots are

made every two or three minutes and these are joined, the result often

is that all the lines are strongly zigzag, many small loops, triangles,

and other figures being also formed. This fact is shown in two parts of

the diagram (Fig. 150) of the movements of Desmodium gyrans. Strephium

floribundum, observed under a high temperature,

made several little triangles at the rate of 43 m. for each. Mimosa

pudica, similarly observed, described three little ellipses in 67 m.;

and the apex of a leaflet crossed 1/500 of an inch in a second, or 0.12

inch in a minute. The leaflets of Averrhoa made a countless number of

little oscillations when the temperature was high and the sun shining.

The zigzag movement may in all cases be considered as an attempt to

form small loops, which are drawn out by a prevailing movement in some

one direction. The rapid gyrations of the little lateral leaflets of

Desmodium belong to the same class of movements, somewhat exaggerated

in rapidity and amplitude. The jerking movements, with a small advance

and still smaller retreat, apparently not exactly in the same line, of

the hypocotyl of the cabbage and of the leaves of Dionaea, as seen

under the microscope, all probably come under this same head. We may

suspect that we here see the energy which is freed during the incessant

chemical changes in progress in the tissues, converted into motion.

Finally, it should be noted that leaflets and probably some leaves,

whilst describing their ellipses, often rotate slightly on their axes;

so that the plane of the leaf is directed first to one and then to

another side. This was plainly seen to be the case with the large

terminal leaflets of Desmodium, Erythrina and Amphicarpæa, and is

probably common to all leaflets provided with a pulvinus.

With respect to the periodicity of the movements of sleeping leaves,

Pfeffer[23] has so clearly shown that this depends on the daily

alternations of light and darkness, that nothing farther need be said

on this

head. But we may recall the behaviour of Mimosa in the North, where the

sun does not set, and the complete inversion of the daily movements by

artificial light and darkness. It has also been shown by us, that

although leaves subjected to darkness for a moderately long time

continue to circumnutate, yet the periodicity of their movements is

soon greatly disturbed, or quite annulled. The presence of light or its

absence cannot be supposed to be the direct cause of the movements, for

these are wonderfully diversified even with the leaflets of the same

leaf, although all have of course been similarly exposed. The movements

depend on innate causes, and are of an adaptive nature. The

alternations of light and darkness merely give notice to the leaves

that the period has arrived for them to move in a certain manner. We

may infer from the fact of several plants (Tropaeolum, Lupinus, etc.)

not sleeping unless they have been well illuminated during the day,

that it is not the actual decrease of light in the evening, but the

contrast between the amount at this hour and during the early part of

the day, which excites the leaves to modify their ordinary mode of

circumnutation.

[23] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, p. 30, et

passim.

As the leaves of most plants assume their proper diurnal position in

the morning, although light be excluded, and as the leaves of some

plants continue to move in the normal manner in darkness during at

least a whole day, we may conclude that the periodicity of their

movements is to a certain extent inherited.[24] The strength of such

inheritance differs

much in different species, and seems never to be rigid; for plants have

been introduced from all parts of the world into our gardens and

greenhouses; and if their movements had been at all strictly fixed in

relation to the alternations of day and night, they would have slept in

this country at very different hours, which is not the case. Moreover,

it has been observed that sleeping plants in their native homes change

their times of sleep with the changing seasons.[25]

[24] Pfeffer denies such inheritance; he attributes (‘Die Period.

Bewegungen,’ pp. 30–56) the periodicity when prolonged for a day or

two in darkness, to “Nachwirkung,” or the after-effects of light and

darkness. But we are unable to follow his train of reasoning. There

does not seem to be any more reason for attributing such movements to

this cause than, for instance, the inherited habit of winter and

summer wheat to grow best at different seasons; for this habit is lost

after a few years, like the movements of leaves in darkness after a

few days. No doubt some effect must be produced on the seeds by the

long-continued cultivation of the parent-plants under different

climates, but no one probably would call this the “Nachwirkung” of the

climates.

[25] Pfeffer, ibid., p. 46.

