MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.
Conditions necessary for these movements—List of Genera and Families,
which include sleeping plants—Description of the movements in the
several Genera—Oxalis: leaflets folded at night—Averrhoa: rapid
movements of the leaflets—Porlieria: leaflets close when plant kept
very dry—Tropaeolum: leaves do not sleep unless well illuminated during
day—Lupinus: various modes of sleeping—Melilotus: singular movements of
terminal leaflet—Trifolium—Desmodium: rudimentary lateral leaflets,
movements of, not developed on young plants, state of their
pulvini—Cassia: complex movements of the leaflets—Bauhinia: leaves
folded at night—Mimosa pudica: compounded movements of leaves, effect
of darkness—Mimosa albida, reduced leaflets of—Schrankia: downward
movement of the pinnae—Marsilea: the only cryptogam known to
sleep—Concluding remarks and summary—Nyctitropism consists of modified
circumnutation, regulated by the alternations of light and
darkness—Shape of first true leaves.
We now come to the nyctitropic or sleep movements of leaves. It should
be remembered that we confine this term to leaves which place their
blades at night either in a vertical position or not more than 30° from
the vertical,—that is, at least 60° above or beneath the horizon. In
some few cases this is effected by the rotation of the blade, the
petiole not being either raised or lowered to any considerable extent.
The limit of 30° from the vertical is obviously an arbitrary one, and
has been selected for reasons previously assigned, namely, that when
the blade approaches the perpendicular as nearly as this, only half as
much of the surface is exposed at night to the
zenith and to free radiation as when the blade is horizontal.
Nevertheless, in a few instances, leaves which seem to be prevented by
their structure from moving to so great an extent as 60° above or
beneath the horizon, have been included amongst sleeping plants.
It should be premised that the nyctitropic movements of leaves are
easily affected by the conditions to which the plants have been
subjected. If the ground is kept too dry, the movements are much
delayed or fail: according to Dassen,[1] even if the air is very dry
the leaves of Impatiens and Malva are rendered motionless. Carl Kraus
has also lately insisted[2] on the great influence which the quantity
of water absorbed has on the periodic movements of leaves; and he
believes that this cause chiefly determines the variable amount of
sinking of the leaves of Polygonum convolvulus at night; and if so,
their movements are not in our sense strictly nyctitropic. Plants in
order to sleep must have been exposed to a proper temperature:
Erythrina crista-galli, out of doors and nailed against a wall, seemed
in fairly good health, but the leaflets did not sleep, whilst those on
another plant kept in a warm greenhouse were all vertically dependent
at night. In a kitchen-garden the leaflets of Phaseolus vulgaris did
not sleep during the early part of the summer. Ch. Royer says,[3]
referring I suppose to the native plants in France, that they do not
sleep when the temperature is below 5° C. or 41° F. In the case of
several sleeping plants, viz., species of
Tropaeolum, Lupinus, Ipomœa, Abutilon, Siegesbeckia, and probably other
genera, it is indispensable that the leaves should be well illuminated
during the day in order that they may assume at night a vertical
position; and it was probably owing to this cause that seedlings of
Chenopodium album and Siegesbeckia orientalis, raised by us during the
middle of the winter, though kept at a proper temperature, did not
sleep. Lastly, violent agitation by a strong wind, during a few
minutes, of the leaves of Maranta arundinacea (which previously had not
been disturbed in the hot-house), prevented their sleeping during the
two next nights.
[1] Dassen,’Tijdschrift vor. Naturlijke Gesch. en Physiologie,’ 1837,
vol. iv. p. 106. See also Ch. Royer on the importance of a proper
state of turgescence of the cells, in ‘Annal. des Sc. Nat. Bot.’ (5th
series), ix. 1868, p. 345.
[2] ‘Beiträge zur Kentniss der Bewegungen,’ etc., in ‘Flora,’ 1879,
pp. 42, 43, 67, etc.
[3] ‘Annal. des Sc. Nat. Bot.’ (5th Series), ix. 1868, p. 366.
We will now give our observations on sleeping plants, made in the
manner described in the Introduction. The stem of the plant was always
secured (when not stated to the contrary) close to the base of the
leaf, the movements of which were being observed, so as to prevent the
stem from circumnutating. As the tracings were made on a vertical glass
in front of the plant, it was obviously impossible to trace its course
as soon as the leaf became in the evening greatly inclined either
upwards or downwards; it must therefore be understood that the broken
lines in the diagrams, which represent the evening and nocturnal
courses, ought always to be prolonged to a much greater distance,
either upwards or downwards, than appears in them. The conclusions
which may be deduced from our observations will be given near the end
of this chapter.
In the following list all the genera which include sleeping plants are
given, as far as known to us. The same arrangement is followed as in
former cases, and the number of the Family is appended. This list
possesses some interest, as it shows that the habit of
sleeping is common to some few plants throughout the whole vascular
series. The greater number of the genera in the list have been observed
by ourselves with more or less care; but several are given on the
authority of others (whose names are appended in the list), and about
these we have nothing more to say. No doubt the list is very imperfect,
and several genera might have been added from the ‘Somnus Plantarum’ by
Linnæus; but we could not judge in some of his cases, whether the
blades occupied at night a nearly vertical position. He refers to some
plants as sleeping, for instance, Lathyrus odoratus and Vicia faba, in
which we could observe no movement deserving to be called sleep, and as
no one can doubt the accuracy of Linnæus, we are left in doubt.
[List of Genera, including species the leaves of which sleep.
CLASS I. DICOTYLEDONS.
Sub-class I. ANGIOSPERMS.
Genus Family.
Githago Caryophylleae (26). Stellaria (Batalin). ” Portulaca
(Ch.Royer). Portulaceae (27). Sida Malvaceae (36). Abutilon. ” Malva
(Linnæus and Pfeffer). ” Hibiscus (Linnæus). ” Anoda. ” Gossypium. ”
Ayenia (Linnæus). Sterculaceae (37). Triumfetta (Linnæus). Tiliaceae
(38). Linum (Batalin). Lineae (39). Oxalis. Oxalidæ (41). Averrhoa. ”
Porlieria. Zygophylleæ (45). Guiacum. ” Impatiens (Linnæus, Pfeffer,
Batalin). Balsamineae (48). Tropaeolum. Tropaeoleae (49). Crotolaria
(Thiselton Dyer). Leguminosae (75) Tribe II. Lupinus. ” ” Cytisus. ” ”
Trigonella. ” Tr. III. Medicago. ” Melilotus. ” ” Trifolium. ” ”
Securigera. ” Tr. IV. Lotus. ” ” Psoralea. ” Tr. V. Amorpha
(Cuchartre). ” ” Daelea. ” ” Indigofera. ” ” Tephrosia. ” ” Wistaria. ”
” Robinia. ” ” Sphaerophysa. ” ” Colutea. ” ” Astragalus. ” ”
Glycyrrhiza. ” ” Coronilla. ” Tr. VI. Hedysarum. ” ”
Genus Family. Onobrychis. Leguminosae (75) Tr. VI. Smithia. ” ”
Arachis. ” ” Desmodium. ” ” Urania. ” ” Vicia. ” Tr. VII. Centrosema. ”
Tr. VIII. Amphicarpæa. ” ” Glycine. ” ” Erythrina. ” ” Apios. ” ”
Phaseolus. ” ” Sophora. ” Tr. X. Caesalpinia. ” Tr. XIII. Haematoxylon.
” ” Gleditschia (Duchartre). ” ” Poinciana. ” ” Cassia. ” Tr. XIV.
Bauhinia. ” Tr. XV. Tamarindus. ” Tr. XVI. Adenanthera. ” Tr. XX.
Prosopis. ” ” Neptunia. ” ” Mimosa. ” ” Schrankia. ” ” Acacia. ” Tr.
XXII. Albizzia. ” Tr. XXIII. Melaleuca (Bouché). Myrtaceae (94). Genus
Family. Aenothera (Linnæus). Omagrarieae (100). Passiflora.
Passifloracea (105). Siegesbeckia. Compositæ (122). Ipomœa.
Convolvulacea (151). Nicotiana. Solaneae (157). Mirabilis. Nyctagineae
(177). Polygonum (Batalin). Polygoneae (179). Amaranthus. Amaranthaceae
(180). Chenopodium. Chenopodieae (181). Pimelia (Bouché). Thymeteae
(188). Euphorbia. Euphorbiaceae (202) Phyllanthus (Pfeffer). ”
Sub-class II. GYMNOSPERMS. Aies (Chatin).
CLASS II. MONOCOTYLEDONS.
Thalia. Cannaceae (21). Maranta. ” Colocasia. Aroideae (30). Strephium.
Gramineæ (55).
CLASS III. ACOTYLEDONS.
Marsilea. Marsileaceae (4).
Githago segetum (Caryophylleae).—The first leaves produced by young
seedlings, rise up and close together at night. On a rather older
seedling, two young leaves stood at noon at 55° above the horizon, and
at night at 86°, so each had risen 31°. The angle, however, was less in
some cases. Similar observations were occasionally made on young leaves
(for the older ones moved very little) produced by nearly full-grown
plants. Batalin says (‘Flora,’ Oct. 1st, 1873, p. 437) that the young
leaves of Stellaria close up so completely at night that they form
together great buds.
Sida (Malvaceae).—the nyctitropic movements of the leaves in this genus
are remarkable in some respects. Batalin informs
us (see also ‘Flora,’ Oct. 1st, 1873, p. 437) that those of S. napaea
fall at night, but to what angle he cannot remember. The leaves of S.
rhombifolia and retusa, on the other hand, rise up vertically, and are
pressed against the stem. We have therefore here within the same genus,
directly opposite movements. Again, the leaves of S. rhombifolia are
furnished with a pulvinus, formed of a mass of small cells destitute of
chlorophyll, and with their longer axes perpendicular to the axis of
the petiole. As measured along this latter line, these cells are only
1/5th of the length of those of the petiole; but instead of being
abruptly separated from them (as is usual with the pulvinus in most
plants), they graduate into the larger cells of the petiole. On the
other hand, S. napaea, according to Batalin, does not possess a
pulvinus; and he informs us that a gradation may be traced in the
several species of the genus between these two states of the petiole.
Sida rhombifolia presents another peculiarity, of which we have seen no
other instance with leaves that sleep: for those on very young plants,
though they rise somewhat in the evening, do not go to sleep, as we
observed
on several occasions; whilst those on rather older plants sleep in a
conspicuous manner. For instance a leaf (.85 of an inch in length) on a
very young seedling 2 inches high, stood at noon 9° above the horizon,
and at 10 P.M. at 28°, so it had risen only 19°; another leaf (1.4 inch
in length) on a seedling of the same height, stood at the same two
periods at 7° and 32°, and therefore had risen 25°. These leaves, which
moved so little, had a fairly well-developed pulvinus. After an
interval of some weeks, when the same seedlings were 2½ and 3 inches in
height, some of the young leaves stood up at night quite vertically,
and others were highly inclined; and so it was with bushes which were
fully grown and were flowering.
Fig. 126. Sida rhombifolia: circumnutation and nyctitropic (or sleep)
movements of a leaf on a young plant, 9½ inches high; filament fixed to
midrib of nearly full-grown leaf, 2 3/8 inches in length; movement
traced under a sky-light. Apex of leaf 5 5/8 inches from the vertical
glass, so diagram not greatly enlarged.
The movement of a leaf was traced from 9.15 A.M. on May 28th to 8.30
A.M. on the 30th. The temperature was too low (15°–16° C.), and the
illumination hardly sufficient; consequently the leaves did not become
quite so highly inclined at night, as they had done previously and as
they did subsequently in the hot-house: but the movements did not
appear otherwise disturbed. On the first day the leaf sank till 5.15
P.M.; it then rose rapidly and greatly till 10.5 P.M., and only a
little higher during the rest of the night (Fig. 126). Early on the
next day (29th) it fell in a slightly zigzag line rapidly until 9 A.M.,
by which time it had reached nearly the same place as on the previous
morning. During the remainder of the day it fell slowly, and zigzagged
laterally. The evening rise began after 4 P.M. in the same manner as
before, and on the second morning it again fell rapidly. The ascending
and descending lines do not coincide, as may be seen in the diagram. On
the 30th a new tracing was made (not here given) on a rather enlarged
scale, as the apex of the leaf now stood 9 inches from the vertical
glass. In order to observe more carefully the course pursued at the
time when the diurnal fall changes into the nocturnal rise, dots were
made every half-hour between 4 P.M. and 10.30 P.M. This rendered the
lateral zigzagging movement during the evening more conspicuous than in
the diagram given, but it was of the same nature as there shown. The
impression forced on our minds was that the leaf was expending
superfluous movement, so that the great nocturnal rise might not occur
at too early an hour.
Abutilon Darwinii (Malvaceae).—The leaves on some very young plants
stood almost horizontally during the day, and hung down vertically at
night. Very fine plants kept in a
large hall, lighted only from the roof, did not sleep at night for in
order to do so the leaves must be well illuminated during the day. The
cotyledons do not sleep. Linnæus says that the leaves of his Sida
abutilon sink perpendicularly down at night, though the petioles rise.
Prof. Pfeffer informs us that the leaves of a Malva, allied to M.
sylvestris, rise greatly at night; and this genus, as well as that of
Hibiscus, are included by Linnæus in his list of sleeping plants.
Anoda Wrightii (Malvaceae).—The leaves, produced by very young plants,
when grown to a moderate size, sink at night either almost vertically
down or to an angle of about 45° beneath the horizon; for there is a
considerable degree of variability in the amount of sinking at night,
which depends in part on the degree to which they have been illuminated
during the day. But the leaves, whilst quite young, do not sink down at
night, and this is a very unusual circumstance. The summit of the
petiole, where it joins the blade, is developed into a pulvinus, and
this is present in very young leaves which do not sleep; though it is
not so well defined as in older leaves.
Gossypium (var. Nankin cotton, Malvaceae).—Some young leaves, between 1
and 2 inches in length, borne by two seedlings 6 and 7½ inches in
height, stood horizontally, or were raised a little above the horizon
at noon on July 8th and 9th; but by 10 P.M. they had sunk down to
between 68° and 90° beneath the horizon. When the same plants had grown
to double the above height, their leaves stood at night almost or quite
vertically dependent. The leaves on some large plants of G. maritimum
and Brazilense, which were kept in a very badly lighted hot-house, only
occasionally sank much downwards at night, and hardly enough to be
called sleep.
Oxalis (Oxalidæ).—In most of the species in this large genus the three
leaflets sink vertically down at night; but as their sub-petioles are
short the blades could not assume this position from the want of space,
unless they were in some manner rendered narrower; and this is effected
by their becoming more or less folded (Fig. 127). The angle formed by
the two halves of the same leaflet was found to vary in different
individuals of several species between 92° and 150°; in three of the
best folded leaflets of O. fragrans it was 76°, 74°, and 54°. The angle
is often different in the three leaflets of the same leaf. As the
leaflets sink down at night and become folded, their lower surfaces are
brought near together (see B), or even into
close contact; and from this circumstance it might be thought that the
object of the folding was the protection of their lower surfaces. If
this had been the case, it would have formed a strongly marked
exception to the rule, that when there is any difference in the degree
of protection from radiation of the two surfaces of the leaves, it is
always the upper surface which is the best protected. But that the
folding of the leaflets, and consequent mutual approximation of their
lower surfaces, serves merely to allow them to sink down vertically,
may be inferred from the fact that when the leaflets do not radiate
from the summit of a common petiole, or, again, when there is plenty of
room from the sub-petioles not being very short, the leaflets sink down
without becoming folded. This occurs with the leaflets of O. sensitiva,
Plumierii, and bupleurifolia.
Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B,
diagram of leaf asleep, also seen from vertically above.
There is no use in giving a long list of the many species which sleep
in the above described manner. This holds good with species having
rather fleshy leaves, like those of O. carnosa, or large leaves like
those of O. Ortegesii, or four leaflets like those of O. variabilis.
There are, however, some species which show no signs of sleep, viz., O.
pentaphylla, enneaphylla, hirta, and rubella. We will now describe the
nature of the movements in some of the species.
Oxalis acetosella.—The movement of a leaflet, together with that of the
main petiole, are shown in the following diagram (Fig. 128), traced
between 11 A.M. on October 4th and 7.45 A.M. on the 5th. After 5.30
P.M. on the 4th the leaflet sank rapidly, and at 7 P.M. depended
vertically. for some time before it assumed this latter position, its
movements could, of course, no longer be traced on the vertical glass,
and the broken line in the diagram ought to be extended much further
down in this and all other cases. By 6.45 A.M. on the following morning
it had risen considerably, and continued to rise for the next hour;
but, judging from other observations, it would soon have begun to fall
again. Between 11 A.M. and 5.30 P.M. the leaflet moved at least four
times up and four times down before the great nocturnal fall commenced;
it reached its highest point at noon. Similar observations were made on
two other leaflets, with nearly the same results. Sachs and Pfeffer
have also described briefly[4] the autonomous movements of the leaves
of this plant.
[4] Sachs in ‘Flora,’ 1863, p. 470, etc; Pfeffer, ‘Die Period.
Bewegungen,’ etc., 1875, p. 53.
Fig 128. Oxalis acetosella: circumnutation and nyctitropic movements of
a nearly full-grown leaf, with filament attached to the midrib of one
of the leaflets; traced on vertical glass during 20 h. 45m.
On another occasion the petiole of a leaf was secured to a little stick
close beneath the leaflets, and a filament tipped with a bead of
sealing-wax was affixed to the mid-rib of one of them, and a mark was
placed close behind. At 7 P.M., when the leaflets were asleep, the
filament depended vertically down, and the movements of the bead were
then traced till 10.40 P.M., as shown in the following diagram (Fig.
129). We here see that the leaflet moved a little from side to side, as
well as a little up and down, whilst asleep.
Oxalis Valdiviana.—The leaves resemble those of the last species, and
the movements of two leaflets (the main petioles of both having been
secured) were traced during two days; but the tracings are not given,
as they resembled that of O. acetosella, with the exception that the up
and down oscillations were not so frequent during the day, and there
was more lateral movement, so that broader ellipses were described. The
leaves awoke early in the morning, for by 6.45 A.M. on June 12th and
13th they had not only risen to their full height, but had already
begun to fall, that is, they were circumnutating. We have seen in the
last chapter that the cotyledons, instead of sinking, rise up
vertically at night.
Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep;
traced on vertical glass during 3 h. 40 m.
Oxalis Ortegesii.—The large leaves of this plant sleep like those of
the previous species. The main petioles are long, and that of a young
leaf rose 20° between noon and 10 P.M., whilst the petiole of an older
leaf rose only 13°. Owing to this rising of the petioles, and the
vertical sinking of the large leaflets, the leaves become crowded
together at night, and the whole plant then exposes a much smaller
surface to radiation than during the day.
Oxalis Plumierii.—In this species the three leaflets do not surround
the summit of the petiole, but the terminal leaflet projects in the
line of the petiole, with a lateral leaflet on each side. They all
sleep by bending vertically downwards, but do not become at all folded.
The petiole is rather long, and, one having been secured to a stick,
the movement of the terminal leaflet was traced during 45 h. on a
vertical glass. It moved in a very simple manner, sinking rapidly after
5 P.M., and rising rapidly early next morning. During the middle of the
day it moved slowly and a little laterally. Consequently the ascending
and descending lines did not coincide, and a single great ellipse was
formed each day. There was no other evidence of circumnutation, and
this fact is of interest, as we shall hereafter see.