We may now turn to the systematic list. This contains the names of all

the sleeping plants known to us, though the list undoubtedly is very

imperfect. It may be premised that, as a general rule, all the species

in the same genus sleep in nearly the same manner. But there are some

exceptions; in several large genera including many sleeping species

(for instance, Oxalis), some do not sleep. One species of Melilotus

sleeps like a Trifolium, and therefore very differently from its

congeners; so does one species of Cassia. In the genus Sida, the leaves

either rise or fall at night; and with Lupinus they sleep in three

different methods. Returning to the list, the first point which strikes

us, is that there are many more genera amongst the Leguminosae (and in

almost every one of the Leguminous tribes) than in all the other

families put together; and we are tempted to connect this fact with the

great

mobility of the stems and leaves in this family, as shown by the large

number of climbing species which it contains. Next to the Leguminosae

come the Malvaceae, together with some closely allied families. But by

far the most important point in the list, is that we meet with sleeping

plants in 28 families, in all the great divisions of the Phanerogamic

series, and in one Cryptogam. Now, although it is probable that with

the Leguminosae the tendency to sleep may have been inherited from one

or a few progenitors, and possibly so in the cohorts of the Malvales

and Chenopodiales, yet it is manifest that the tendency must have been

acquired by the several genera in the other families, quite

independently of one another. Hence the question naturally arises, how

has this been possible? and the answer, we cannot doubt is that leaves

owe their nyctitropic movements to their habit of circumnutating,—a

habit common to all plants, and everywhere ready for any beneficial

development or modification.

It has been shown in the previous chapters that the leaves and

cotyledons of all plants are continually moving up and down, generally

to a slight but sometimes to a considerable extent, and that they

describe either one or several ellipses in the course of twenty-four

hours; they are also so far affected by the alternations of day and

night that they generally, or at least often, move periodically to a

small extent; and here we have a basis for the development of the

greater nyctitropic movements. That the movements of leaves and

cotyledons which do not sleep come within the class of circumnutating

movements cannot be doubted, for they are closely similar to those of

hypocotyls, epicotyls, the stems of mature plants, and of various other

organs. Now, if we take the simplest

case of a sleeping leaf, we see that it makes a single ellipse in the

twenty-four hours, which resembles one described by a non-sleeping leaf

in every respect, except that it is much larger. In both cases the

course pursued is often zigzag. As all non-sleeping leaves are

incessantly circumnutating, we must conclude that a part at least of

the upward and downward movement of one that sleeps, is due to ordinary

circumnutation; and it seems altogether gratuitous to rank the

remainder of the movement under a wholly different head. With a

multitude of climbing plants the ellipses which they describe have been

greatly increased for another purpose, namely, catching hold of a

support. With these climbing plants, the various circumnutating organs

have been so far modified in relation to light that, differently from

all ordinary plants, they do not bend towards it. with sleeping plants

the rate and amplitude of the movements of the leaves have been so far

modified in relation to light, that they move in a certain direction

with the waning light of the evening and with the increasing light of

the morning more rapidly, and to a greater extent, than at other hours.

But the leaves and cotyledons of many non-sleeping plants move in a

much more complex manner than in the cases just alluded to, for they

describe two, three, or more ellipses in the course of a day. Now, if a

plant of this kind were converted into one that slept, one side of one

of the several ellipses which each leaf daily describes, would have to

be greatly increased in length in the evening, until the leaf stood

vertically, when it would go on circumnutating about the same spot. On

the following morning, the side of another ellipse would have to be

similarly increased in length so as to bring the leaf back again into

its diurnal position, when it would again circumnutate

until the evening. If the reader will look, for instance, at the

diagram (Fig. 142, p. 351), representing the nyctitropic movements of

the terminal leaflet of Trifolium subterraneum, remembering that the

curved broken lines at the top ought to be prolonged much higher up, he

will see that the great rise in the evening and the great fall in the

morning together form a large ellipse like one of those described

during the daytime, differing only in size. Or, he may look at the

diagram (Fig. 103, p. 236) of the 3½ ellipses described in the course

of 6 h. 35 m. by a leaf of Lupinus speciosus, which is one of the

species in this genus that does not sleep; and he will see that by

merely prolonging upwards the line which was already rising late in the

evening, and bringing it down again next morning, the diagram would

represent the movements of a sleeping plant.

With those sleeping plants which describe several ellipses in the

daytime, and which travel in a strongly zigzag line, often making in

their course minute loops, triangles, etc., if as soon as one of the

ellipses begins in the evening to be greatly increased in size, dots

are made every 2 or 3 minutes and these are joined, the line then

described is almost strictly rectilinear, in strong contrast with the

lines made during the daytime. This was observed with Desmodium gyrans

and Mimosa pudica. With this latter plant, moreover, the pinnae

converge in the evening by a steady movement, whereas during the day

they are continually converging and diverging to a slight extent. In

all such cases it was scarcely possible to observe the difference in

the movement during the day and evening, without being convinced that

in the evening the plant saves the expenditure of force by not moving

laterally, and that its whole energy is now expended

in gaining quickly its proper nocturnal position by a direct course. In

several other cases, for instance, when a leaf after describing during

the day one or more fairly regular ellipses, zigzags much in the

evening, it appears as if energy was being expended, so that the great

evening rise or fall might coincide with the period of the day proper

for this movement.

The most complex of all the movements performed by sleeping plants, is

that when leaves or leaflets, after describing in the daytime several

vertically directed ellipses, rotate greatly on their axes in the

evening, by which twisting movement they occupy a wholly different

position at night to what they do during the day. For instance, the

terminal leaflets of Cassia not only move vertically downwards in the

evening, but twist round, so that their lower surfaces face outwards.

Such movements are wholly, or almost wholly, confined to leaflets

provided with a pulvinus. But this torsion is not a new kind of

movement introduced solely for the purpose of sleep; for it has been

shown that some leaflets whilst describing their ordinary ellipses

during the daytime rotate slightly, causing their blades to face first

to one side and then to another. Although we can see how the slight

periodical movements of leaves in a vertical plane could be easily

converted into the greater yet simple nyctitropic movements, we do not

at present know by what graduated steps the more complex movements,

effected by the torsion of the pulvini, have been acquired. A probable

explanation could be given in each case only after a close

investigation of the movements in all the allied forms.

From the facts and considerations now advanced we may conclude that

nyctitropism, or the sleep of leaves

and cotyledons, is merely a modification of their ordinary

circumnutating movement, regulated in its period and amplitude by the

alternations of light and darkness. The object gained is the protection

of the upper surfaces of the leaves from radiation at night, often

combined with the mutual protection of the several parts by their close

approximation. In such cases as those of the leaflets of Cassia—of the

terminal leaflets of Melilotus—of all the leaflets of Arachis,

Marsilea, etc.—we have ordinary circumnutation modified to the extreme

extent known to us in any of the several great classes of modified

circumnutation. On this view of the origin of nyctitropism we can

understand how it is that a few plants, widely distributed throughout

the Vascular series, have been able to acquire the habit of placing the

blades of their leaves vertically at night, that is, of sleeping,—a

fact otherwise inexplicable.

The leaves of some plants move during the day in a manner, which has

improperly been called diurnal sleep; for when the sun shines brightly

on them, they direct their edges towards it. To such cases we shall

recur in the following chapter on Heliotropism. It has been shown that

the leaflets of one form of Porlieria hygrometrica keep closed during

the day, as long as the plant is scantily supplied with water, in the

same manner as when asleep; and this apparently serves to check

evaporation. There is only one other analogous case known to us,

namely, that of certain Gramineæ, which fold inwards the sides of their

narrow leaves, when these are exposed to the sun and to a dry

atmosphere, as described by Duval-Jouve.[26] We have also observed the

same phenomenon in Elymus arenareus.

[26] ‘Annal. des Sc. Nat. (Bot.),’ 1875, tom. i. pp. 326–329.

There is another movement, which since the time of Linnæus has

generally been called sleep, namely, that of the petals of the many

flowers which close at night. These movements have been ably

investigated by Pfeffer, who has shown (as was first observed by

Hofmeister) that they are caused or regulated more by temperature than

by the alternations of light and darkness. Although they cannot fail to

protect the organs of reproduction from radiation at night, this does

not seem to be their chief function, but rather the protection of the

organs from cold winds, and especially from rain, during the day. the

latter seems probable, as Kerner[27] has shown that a widely different

kind of movement, namely, the bending down of the upper part of the

peduncle, serves in many cases the same end. The closure of the flowers

will also exclude nocturnal insects which may be ill-adapted for their

fertilisation, and the well-adapted kinds at periods when the

temperature is not favourable for fertilisation. Whether these

movements of the petals consist, as is probable, of modified

circumnutation we do not know.