Oxalis sensitiva.—The leaflets, as in the last species, bend vertically
down at night, without becoming folded. The much elongated main petiole
rises considerably in the evening, but in
some very young plants the rise did not commence until late at night.
We have seen that the cotyledons, instead of sinking like the leaflets,
rise up vertically at night.
Oxalis bupleurifolia.—This species is rendered remarkable by the
petioles being foliaceous, like the phyllodes of many Acacias. The
leaflets are small, of a paler green and more tender consistence than
the foliaceous petioles. The leaflet which was observed was .55 inch in
length, and was borne by a petiole 2 inches long and .3 inch broad. It
may be suspected that the leaflets are on the road to abortion or
obliteration, as has actually occurred with those of another Brazilian
species, O. rusciformis. Nevertheless, in the present species the
nyctitropic movements are perfectly performed. The foliaceous petiole
was first observed during 48 h., and found to be in continued
circumnutation, as shown in the accompanying figure (Fig. 130). It rose
during the day and early part of the night, and fell during the
remainder of the night and early morning; but the movement was not
sufficient to be called sleep. The ascending and descending lines did
not coincide, so that an ellipse was formed each day. There was but
little zigzagging; if the filament had been fixed longitudinally, we
should probably have seen that there was more lateral movement than
appears in the diagram.
Fig. 130. Oxalis bupleurifolia: circumnutation of foliaceous petiole,
filament fixed obliquely across end of petiole; movements traced on
vertical glass from 9 A.M. June 26th to 8.50 A.M. 28th. Apex of leaflet
4½ inches from the glass, so movement not much magnified. Plant 9
inches high, illuminated from above. Temp. 23½°–24½° C.
A terminal leaflet on another leaf was next observed (the petiole being
secured), and its movements are shown in Fig. 131. During the day the
leaflets are extended horizontally, and at night depend vertically; and
as the petiole rises during the day the leaflets have to bend down in
the evening
more than 90°, so as to assume at night their vertical position. On the
first day the leaflet simply moved up and down; on the
second day it plainly circumnutated between 8 A.M. and 4.30 P.M., after
which hour the great evening fall commenced.
Fig. 131. Oxalis bupleurifolia: circumnutation and nyctitropic movement
of terminal leaflet, with filament affixed along the midrib; traced on
a vertical glass from 9 A.M. on June 26th to 8.45 A.M. 28th. Conditions
the same as in the last case.
Averrhoa bilimbi (Oxalidæ).—It has long been known,[5] firstly, that
the leaflets in this genus sleep; secondly, that they move
spontaneously during the day; and thirdly, that they are sensitive to a
touch; but in none of these respects do they differ essentially from
the species of Oxalis. They differ, however, as Mr. R. I. Lynch[6] has
lately shown, in their spontaneous movements being strongly marked. In
the case of A. bilimbi, it is a wonderful spectacle to behold on a warm
sunny day the leaflets one after the other sinking rapidly downwards,
and again ascending slowly. Their movements rival those of Desmodium
gyrans. At night the leaflets hang vertically down; and now they are
motionless, but this may be due to the opposite ones being pressed
together (Fig. 132). The main petiole is in constant movement during
the day, but no careful observations were made on it. The following
diagrams are graphic representations of the variations in the angle,
which a given leaflet makes with the vertical. The observations were
made as follows. The plant growing in a pot was kept in a high
temperature, the petiole of the leaf to be observed pointing straight
at the observer, being separated from him by a vertical pane of glass.
The petiole was secured so that the basal joint, or pulvinus, of one of
the lateral leaflets was at the centre of a graduated arc placed close
behind the leaflet. A fine glass filament was fixed to the leaf, so as
to project like a continuation of the
midrib. This filament acted as an index; and as the leaf rose and fell,
rotating about its basal joint, its angular movement could be recorded
by reading off at short intervals of time the position of the glass
filament on the graduated arc. In order
to avoid errors of parallax, all readings were made by looking through
a small ring painted on the vertical glass, in a line with the joint of
the leaflet and the centre of the graduated arc. In the following
diagrams the ordinates represent the angles which the leaflet made with
the vertical at successive instants.[7] It follows that a fall in the
curve represents an actual dropping of the leaf, and that the zero line
represents a vertically dependent position. Fig. 133 represents the
nature of the movements which occur in the evening, as soon as the
leaflets begin to assume their nocturnal position. At 4.55 P.M. the
leaflet formed an angle of 85° with the vertical, or was only 5° below
the horizontal; but in order that the diagram might get into our page,
the leaflet is represented falling from 75° instead of 85°. Shortly
after 6 P.M. it hung vertically down, and had attained its nocturnal
position. Between 6.10 and 6.35 P.M. it performed a number of minute
oscillations of about 2° each, occupying periods of 4 or 5 m. The
complete state of rest of the leaflet which ultimately followed is not
shown in the diagram. It is manifest that each oscillation consists of
a gradual rise, followed by a sudden fall. Each time the leaflet fell,
it approached nearer to the nocturnal position than it did on the
previous fall. The amplitude of the oscillations diminished, while the
periods of oscillation became shorter.
[5] Dr. Bruce, ‘Philosophical Trans.,’ 1785, p. 356.
[6] ‘Journal Linn. Soc.,’ vol. xvi. 1877, p. 231.
[7] In all the diagrams 1 mm. in the horizontal direction represents
one minute of time. Each mm. in the vertical direction represents one
degree of angular movement. In Figs. 133 and 134 the temperature is
represented (along the ordinates) in the scale of 1 mm. to each 0.1
degree C. In Fig. 135 each mm. equals 0.2° F.
Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced.
Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during its
evening descent, when going to sleep. Temp. 78°–81° F.
In bright sunshine the leaflets assume a highly inclined dependent
position. A leaflet in diffused light was observed rising for 25 m. A
blind was then pulled up so that the plant was brightly illuminated (BR
in Fig. 134), and within a minute it began to fall, and ultimately fell
47°, as shown in the diagram. This descent was performed by six
descending steps, precisely similar to those by which the nocturnal
fall is effected. The plant was then again shaded (SH), and a long slow
rise occurred until another series of falls commenced at BR’, when the
sun was again admitted. In this experiment cool air was allowed to
enter by the windows being opened at the same time that the blinds were
pulled up, so that in spite of the sun shining on the plant the
temperature was not raised.
The effect of an increase of temperature in diffused light is
shown in Fig. 135. The temperature began to rise at 11.35 A.M. (in
consequence of the fire being lighted), but by 12.42 a marked fall had
occurred. It may be seen in the diagram that when the temperature was
highest there were rapid oscillations of small amplitude, the mean
position of the leaflet being at the time nearer the vertical. When the
temperature began to fall, the oscillations became slower and larger,
and the mean position of the leaf again approached the horizontal. The
rate of oscillation was sometimes quicker than is represented in the
above diagram. Thus, when the temperature was between 31° and
32° C., 14 oscillations of a few degrees occurred in 19 m. On the other
hand, an oscillation may be much slower; thus a leaflet was observed
(temperature 25° C.) to rise during 40 m. before it fell and completed
its oscillation.
Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a
change from bright illumination to shade; temperature (broken line)
remaining nearly the same.
Fig. 135. Averrhoa bilimbi: angular movement of leaflet during a change
of temperature; light remaining the same. The broken line shows the
change of temperature.
Fig. 136. Porlieria hygrometrica: circumnutation and nyctitropic
movements of petiole of leaf, traced from 9.35 A.M. July 7th to about
midnight on the 8th. Apex of leaf 7½ inches from the vertical glass.
Temp. 19½°–20½° C.
Porlieria hygrometrica (Zygophylleæ).—The leaves of this plant (Chilian
form) are from 1 to 1½ inch in length, and bear as many as 16 or 17
small leaflets on each side, which do not stand opposite one another.
They are articulated to the petiole, and the petiole to the branch by a
pulvinus. We must premise that apparently two forms are confounded
under the same name: the leaves on a bush from Chili, which was sent to
us from Kew, bore many leaflets, whilst those on plants in the Botanic
Garden at Würzburg bore only 8 or 9 pairs; and the whole character of
the bushes appeared somewhat different. We shall also see that they
differ in a remarkable physiological peculiarity. On the Chilian plant
the petioles of the younger leaves on upright branches, stood
horizontally during the day, and at night sank down vertically so as to
depend parallel and close to the branch beneath. The petioles of rather
older leaves did not become at night vertically depressed, but only
highly inclined. In one instance we found a branch which had grown
perpendicularly downwards, and the petioles on it moved in the same
direction relatively to the branch as just stated, and therefore moved
upwards. On horizontal branches the younger petioles likewise move at
night in the same direction as before, that is, towards the branch, and
are consequently then extended horizontally; but it is remarkable that
the older petioles on the
same branch, though moving a little in the same direction, also bend
downwards; they thus occupy a somewhat different position, relatively
to the centre of the earth and to the branch, from that of the petioles
on the upright branches. With respect to the leaflets, they move at
night towards the apex of the petiole until their midribs stand nearly
parallel to it; and they then lie neatly imbricated one over the other.
Thus half of the upper surface of each leaflet is in close contact with
half of the lower surface of the one next in advance; and all the
leaflets, excepting the basal ones, have the whole of their upper
surfaces and half of their lower surfaces well protected. Those on the
opposite sides of the same petiole do not come into close contact at
night, as occurs with the leaflets of so many Leguminosae but are
separated by an open furrow; nor could they exactly coincide, as they
stand alternately with respect to one another.
The circumnutation of the petiole of a leaf 3/4 of an inch in length,
on an upright branch, was observed during 36h., and is shown in the
preceding diagram (Fig. 136). On the first morning, the leaf fell a
little and then rose until 1 P.M., and this was probably due to its
being now illuminated through a skylight from above; it then
circumnutated on a very small scale round the same spot until about 4
P.M., when the great evening fall commenced. During the latter part of
the night or very early on the next morning the leaf rose again. On the
second day it fell during the morning till 1 P.M., and this no doubt is
its normal habit. From 1 to 4 P.M. it rose in a zigzag line, and soon
afterwards the great evening fall commenced. It thus completed a double
oscillation during the 24 h.