[27] ‘Die Schutzmittel des Pollens,’ 1873, pp. 30–39.

Embryology of Leaves.—A few facts have been incidentally given in this

chapter on what may be called the embryology of leaves. With most

plants the first leaf which is developed after the cotyledons,

resembles closely the leaves produced by the mature plant, but this is

not always the case. the first leaves produced by some species of

Drosera, for instance by D. Capensis, differ widely in shape from those

borne by the mature plant, and resemble closely the leaves of D.

rotundifolia, as was shown to us by Prof. Williamson of Manchester. The

first true leaf of

the gorse, or Ulex, is not narrow and spinose like the older leaves. On

the other hand, with many Leguminous plants, for instance, Cassia,

Acacia lophantha, etc., the first leaf has essentially the same

character as the older leaves, excepting that it bears fewer leaflets.

In Trifolium the first leaf generally bears only a single leaflet

instead of three, and this differs somewhat in shape from the

corresponding leaflet on the older leaves. Now, with Trifolium

Pannonicum the first true leaf on some seedlings was unifoliate, and on

others completely trifoliate; and between these two extreme states

there were all sorts of gradations, some seedlings bearing a single

leaflet more or less deeply notched on one or both sides, and some

bearing a single additional and perfect lateral leaflet. Here, then, we

have the rare opportunity of seeing a structure proper to a more

advanced age, in the act of gradually encroaching on and replacing an

earlier or embryological condition.

The genus Melilotus is closely allied to Trifolium, and the first leaf

bears only a single leaflet, which at night rotates on its axis so as

to present one lateral edge to the zenith. Hence it sleeps like the

terminal leaflet of a mature plant, as was observed in 15 species, and

wholly unlike the corresponding leaflet of Trifolium, which simply

bends upwards. It is therefore a curious fact that in one of these 15

species, viz., M. Taurica (and in a lesser degree in two others),

leaves arising from young shoots, produced on plants which had been cut

down and kept in pots during the winter in the green-house, slept like

the leaves of a Trifolium, whilst the leaves on the fully-grown

branches on these same plants afterwards slept normally like those of a

Melilotus. If young shoots rising from the ground may be considered as

new individuals, partaking to a certain extent of the nature of

seedlings, then the peculiar manner in which their leaves slept may be

considered

as an embryological habit, probably the result of Melilotus being

descended from some form which slept like a Trifolium. This view is

partially supported by the leaves on old and young branches of another

species, M. Messanensis (not included in the above 15 species), always

sleeping like those of a Trifolium.

The first true leaf of Mimosa albida consists of a simple petiole,

often bearing three pairs of leaflets, all of which are of nearly equal

size and of the same shape: the second leaf differs widely from the

first, and resembles that on a mature plant (see Fig. 159, p. 379), for

it consists of two pinnae, each of which bears two pairs of leaflets,

of which the inner basal one is very small. But at the base of each

pinna there is a pair of minute points, evidently rudiments of

leaflets, for they are of unequal sizes, like the two succeeding

leaflets. These rudiments are in one sense embryological, for they

exist only during the youth of the leaf, falling off and disappearing

as soon as it is fully grown.

With Desmodium gyrans the two lateral leaflets are very much smaller

than the corresponding leaflets in most of the species in this large

genus; they vary also in position and size; one or both are sometimes

absent; and they do not sleep like the fully-developed leaflets. They

may therefore be considered as almost rudimentary; and in accordance

with the general principles of embryology, they ought to be more

constantly and fully developed on very young than on old plants. But

this is not the case, for they were quite absent on some young

seedlings, and did not appear until from 10 to 20 leaves had been

formed. This fact leads to the suspicion that D. gyrans is descended

through a unifoliate form (of which some exist) from a trifoliate

species; and that the little lateral leaflets reappear through

reversion. However this may be,

the interesting fact of the pulvini or organs of movement of these

little leaflets, not having been reduced nearly so much as their

blades—taking the large terminal leaflet as the standard of

comparison—gives us probably the proximate cause of their extraordinary

power of gyration.