The specific name given to this plant by Ruiz and Pavon, indicates that
in its native arid home it is affected in some manner by the dryness or
dampness of the atmosphere.[8] In the Botanic Garden at Würzburg, there
was a plant in a pot out of doors which was daily watered, and another
in the open ground which was never watered. After some hot and dry
weather there was a great difference in the state of the leaflets on
these two plants; those on the unwatered plant in the open ground
remaining half,
or even quite, closed during the day. But twigs cut from this bush,
with their ends standing in water, or wholly immersed in it, or kept in
damp air under a bell-glass, opened their leaves though exposed to a
blazing sun; whilst those on the plant in the ground remained closed.
The leaves on this same plant, after some heavy rain, remained open for
two days; they then became half closed during two days, and after an
additional day were quite closed. This plant was now copiously watered,
and on the following morning the leaflets were fully expanded. The
other plant growing in a pot, after having been exposed to heavy rain,
was placed before a window in the Laboratory, with its leaflets open,
and they remained so during the daytime for 48 h.; but after an
additional day were half closed. The plant was then watered, and the
leaflets on the two following days remained open. On the third day they
were again half closed, but on being again watered remained open during
the two next days. From these several facts we may conclude that the
plant soon feels the want of water; and that as soon as this occurs, it
partially or quite closes its leaflets, which in their then imbricated
condition expose a small surface to evaporation. It is therefore
probable that this sleep-like movement, which occurs only when the
ground is dry, is an adaptation against the loss of moisture.
[8] ‘Systema Veg. Florae Peruvianae et Chilensis,’ tom. i. p. 95,
1798. We cannot understand the account given by the authors of the
behaviour of this plant in its native home. There is much about its
power of foretelling changes in the weather; and it appears as if the
brightness of the sky largely determined the opening and closing of
the leaflets.
A bush about 4 feet in height, a native of Chili, which was thickly
covered with leaves, behaved very differently, for during the day it
never closed its leaflets. On July 6th the earth in the small pot in
which it grew appeared extremely dry, and it was given a very little
water. After 21 and 22 days (on the 27th and 28th), during the whole of
which time the plant did not receive a drop of water, the leaves began
to droop, but they showed no signs of closing during the day. It
appeared almost incredible that any plant, except a fleshy one, could
have kept alive in soil so dry, which resembled the dust on a road. On
the 29th, when the bush was shaken, some leaves fell off, and the
remaining ones were unable to sleep at night. It was therefore
moderately watered, as well as syringed, late in the evening. On the
next morning (30th) the bush looked as fresh as ever, and at night the
leaves went to sleep. It may be added that a small branch while growing
on the bush was enclosed, by means of a curtain of bladder, during 13
days in a large bottle half full of quicklime, so that the air within
must have been intensely dry; yet the leaves on this branch did not
suffer in the
least, and did not close at all during the hottest days. Another trial
was made with the same bush on August 2nd and 6th (the soil appearing
at this latter date extremely dry), for it was exposed out of doors
during the whole day to the wind, but the leaflets showed no signs of
closing. The Chilian form therefore differs widely from the one at
Würzburg, in not closing its leaflets when suffering from the want of
water; and it can live for a surprisingly long time without water.
Tropaeolum majus (?) (cultivated var.) (Tropaeoleae).—Several plants in
pots stood in the greenhouse, and the blades of the leaves which faced
the front-lights were during the day highly inclined and at night
vertical; whilst the leaves on the back of the pots, though of course
illuminated through the roof, did not become vertical at night. We
thought, at first, that this difference in their positions was in some
manner due to heliotropism, for the leaves are highly heliotropic. The
true explanation, however, is that unless they are well illuminated
during at least a part of the day they do not sleep at night; and a
little difference in the degree of illumination determines whether or
not they shall become vertical at night. We have observed no other so
well-marked a case as this, of the influence of previous illumination
on nyctitropic movements. The leaves present also another peculiarity
in their habit of rising or awaking in the morning, being more strongly
fixed or inherited than that of sinking or sleeping at night. The
movements are caused by the bending of an upper part of the petiole,
between ½ and 1 inch in length; but the part close to the blade, for
about 1/4 of an inch in length, does not bend and always remains at
right angles to the blade. The bending portion does not present any
external or internal difference in structure from the rest of the
petiole. We will now give the experiments on which the above
conclusions are founded.
A large pot with several plants was brought on the morning of Sept. 3rd
out of the greenhouse and placed before a north-east window, in the
same position as before with respect to the light, as far as that was
possible. On the front of the plants, 24 leaves were marked with
thread, some of which had their blades horizontal, but the greater
number were inclined at about 45°, beneath the horizon; at night all
these, without exception, became vertical. Early on the following
morning (4th) they reassumed their former positions, and at night again
became vertical. On the 5th the shutters were opened at 6.15 A.M., and
by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m. and
had acquired their diurnal position, they were placed in a dark
cupboard. They were looked at twice during the day and thrice in the
evening, the last time at 10.30 P.M., and not one had become vertical.
At 8 A.M. on the following morning (6th) they still retained the same
diurnal position, and were now replaced before the north-east window.
At night all the leaves which had faced the light had their petioles
curved and their blades vertical; whereas none of the leaves on the
back of the plants, although they had been moderately illuminated by
the diffused light of the room, were vertical. They were now at night
placed in the same dark cupboard; at 9 A.M. on the next morning (7th)
all those which had been asleep had reassumed their diurnal position.
The pot was then placed for 3 h. in the sunshine, so as to stimulate
the plants; at noon they were placed before the same north-east window,
and at night the leaves slept in the usual manner and awoke on the
following morning. At noon on this day (8th) the plants, after having
been left before the north-east window for 5 h. 45 m. and thus
illuminated (though not brightly, as the sky was cloudy during the
whole time), were replaced in the dark cupboard, and at 3 P.M. the
position of the leaves was very little, if at all, altered, so that
they are not quickly affected by darkness; but by 10.15 P.M. all the
leaves which had faced the north-east sky during the 5 h. 45 m. of
illumination stood vertical, whereas those on the back of the plant
retained their diurnal position. On the following morning (9th) the
leaves awoke as on the two former occasions in the dark, and they were
kept in the dark during the whole day; at night a very few of them
became vertical, and this was the one instance in which we observed any
inherited tendency or habit in this plant to sleep at the proper time.
That it was real sleep was shown by these same leaves reassuming their
diurnal position on the following morning (10th) whilst still kept in
the dark.
The pot was then (9.45 A.M. 10th) replaced, after having been kept for
36 h. in darkness, before the north-east window; and at night the
blades of all the leaves (excepting a few on the back of the plants)
became conspicuously vertical.
At 6.45 A.M. (11th) after the plants had been illuminated on the same
side as before during only 25 m., the pot was turned round, so that the
leaves which had faced the light now faced the interior of the room,
and not one of these went to sleep at night;
whilst some, but not many, of those which had formerly stood facing the
back of the room and which had never before been well illuminated or
gone to sleep, now assumed a vertical position at night. On the next
day (12th) the plant was turned round into its original position, so
that the same leaves faced the light as formerly, and these now went to
sleep in the usual manner. We will only add that with some young
seedlings kept in the greenhouse, the blades of the first pair of true
leaves (the cotyledons being hypogean) stood during the day almost
horizontally and at night almost vertically.
A few observations were subsequently made on the circumnutation of
three leaves, whilst facing a north-east window; but the tracings are
not given, as the leaves moved somewhat towards the light. It was,
however, manifest that they rose and fell more than once during the
daytime, the ascending and descending lines being in parts extremely
zigzag. The nocturnal fall commenced about 7 P.M., and the leaves had
risen considerably by 6.45 A.M. on the following morning.
Leguminosae.—This Family includes many more genera with sleeping
species than all the other families put together. The number of the
tribes to which each genus belongs, according to Bentham and Hooker’s
arrangement, has been added.
Crotolaria (sp.?) (Tribe 2).—This plant is monophyllous, and we are
informed by Mr. T. Thiselton Dyer that the leaves rise up vertically at
night and press against the stem.
Lupinus (Tribe 2).—The palmate or digitate leaves of the species in
this large genus sleep in three different manners. One of the simplest,
is that all the leaflets become steeply inclined downwards at night,
having been during the day extended horizontally. This is shown in the
accompanying figures (Fig. 137), of a leaf of L. pilosus, as seen
during the day from vertically above, and of another leaf asleep with
the leaflets inclined downwards. As in this position they are crowded
together, and as they do not become folded like those in the genus
Oxalis, they cannot occupy a vertically dependent position; but they
are often inclined at an angle of 50° beneath the horizon. In this
species, whilst the leaflets are sinking, the petioles rise up, in two
instances when the angles were measured to the extent of 23°. The
leaflets of L. sub-carnosus and arboreus, which were horizontal during
the day, sank down at night in nearly the same manner; the former to an
angle of 38° and the latter of 36°, beneath the horizon; but their
petioles
did not move in any plainly perceptible degree. It is, however, quite
possible, as we shall presently see, that if a large number of plants
of the three foregoing and of the following species were to be observed
at all seasons, some of the leaves would be found to sleep in a
different manner.
Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in
daytime; B, leaf asleep, seen laterally at night.
In the two following species the leaflets, instead of moving downwards,
rise at night. With L. Hartwegii some stood at noon at a mean angle of
36° above the horizon, and at night at 51°, thus forming together a
hollow cone with moderately steep sides. The petiole of one leaf rose
14° and of a second 11° at night. With L. luteus a leaflet rose from
47° at noon to 65° above the horizon at night, and another on a
distinct leaf rose from 45° to 69°. The petioles, however, sink at
night to a small extent, viz., in three instances by 2°, 6°, and 9° 30
seconds. Owing to this movement of the petioles, the outer and longer
leaflets have to bend up a little more than the shorter and inner ones,
in order that all should stand symmetrically at night. We shall
presently see that some leaves on the same individual plants of L.
luteus sleep in a very different manner.
We now come to a remarkable position of the leaves when asleep, which
is common to several species of Lupines. On the same leaf the shorter
leaflets, which generally face the centre of the plant, sink at night,
whilst the longer ones on the opposite side rise; the intermediate and
lateral ones merely twisting on their own axes. But there is some
variability with respect to which leaflets rise or fall. As might have
been expected from such diverse and complicated movements, the
base of each leaflet is developed (at least in the case of L. luteus)
into a pulvinus. The result is that all the leaflets on the same leaf
stand at night more or less highly inclined, or even quite vertically,
forming in this latter case a vertical star. This occurs with the
leaves of a species purchased under the name of L. pubescens; and in
the accompanying figures we see at A (Fig. 138) the leaves in their
diurnal position; and at B the same plant at night with the two upper
leaves having their leaflets almost vertical. At C another leaf, viewed
laterally, is shown with the leaflets quite vertical. It is chiefly or
exclusively the youngest leaves which form at night vertical stars. But
there
is much variability in the position of the leaves at night on the same
plant; some remaining with their leaflets almost horizontal, others
forming more or less highly inclined or vertical stars, and some with
all their leaflets sloping downwards, as in our first class of cases.
It is also a remarkable fact, that although all the plants produced
from the same lot of seeds were identical in appearance, yet some
individuals at night had the leaflets of all their leaves arranged so
as to form more or less highly inclined stars; others had them all
sloping downwards and never forming a star; and others, again, retained
them either in a horizontal position or raised them a little.
Fig. 138. Lupinus pubescens: A, leaf viewed laterally during the day;
B, same leaf at night; C, another leaf with the leaflet forming a
vertical star at night. Figures reduced.
We have as yet referred only to the different positions of the leaflets
of L. pubescens at night; but the petioles likewise differ in their
movements. That of a young leaf which formed a highly inclined star at
night, stood at noon at 42° above the horizon, and during the night at
72°, so had risen 30°. The petiole of another leaf, the leaflets of
which occupied a similar position at night, rose only 6°. On the other
hand, the petiole of a leaf with all its leaflets sloping down at
night, fell at this time 4°. The petioles of two rather older leaves
were subsequently observed; both of which stood during the day at
exactly the same angle, viz., 50° above the horizon, and one of these
rose 7°–8°, and the other fell 3°–4° at night. We meet with cases like
that of L. pubescens with some other species. On a single plant of L.
mutabilis some leaves, which stood horizontally during the day, formed
highly inclined stars at night, and the petiole of one rose 7°. Other
leaves which likewise stood horizontally during the day, had at night
all their leaflets sloping downwards at 46° beneath the horizon, but
their petioles had hardly moved. Again, L. luteus offered a still more
remarkable case, for on two leaves, the leaflets which stood at noon at
about 45° above the horizon, rose at night to 65° and 69°, so that they
formed a hollow cone with steep sides. Four leaves on the same plant,
which had their leaflets horizontal at noon, formed vertical stars at
night; and three other leaves equally horizontal at noon, had all their
leaflets sloping downwards at night. So that the leaves on this one
plant assumed at night three different positions. Though we cannot
account for this fact, we can see that such a stock might readily give
birth to species having widely different nyctitropic habits.
Little more need be said about the sleep of the species of Lupinus;
several, namely, L. polyphyllus, nanus, Menziesii, speciosus,
and albifrons, though observed out of doors and in the greenhouse, did
not change the position of their leaves sufficiently at night to be
said to sleep. From observations made on two sleeping species, it
appears that, as with Tropaeolum majus, the leaves must be well
illuminated during the day in order to sleep at night. For several
plants, kept all day in a sitting-room with north-east windows, did not
sleep at night; but when the pots were placed on the following day out
of doors, and were brought in at night, they slept in the usual manner.
the trial was repeated on the following day and night with the same
result.
Some observations were made on the circumnutation of the leaves of L.
luteus and arboreus. It will suffice to say that the leaflets of the
latter exhibited a double oscillation in the course of 24 h.; for they
fell from the early morning until 10.15 A.M., then rose and zigzagged
greatly till 4 P.M., after which hour the great nocturnal fall
commenced. By 8 A.M. on the following morning the leaflets had risen to
their proper height. We have seen in the fourth chapter, that the
leaves of Lupinus speciosus, which do not sleep, circumnutate to an
extraordinary extent, making many ellipses in the course of the day.
Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3).—Only a few
observations were made on these three genera. The petioles on a young
plant, about a foot in height, of Cytisus fragrans rose at night, on
one occasion 23° and on another 33°. The three leaflets also bend
upwards, and at the same time
approach each other, so that the base of the central leaflet overlaps
the bases of the two lateral leaflets. They bend up so much that they
press against the stem; and on looking down on one of these young
plants from vertically above, the lower surfaces of the leaflets are
visible; and thus their upper surfaces, in accordance with the general
rule, are best protected from radiation. Whilst the leaves on these
young plants were thus behaving, those on an old bush in full flower
did not sleep at night.
Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep
at night.
Trigonella Cretica resembles a Melilotus in its sleep, which will be
immediately described. According to M. Royer,[9] the leaves of Medicago
maculata rise up at night, and “se renversent un peu de manière à
presenter obliquement au ciel leur face inférieure.” A drawing is here
given (Fig. 139) of the leaves of M. marina awake and asleep; and this
would almost serve for Cytisus fragrans in the same two states.
[9] ‘Annales des Sc. Nat. Bot.’ (5th series), ix. 1868, p. 368.
Melilotus (Tribe 3).—The species in this genus sleep in a remarkable
manner. The three leaflets of each leaf twist through an angle of 90°,
so that their blades stand vertically at night with one lateral edge
presented to the zenith (Fig. 140). We shall best understand the other
and more complicated movements, if we imagine ourselves always to hold
the leaf with the tip of the terminal leaflet pointed to the north. The
leaflets in becoming vertical at night could of course twist so that
their upper surfaces should face to either side; but the two lateral
leaflets always twist so that this surface tends to face the north, but
as they move at the same time towards the terminal leaflet, the upper
surface of the one faces about N.N.W., and that of the other N.N.E. The
terminal leaflet behaves differently, for it twists to either side, the
upper surface facing sometimes east and sometimes west, but rather more
commonly west than east. The terminal leaflet also moves in another and
more remarkable manner, for whilst its blade is twisting and becoming
vertical, the whole leaflet bends to one side, and invariably to the
side towards which the upper surface is directed; so that if this
surface faces the west the whole leaflet bends to the west, until it
comes into contact with the upper and vertical surface of the western
lateral leaflet. Thus the upper surface of the terminal and of one of
the two lateral leaflets is well protected.
The fact of the terminal leaflet twisting indifferently to either
side and afterwards bending to the same side, seemed to us so
remarkable, that we endeavoured to discover the cause. We imagined that
at the commencement of the movement it might be determined by one of
the two halves of the leaflet being a little heavier than the other.
Therefore bits of wood were gummed on one side of several leaflets, but
this produced no effect; and they continued to twist in the same
direction as they had previously done. In order to discover whether the
same leaflet twisted permanently in the same direction, black threads
were tied to 20 leaves, the terminal leaflets of which twisted so that
their upper surfaces faced west, and 14 white threads to leaflets which
twisted to the east. These were observed occasionally during 14 days,
and they all continued, with a single exception, to twist and bend in
the same direction; for
one leaflet, which had originally faced east, was observed after 9 days
to face west. The seat of both the twisting and bending movement is in
the pulvinus of the sub-petioles.
Fig. 140. Melilotus officinalis: A, leaf during the daytime. B, another
leaf asleep. C, a leaf asleep as viewed from vertically above; but in
this case the terminal leaflet did not happen to be in such close
contact with the lateral one, as is usual.
We believe that the leaflets, especially the two lateral ones, in
performing the above described complicated movements generally bend a
little downwards; but we are not sure of this, for, as far as the main
petiole is concerned, its nocturnal movement is largely determined by
the position which the leaf happens to occupy during the day. Thus one
main petiole was observed to rise at night 59°, whilst three others
rose only 7° and 9°. The petioles and sub-petioles are continually
circumnutating during the whole 24 h., as we shall presently see.
The leaves of the following 15 species, M. officinalis, suaveolens,
parviflora, alba, infesta, dentata, gracilis, sulcata, elegans,
coerulea, petitpierreana, macrorrhiza, Italica, secundiflora, and
Taurica, sleep in nearly the same manner as just described; but the
bending to one side of the terminal leaflet is apt to fail unless the
plants are growing vigorously. With M. petitpierreana and secundiflora
the terminal leaflet was rarely seen to bend to one side. In young
plants of M. Italica it bent in the usual manner, but with old plants
in full flower, growing in the same pot and observed at the same hour,
viz., 8.30 P.M., none of the terminal leaflets on several scores of
leaves had bent to one side, though they stood vertically; nor had the
two lateral leaflets, though standing vertically, moved towards the
terminal one. At 10.30 P.M., and again one hour after midnight, the
terminal leaflets had become very slightly bent to one side, and the
lateral leaflets had moved a very little towards the terminal one, so
that the position of the leaflets even at this late hour was far from
the ordinary one. Again, with M. Taurica the terminal leaflets were
never seen to bend towards either of the two lateral leaflets, though
these, whilst becoming vertical, had bent towards the terminal one. The
sub-petiole of the terminal leaflet in this species is of unusual
length, and if the leaflet had bent to one side, its upper surface
could have come into contact only with the apex of either lateral
leaflet; and this, perhaps, is the meaning of the loss of the lateral
movement.
The cotyledons do not sleep at night. the first leaf consists of a
single orbicular leaflet, which twists at night so that the blade
stands vertically. It is a remarkable fact that with M. Taurica, and in
a somewhat less degree with M. macrorrhiza and petitpierreana, all the
many small and young leaves produced during
the early spring from shoots on some cut-down plants in the greenhouse,
slept in a totally different manner from the normal one; for the three
leaflets, instead of twisting on their own axes so as to present their
lateral edges to the zenith, turned upwards and stood vertically with
their apices pointing to the zenith. They thus assumed nearly the same
position as in the allied genus Trifolium; and on the same principle
that embryological characters reveal the lines of descent in the animal
kingdom, so the movements of the small leaves in the above three
species of Melilotus, perhaps indicate that this genus is descended
from a form which was closely allied to and slept like a Trifolium.
Moreover, there is one species, M. messanensis, the leaves of which, on
full-grown plants between 2 and 3 feet in height, sleep like the
foregoing small leaves and like those of a Trifolium. We were so much
surprised at this latter case that, until the flowers and fruit were
examined, we thought that the seeds of some Trifolium had been sown by
mistake instead of those of a Melilotus. It appears therefore probable
that M. messanensis has either retained or recovered a primordial
habit.
The circumnutation of a leaf of M. officinalis was traced, the stem
being left free; and the apex of the terminal leaflet described three
laterally extended ellipses, between 8 A.M. and 4 P.M.; after the
latter hour the nocturnal twisting movement commenced. It was
afterwards ascertained that the above movement was compounded of the
circumnutation of the stem on a small scale, of the main petiole which
moved most, and of the sub-petiole of the terminal leaflet. The main
petiole of a leaf having been secured to a stick, close to the base of
the sub-petiole of the terminal leaflet, the latter described two small
ellipses between 10.30 A.M., and 2 P.M. At 7.15 P.M., after this same
leaflet (as well as another) had twisted themselves into their vertical
nocturnal position, they began to rise slowly, and continued to do so
until 10.35 P.M., after which hour they were no longer observed.
As M. messanensis sleeps in an anomalous manner, unlike that of any
other species in the genus, the circumnutation of a terminal leaflet,
with the stem secured, was traced during two days. On each morning the
leaflet fell, until about noon, and then began to rise very slowly; but
on the first day the rising movement was interrupted between 1 and 3
P.M. by the formation of a laterally extended ellipse, and on the
second day, at the same time, by two smaller ellipses. The rising
movement then
recommenced, and became rapid late in the evening, when the leaflet was
beginning to go to sleep. The awaking or sinking movement had already
commenced by 6.45 A.M. on both mornings.
Trifolium (Tribe 3).—The nyctitropic movements of 11 species were
observed, and were found to be closely similar. If we select a leaf of
T. repens having an upright petiole, and with the three leaflets
expanded horizontally, the two lateral leaflets will be seen in the
evening to twist and approach each other, until their upper surfaces
come into contact. At the same time they bend downwards in a plane at
right angles to that of their former position, until their midribs form
an angle of about 45° with the upper part of the petiole. This peculiar
change of position requires a considerable amount of torsion in the
pulvinus. The terminal leaflet merely rises up without any twisting and
bends over until it rests on and forms a roof over the edges of the now
vertical and united lateral leaflets. Thus the terminal leaflet always
passes through an angle of at least 90°, generally of 130° or 140°, and
not rarely—as was often observed with T. subterraneum—of 180°. In this
latter case the terminal leaflet stands at night horizontally (as in
Fig. 141), with its lower surface fully exposed to the zenith. Besides
the difference in the angles, at which the terminal leaflets stand at
night in the individuals of the same species, the degree to which the
lateral leaflets approach each other often likewise differs.
Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at
night.
We have seen that the cotyledons of some species and not of others rise
up vertically at night. The first true leaf is generally unifoliate and
orbicular; it always rises, and either stands vertically at night or
more commonly bends a little over so as to expose the lower surface
obliquely to the zenith, in the same manner as does the terminal
leaflet of the mature leaf. But it does not twist itself like the
corresponding first simple leaf of Melilotus.
With T. Pannonicum the first true leaf was generally unifoliate, but
sometimes trifoliate, or again partially lobed and in an intermediate
condition.
Circumnutation.—Sachs described in 1863[10] the spontaneous up and down
movements of the leaflets of T. incarnatum, when kept in darkness.
Pfeffer made many observations on the similar movements in T.
pratense.[11] He states that the terminal leaflet of this species,
observed at different times, passed through angles of from 30° to 120°
in the course of from 1½ to 4 h. We observed the movements of T.
subterraneum, resupinatum, and repens.
[10] ‘Flora,’ 1863, p. 497.
[11] ‘Die Period. Bewegungen,’ 1875, pp. 35, 52.
Trifolium subterraneum.—A petiole was secured close to the base of the
three leaflets, and the movement of the terminal leaflet was traced
during 26½ h., as shown in the figure on the next page.
Between 6.45 A.M. and 6 P.M. the apex moved 3 times up and 3 times
down, completing 3 ellipses in 11 h. 15 m. The ascending and descending
lines stand nearer to one another than is usual with most plants, yet
there was some lateral motion. At 6 P.M. the great nocturnal rise
commenced, and on the next morning the sinking of the leaflet was
continued until 8.30 A.M., after which hour it circumnutated in the
manner just described. In the figure the great nocturnal rise and the
morning fall are greatly abbreviated, from the want of space, and are
merely represented by a short curved line. The leaflet stood
horizontally when at a point a little beneath the middle of the
diagram; so that during the daytime it oscillated almost equally above
and beneath a horizontal position. At 8.30 A.M. it stood 48° beneath
the horizon, and by 11.30 A.M. it had risen 50° above the horizon; so
that it passed through 98° in 3 h. By the aid of the tracing we
ascertained that the distance travelled in the 3 h. by the apex of this
leaflet was 1.03 inch. If we look at the figure, and prolong upwards in
our mind’s eye the short curved broken line, which represents the
nocturnal course, we see that the latter movement is merely an
exaggeration or prolongation of one of the diurnal ellipses. The same
leaflet had been observed on the previous day, and the course then
pursued was almost identically the same as that here described.
Fig. 142. Trifolium subterraneum: circumnutation and nyctitropic
movement of terminal leaflet (.68 inch in length), traced from 6.45
A.M. July 4th to 9.15 A.M. 5th. Apex of leaf 3 7/8 inches from the
vertical glass, and movement, as here shown, magnified 5 1/4 times,
reduced to one-half of original scale. Plant illuminated from above;
temp. 16°–17° C.
Trifolium resupinatum.—A plant left entirely free was placed before a
north-east window, in such a position that a terminal leaflet projected
at right angles to the source of the light, the sky being uniformly
clouded all day. The movements of this leaflet were traced during two
days, and on both were closely similar. Those executed on the second
day are shown in Fig. 143. The obliquity of the several lines is due
partly to the manner in which the leaflet was viewed, and partly to its
having moved a little towards the light. From 7.50 A.M. to 8.40 A.M.
the leaflet fell, that is, the awakening movement was continued. It
then rose and moved a little laterally towards the light. At 12.30 it
retrograded, and at 2.30 resumed its original course, having thus
completed a small ellipse during the middle of the day. In the evening
it rose rapidly, and by 8 A.M. on the following morning had returned to
exactly the same spot as on the previous morning. The line representing
the nocturnal course ought to be extended much higher up, and is here
abbreviated into a short,
curved, broken line. The terminal leaflet, therefore, of this species
described during the daytime only a single additional ellipse, instead
of two additional ones, as in the case of T. subterraneum. But we
should remember that it was shown in the fourth chapter that the stem
circumnutates, as no doubt does the main petiole and the sub-petioles;
so that the movement represented in Fig. 143 is a compounded one. We
tried to observe the movements of a leaf kept during the day in
darkness, but it began to go to sleep after 2 h. 15 m., and this was
well pronounced after 4 h. 30 m.
Fig 143. Trifolium resupinatum: circumnutation and nyctitropic
movements of the terminal leaflet during 24 hours.
Trifolium repens.—A stem was secured close to the base of a moderately
old leaf, and the movement of the terminal leaflet was observed during
two days. This case is interesting solely from the simplicity of the
movements, in contrast with those of the two preceding species. On the
first day the leaflet fell between 8 A.M. and 3 P.M., and on the second
between 7 A.M. and 1 P.M. On both days the descending course was
somewhat zigzag, and this evidently represents the circumnutating
movement of the two previous species during the middle of the day.
After 1 P.M., Oct. 1st (Fig. 144), the leaflet began to rise, but the
movement was slow on both days, both before and after this hour, until
4 P.M. The rapid evening and nocturnal rise then commenced. Thus in
this species the course during 24 h. consists of a single great
ellipse; in T. resupinatum of two ellipses, one of which includes the
nocturnal movement and is much elongated; and in T. subterraneum of
three ellipses, of which the nocturnal one is likewise of great length.
Securigera coronilla (Tribe 4).—The leaflets, which stand opposite one
another and are numerous, rise up at night, come into close contact,
and bend backwards at a moderate angle towards the base of the petiole.
Fig. 144. Trifolium repens: circumnutation and nyctitropic movements of
a nearly full-grown terminal leaflet, traced on a vertical glass from 7
A.M. Sept. 30th to 8 A.M. Oct. 1st. Nocturnal course, represented by
curved broken line, much abbreviated.
Lotus (Tribe 4).—The nyctitropic movements of 10 species in this genus
were observed, and found to be the same. The main petiole rises a
little at night, and the three leaflets rise till they become vertical,
and at the same time approach each other. This was conspicuous with L.
Jacoboeus, in which the leaflets are almost linear. In most of the
species the leaflets rise so much as to press against the stem, and not
rarely they become inclined a little inwards with their lower surfaces
exposed obliquely to the zenith. This was clearly the case with L.
major, as its petioles are unusually long, and the leaflets are thus
enabled to bend further inwards. The young leaves on the summits of the
stems close up at night so much, as often to resemble large buds. The
stipule-like leaflets, which are often of large size, rise up like the
other leaflets, and press against the stem (Fig. 145). All the leaflets
of L. Gebelii, and probably of the other species, are provided at their
bases with distinct pulvini, of a yellowish colour, and formed of very
small cells. The circumnutation of a terminal leaflet of L. peregrinus
(with the stem secured) was traced during two days, but the movement
was so simple that it is not worth while to give the diagram. The
leaflet fell slowly from the early morning till about 1 P.M. It then
rose gradually at first, but rapidly late in the evening. It
occasionally stood still for about 20 m. during the day, and sometimes
zigzagged a little. The movement of one of the basal, stipule-like
leaflets was likewise traced in the same manner and at the same time,
and its course was closely similar to that of the terminal leaflet.
Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B,
with leaves asleep at night. SS, stipule-like leaflets.
In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12
genera have been observed by ourselves and
others, but only in Robinia with any care. Psoralea acaulis raises its
three leaflets at night; whilst Amorpha fruticosa,[12] Dalea
alopecuroides, and Indigofera tinctoria depress them. Ducharte[13]
states that Tephrosia caribaea is the sole example of “folioles
couchées le long du pétiole et vers la base;” but a similar movement
occurs, as we have already seen, and shall again see in other cases.
Wistaria Sinensis, according to Royer,[14] “abaisse les folioles qui
par une disposition bizarre sont inclinées dans la même feuille, les
supérieures vers
le sommet, les inférieures vers la base du petiole commun;” but the
leaflets on a young plant observed by us in the greenhouse merely sank
vertically downwards at night. The leaflets are raised in Sphaerophysa
salsola, Colutea arborea, and Astragalus uliginosus, but are depressed,
according to Linnæus, in Glycyrrhiza. The leaflets of Robinia
pseudo-acacia likewise sink vertically down at night, but the petioles
rise a little, viz., in one case 3°, and in another 4°. The
circumnutating movements of a terminal leaflet on a rather old leaf
were traced during two days, and were simple. The leaflet fell slowly,
in a slightly zigzag line, from 8 A.M. to 5 P.M., and then more
rapidly; by 7 A.M. on the following morning it had risen to its diurnal
position. There was only one peculiarity in the movement, namely, that
on both days there was a distinct though small oscillation up and down
between 8.30 and 10 A.M., and this would probably have been more
strongly pronounced if the leaf had been younger.
[12] Ducharte, ‘Eléments de Botanique’, 1867, p. 349.
[13] Ibid., p. 347.
[14] ‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868.
Coronilla rosea (Tribe 6).—the leaves bear 9 or 10 pairs of opposite
leaflets, which during the day stand horizontally, with their midribs
at right angles to the petiole. At night they rise up so that the
opposite leaflets come nearly into contact, and those on the younger
leaves into close contact. At the same time they bend back towards the
base of the petiole, until their midribs form with it angles of from
40° to 50° in a vertical plane, as here figured (Fig. 146). The
leaflets, however, sometimes bend so much back that their midribs
become parallel to and lie on the petiole. They thus occupy a reversed
position to what they do in several Leguminosae, for instance, in
Mimosa
pudica; but, from standing further apart, they do not overlap one
another nearly so much as in this latter plant. The main petiole is
curved slightly downwards during the day, but straightens itself at
night. In three cases it rose from 3° above the horizon at noon, to 9°
at 10 P.M.; from 11° to 33°; and from 5° to 33°—the amount of angular
movement in this latter case amounting to 28°. In several other species
of Coronilla the leaflets showed only feeble movements of a similar
kind.
Fig. 146. Coronilla rosea: leaf asleep.
Hedysarum coronarium (Tribe 6).—The small lateral leaflets on plants
growing out of doors rose up vertically at night, but the large
terminal one became only moderately inclined. The petioles apparently
did not rise at all.
Smithia Pfundii (Tribe 6).—The leaflets rise up vertically, and the
main petiole also rises considerably.
Arachis hypogoea (Tribe 6).—The shape of a leaf, with its two pairs of
leaflets, is shown at A (Fig. 147); and a leaf asleep, traced from a
photograph (made by the aid of aluminium light), is given at B. The two
terminal leaflets twist round at night until their blades stand
vertically, and approach each other until they meet, at the same time
moving a little upwards and backwards. The two lateral leaflets meet
each other in this same manner, but move to a greater extent forwards,
that is, in a contrary direction to the two terminal leaflets, which
they partially embrace. Thus all four leaflets form together a single
packet, with their edges directed to the zenith, and with their lower
surfaces turned outwards. On a plant which was not growing vigorously
the closed leaflets seemed too heavy for the
petioles to support them in a vertical position, so that each night the
main petiole became twisted, and all the packets were extended
horizontally, with the lower surfaces of the leaflets on one side
directed to the zenith in a most anomalous manner. This fact is
mentioned solely as a caution, as it surprised us greatly, until we
discovered that it was an anomaly. The petioles are inclined upwards
during the day, but sink at night, so as to stand at about right angles
with the stem. The amount of sinking was measured only on one occasion,
and found to be 39°. A petiole was secured to a stick at the base of
the two terminal leaflets, and the circumnutating movement of one of
these leaflets was traced from 6.40 A.M. to 10.40 P.M., the plant being
illuminated from above. The temperature was 17°–17½° C., and therefore
rather too low. During the 16 h. the leaflet moved thrice up and thrice
down, and as the ascending and descending lines did not coincide, three
ellipses were formed.
Fig. 147. Arachis hypogoea: A, leaf during the day, seen from
vertically above; B, leaf asleep, seen laterally, copied from a
photograph. Figures much reduced.
Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-half
natural size. The minute stipules unusually large.
Desmodium gyrans (Tribe 6).—A large and full-grown leaf of this plant,
so famous for the spontaneous movements of the two little lateral
leaflets, is here represented (Fig. 148). The large terminal leaflet
sleeps by sinking vertically down, whilst the petiole rises up. The
cotyledons do not sleep, but the first-formed leaf sleeps equally well
as the older ones. The appearance presented by a sleeping branch and
one in the day-time, copied from two photographs, are shown at A and B
(Fig. 149), and we see how at night the leaves are crowded together, as
if for mutual protection, by the rising of the petioles. The petioles
of the younger leaves near the summits of the shoots rise up at night,
so as to stand vertical and parallel to the stem; whilst those on the
sides were found in four cases to have risen respectively 46½°, 36°,
20°, and 19.5° above the inclined positions which they had occupied
during the day. For instance, in the first of these four cases the
petiole stood in the day at 23°, and at night at 69½° above the
horizon. In the evening the rising of the petioles is almost completed
before the leaflets sink perpendicularly downwards.
Circumnutation.—The circumnutating movements of four young shoots were
observed during 5 h. 15 m.; and in this time each completed an oval
figure of small size. The main petiole also circumnutates rapidly, for
in the course of 31 m. (temp. 91° F.) it changed its course by as much
as a rectangle six times, describing a figure which apparently
represented two ellipses. The movement of the terminal leaflet by means
of its sub-petiole or pulvinus is quite as rapid, or even more so, than
that of the main petiole, and has much greater amplitude. Pfeffer has
seen[15] these leaflets move through an angle of 8° in the course of
from 10 to 30 seconds.
[15] ‘Die Period. Beweg.,’ p. 35.
Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves
asleep. Figures reduced.
A fine, nearly full-grown leaf on a young plant, 8 inches in height,
with the stem secured to a stick at the base of the leaf, was observed
from 8.30 A.M. June 22nd to 8 A.M. June 24th.
In the diagram given on the next page (Fig. 150), the two curved broken
lines at the base, which represent the nocturnal courses, ought to be
prolonged far downwards. On the first day the leaflet moved thrice down
and thrice up, and to a considerable distance laterally; the course was
also remarkably crooked. The dots were generally made every hour; if
they had been made every few minutes all the lines would have been
zigzag to an extraordinary degree, with here and there a loop formed.
We may infer that this would have been the case, because five dots were
made in the course of 31 m. (between 12.34 and 1.5 P.M.), and we see in
the upper part of the diagram how crooked the course here is; if only
the first and last dots had been joined we should have had a straight
line. Exactly the same fact may be seen in the lines representing the
course between 2.24 P.M. and 3 P.M., when six intermediate dots were
made; and again at 4.46 and 4.50. But the result was widely different
after 6 P.M.,—that is, after the great nocturnal descent had commenced;
for though nine dots were then made in the course of 32 m., when these
were joined (see Figure) the line thus formed was almost straight. The
leaflets, therefore, begin to descend in the afternoon by zigzag lines,
but as soon as the descent becomes rapid their whole energy is expended
in thus moving, and their course becomes rectilinear. After the
leaflets are completely asleep they move very little or not at all.
Fig. 150. Desmodium gyrans: circumnutation and nyctitropic movement of
leaf (3 3/4 inches in length, petiole included) during 48 h. Filament
affixed to midrib of terminal leaflet; its apex 6 inches from the
vertical glass. Diagram reduced to one-third of original scale. Plant
illuminated from above. Temp. 19°–20° C.
Had the above plant been subjected to a higher temperature than 67°–70°
F., the movements of the terminal leaflet would probably have been even
more rapid and wider in extent than those shown in the diagram; for a
plant was kept for some time in the hot-house at from 92°–93° F., and
in the course of 35 m. the apex of a leaflet twice descended and once
ascended, travelling over a space of 1.2 inch in a vertical direction
and of .82 inch in a horizontal direction. Whilst thus moving the
leaflet also rotated on its own axis (and this was a point to which no
attention had been before paid), for the plane of the blade differed by
41° after an interval of only a few minutes. Occasionally the leaflet
stood still for a short time. There was no jerking movement, which is
so characteristic of the little lateral leaflets. A sudden and
considerable fall of temperature causes the terminal leaflet to sink
downwards; thus a cut-off leaf was immersed in water at 95° F., which
was slowly raised to 103° F., and afterwards allowed to sink to 70° F.,
and the sub-petiole of the terminal leaflet then curved downwards. The
water was afterwards
raised to 120° F., and the sub-petiole straightened itself. Similar
experiments with leaves in water were twice repeated, with nearly the
same result. It should be added, that water raised to even 122° F. does
not soon kill a leaf. A plant was placed in darkness at 8.37 A.M., and
at 2 P.M. (i.e. after 5 h. 23 m.), though the leaflets had sunk
considerably, they had by no means acquired their nocturnal vertically
dependent position. Pfeffer, on the other hand, says[16] that this
occurred with him in from 3/4 h. to 2 h.; perhaps the difference in our
results may be due to the plant on which we experimented being a very
young and vigorous seedling.
[16] ‘Die Period. Beweg.,’ p. 39.
The Movements of the little Lateral Leaflets .—These have been so often
described, that we will endeavour to be as brief as possible in giving
a few new facts and conclusions. The leaflets sometimes quickly change
their position by as much as nearly 180°; and their sub-petioles can
then be seen to become greatly curved. They rotate on their own axes,
so that their upper surfaces are directed to all points of the compass.
The figure described by the apex is an irregular oval or ellipse. They
sometimes remain stationary for a period. In these several respects
there is no difference, except in rapidity and extent, between their
movements and the lesser ones performed by the large terminal leaflet
whilst making its great oscillations. The movements of the little
leaflets are much influenced, as is well known, by temperature. This
was clearly shown by immersing leaves with motionless leaflets in cold
water, which was slowly raised to 103° F., and the leaflets then moved
quickly, describing about a dozen little irregular circles in 40 m. By
this time the water had become much cooler, and the movements became
slower or almost ceased; it was then raised to 100° F., and the
leaflets again began to move quickly. On another occasion a tuft of
fine leaves was immersed in water at 53° F., and the leaflets were of
course motionless. The water was raised to 99°, and the leaflets soon
began to move; it was raised to 105°, and the movements became much
more rapid; each little circle or oval being completed in from 1 m. 30
s. to 1 m. 45 s. There was, however, no jerking, and this fact may
perhaps be attributed to the resistance of the water.
Sachs states that the leaflets do not move until the surrounding air is
as high as 71°–72° F., and this agrees with our
experience on full-grown, or nearly full-grown, plants. But the
leaflets of young seedlings exhibit a jerking movement at much lower
temperatures. A seedling was kept (April 16th) in a room for half the
day where the temperature was steady at 64° F., and the one leaflet
which it bore was continually jerking, but not so rapidly as in the
hot-house. The pot was taken in the evening into a bed-room where the
temperature remained at 62° during nearly the whole night; at 10 and 11
P.M. and at 1 A.M. the leaflet was still jerking rapidly; at 3.30 A.M.
it was not seen to jerk, but was observed during only a short time. It
was, however, now inclined at a much lower angle than that occupied at
1 A.M. At 6.30 A.M. (temp. 61° F.) its inclination was still less than
before, and again less at 6.45 A.M.; by 7.40 A.M. it had risen, and at
8.30 A.M. was again seen to jerk. This leaflet, therefore, was moving
during the whole night, and the movement was by jerks up to 1 A.M. (and
possibly later) and again at 8.30 A.M., though the temperature was only
61° to 62° F. We must therefore conclude that the lateral leaflets
produced by young plants differ somewhat in constitution from those on
older plants.
In the large genus Desmodium by far the greater number of the species
are trifoliate; but some are unifoliate, and even the same plant may
bear uni- and trifoliate leaves. In most of the species the lateral
leaflets are only a little smaller than the terminal one. Therefore the
lateral leaflets of D. gyrans (see Fig. 148) must be considered as
almost rudimentary. They are also rudimentary in function, if this
expression may be used; for they certainly do not sleep like the
full-sized terminal leaflets. It is, however, possible that the sinking
down of the leaflets between 1 A.M. and 6.45 A.M., as above described,
may represent sleep. It is well known that the leaflets go on jerking
during the early part of the night; but my gardener observed (Oct.
13th) a plant in the hot-house between 5 and 5.30 A.M., the temperature
having been kept up to 82° F., and found that all the leaflets were
inclined, but he saw no jerking movement until 6.55 A.M., by which time
the terminal leaflet had risen and was awake. Two days afterwards (Oct.
15th) the same plant was observed by him at 4.47 A.M. (temp. 77° F.),
and he found that the large terminal leaflets were awake, though not
quite horizontal; and the only cause which we could assign for this
anomalous wakefulness was that the plant had been kept for experimental
purposes during
the previous day at an unusually high temperature; the little lateral
leaflets were also jerking at this hour, but whether there was any
connection between this latter fact and the sub-horizontal position of
the terminal leaflets we do not know. Anyhow, it is certain that the
lateral leaflets do not sleep like the terminal leaflets; and in so far
they may be said to be in a functionally rudimentary condition. They
are in a similar condition in relation to irritability; for if a plant
be shaken or syringed, the terminal leaflets sink down to about 45°
beneath the horizon; but we could never detect any effect thus produced
on the lateral leaflets; yet we are not prepared to assert positively
that rubbing or pricking the pulvinus produces no effect.
As in the case of most rudimentary organs, the leaflets are variable in
size; they often depart from their normal position and do not stand
opposite one another; and one of the two is frequently absent. This
absence appeared in some, but not in all the cases, to be due to the
leaflet having become completely confluent with the main petiole, as
might be inferred from the presence of a slight ridge along its upper
margin, and from the course of the vessels. In one instance there was a
vestige of the leaflet, in the shape of a minute point, at the further
end of the ridge. The frequent, sudden and complete disappearance of
one or both of the rudimentary leaflets is a rather singular fact; but
it is a much more surprising one that the leaves which are first
developed on seedling plants are not provided with them. Thus, on one
seedling the seventh leaf above the cotyledons was the first which bore
any lateral leaflets, and then only a single one. On another seedling,
the eleventh leaf first bore a leaflet; of the nine succeeding leaves
five bore a single lateral leaflet, and four bore none at all; at last
a leaf, the twenty-first above the cotyledons, was provided with two
rudimentary lateral leaflets. From a widespread analogy in the animal
kingdom, it might have been expected that these rudimentary leaflets
would have been better developed and more regularly present on very
young than on older plants. But bearing in mind, firstly, that
long-lost characters sometimes reappear late in life, and secondly,
that the species of Desmodium are generally trifoliate, but that some
are unifoliate, the suspicion arises that D. gyrans is descended from a
unifoliate species, and that this was descended from a trifoliate one;
for in this case both the absence of the little lateral leaflets on
very young seedlings, and their
subsequent appearance, may be attributed to reversion to more or less
distant progenitors.[17]
[17] Desmodium vespertilionis is closely allied to D. gyrans, and it
seems only occasionally to bear rudimentary lateral leaflets.
Duchartre, ‘Eléments de Botanique,’ 1867, p. 353.
No one supposes that the rapid movements of the lateral leaflets of ‘D.
gyrans’ are of any use to the plant; and why they should behave in this
manner is quite unknown. We imagined that their power of movement might
stand in some relation with their rudimentary condition, and therefore
observed the almost rudimentary leaflets of Mimosa albida vel sensitiva
(of which a drawing will hereafter be given, Fig. 159); but they
exhibited no extraordinary movements, and at night they went to sleep
like the full-sized leaflets. There is, however, this remarkable
difference in the two cases; in Desmodium the pulvinus of the
rudimentary leaflets has not been reduced in length, in correspondence
with the reduction of the blade, to the same extent as has occurred in
the Mimosa; and it is on the length and degree of curvature of the
pulvinus that the amount of movement of the blade depends. Thus the
average length of the pulvinus in the large terminal leaflets of
Desmodium is 3 mm., whilst that of the rudimentary leaflets is 2.86
mm.; so that they differ only a little in length. But in diameter they
differ much, that of the pulvinus of the little leaflets being only 0.3
mm. to 0.4 mm.; whilst that of the terminal leaflets is 1.33 mm. If we
now turn to the Mimosa, we find that the average length of the pulvinus
of the almost rudimentary leaflets is only 0.466 mm., or rather more
than a quarter of the length of the pulvinus of the full-sized
leaflets, namely, 1.66 mm. In this small reduction in length of the
pulvinus of the rudimentary leaflets of Desmodium, we apparently have
the proximate cause of their great and rapid circumnutating movement,
in contrast with that of the almost rudimentary leaflets of the Mimosa.
The small size and weight of the blade, and the little resistance
opposed by the air to its movement, no doubt also come into play; for
we have seen that these leaflets if immersed in water, when the
resistance would be much greater, were prevented from jerking forwards.
Why, during the reduction of the lateral leaflets of Desmodium, or
during their reappearance—if they owe their origin to reversion—the
pulvinus should have been so much less affected than the blade, whilst
with the
Mimosa the pulvinus has been greatly reduced, we do not know.
Nevertheless, it deserves notice that the reduction of the leaflets in
these two genera has apparently been effected by a different process
and for a different end; for with the Mimosa the reduction of the inner
and basal leaflets was necessary from the want of space; but no such
necessity exists with Desmodium, and the reduction of its lateral
leaflets seems to have been due to the principle of compensation, in
consequence of the great size of the terminal leaflet. Uraria (Tribe 6)
and Centrosema (Tribe 8).—The leaflets of Uraria lagopus and the leaves
of a Centrosema from Brazil both sink vertically down at night. In the
latter plant the petiole at the same time rose 16½°.