MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE:
SLEEP OF COTYLEDONS.
Preliminary sketch of the sleep or nyctitropic movements of
leaves—Presence of pulvini—The lessening of radiation the final cause
of nyctitropic movements—Manner of trying experiments on leaves of
Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the
cotyledons of Mimosa—Concluding remarks on radiation from leaves—Small
differences in the conditions make a great difference in the
result—Description of the nyctitropic position and movements of the
cotyledons of various plants—List of species—Concluding
remarks—Independence of the nyctitropic movements of the leaves and
cotyledons of the same species—Reasons for believing that the movements
have been acquired for a special purpose.
The so-called sleep of leaves is so conspicuous a phenomenon that it
was observed as early as the time of Pliny;[1] and since Linnæus
published his famous Essay, ‘Somnus Plantarum,’ it has been the subject
of several memoirs. Many flowers close at night, and these are likewise
said to sleep; but we are not here concerned with their movements, for
although effected by the same mechanism as in the case of young leaves,
namely, unequal growth on the opposite sides (as first proved by
Pfeffer), yet they differ essentially in being excited chiefly by
changes of temperature instead of light; and in being effected, as far
as we can judge, for a different purpose. Hardly any one supposes that
there is any real analogy
between the sleep of animals and that of plants,[2] whether of leaves
or flowers. It seems therefore, advisable to give a distinct name to
the so-called sleep-movements of plants. These have also generally been
confounded, under the term “periodic,” with the slight daily rise and
fall of leaves, as described in the fourth chapter; and this makes it
all the more desirable to give some distinct name to sleep-movements.
Nyctitropism and nyctitropic, i.e. night-turning, may be applied both
to leaves and flowers, and will be occasionally used by us; but it
would be best to confine the term to leaves. The leaves of some few
plants move either upwards or downwards when the sun shines intensely
on them, and this movement has sometimes been called diurnal sleep; but
we believe it to be of an essentially different nature from the
nocturnal movement, and it will be briefly considered in a future
chapter.
[1] Pfeffer has given a clear and interesting sketch of the history of
this subject in his ‘Die Periodischen Bewegungen der Blattorgane,’
1875, P. 163.
[2] Ch. Royer must, however, be excepted; see ‘Annales des Sc. Nat.’
(5th series), Bot. vol. ix. 1868, p. 378.
The sleep or nyctitropism of leaves is a large subject, and we think
that the most convenient plan will be first to give a brief account of
the position which leaves assume at night, and of the advantages
apparently thus gained. Afterwards the more remarkable cases will be
described in detail, with respect to cotyledons in the present chapter,
and to leaves in the next chapter. Finally, it will be shown that these
movements result from circumnutation, much modified and regulated by
the alternations of day and night, or light and darkness; but that they
are also to a certain extent inherited.
Leaves, when they go to sleep, move either upwards or downwards, or in
the case of the leaflets of
compound leaves, forwards, that is, towards the apex of the leaf, or
backwards, that is, towards its base; or, again, they may rotate on
their own axes without moving either upwards or downwards. But in
almost every case the plane of the blade is so placed as to stand
nearly or quite vertically at night. Therefore the apex, or the base,
or either lateral edge, may be directed towards the zenith. Moreover,
the upper surface of each leaf, and more especially of each leaflet, is
often brought into close contact with that of the opposite one; and
this is sometimes effected by singularly complicated movements. This
fact suggests that the upper surface requires more protection than the
lower one. For instance, the terminal leaflet in Trifolium, after
turning up at night so as to stand vertically, often continues to bend
over until the upper surface is directed downwards whilst the lower
surface is fully exposed to the sky; and an arched roof is thus formed
over the two lateral leaflets, which have their upper surfaces pressed
closely together. Here we have the unusual case of one of the leaflets
not standing vertically, or almost vertically, at night.
Considering that leaves in assuming their nyctitropic positions often
move through an angle of 90°; that the movement is rapid in the
evening; that in some cases, as we shall see in the next chapter, it is
extraordinarily complicated; that with certain seedlings, old enough to
bear true leaves, the cotyledons move vertically upwards at night,
whilst at the same time the leaflets move vertically downwards; and
that in the same genus the leaves or cotyledons of some species move
upwards, whilst those of other species move downwards;—from these and
other such facts, it is hardly possible to doubt that plants must
derive some
great advantage from such remarkable powers of movement.
The nyctitropic movements of leaves and cotyledons are effected in two
ways,[3] firstly, by means of pulvini which become, as Pfeffer has
shown, alternately more turgescent on opposite sides; and secondly, by
increased growth along one side of the petiole or midrib, and then on
the opposite side, as was first proved by Batalin.[4] But as it has
been shown by De Vries[5] that in these latter cases increased growth
is preceded by the increased turgescence of the cells, the difference
between the above two means of movement is much diminished, and
consists chiefly in the turgescence of the cells of a fully developed
pulvinus, not being followed by growth. When the movements of leaves or
cotyledons, furnished with a pulvinus and destitute of one, are
compared, they are seen to be closely similar, and are apparently
effected for the same purpose. Therefore, with our object in view, it
does not appear advisable to separate the above two sets of cases into
two distinct classes. There is, however, one important distinction
between them, namely, that movements effected by growth on the
alternate sides, are confined to young growing leaves, whilst those
effected by means of a pulvinus last for a long time. We have already
seen well-marked instances of this latter fact with cotyledons, and so
it is with leaves, as has been observed by Pfeffer and by ourselves.
The long endurance of the nyctitropic movements when effected by the
aid of pulvini indicates, in addition to the evidence already advanced,
the functional
importance of such movements to the plant. There is another difference
between the two sets of cases, namely, that there is never, or very
rarely, any torsion of the leaves, excepting when a pulvinus is
present;[6] but this statement applies only to periodic and nyctitropic
movements as may be inferred from other cases given by Frank.[7] The
fact that the leaves of many plants place themselves at night in widely
different positions from what they hold during the day, but with the
one point in common, that their upper surfaces avoid facing the zenith,
often with the additional fact that they come into close contact with
opposite leaves or leaflets, clearly indicates, as it seems to us, that
the object gained is the protection of the upper surfaces from being
chilled at night by radiation. There is nothing improbable in the upper
surface needing protection more than the lower, as the two differ in
function and structure. All gardeners know that plants suffer from
radiation. It is this and not cold winds which the peasants of Southern
Europe fear for their olives.[8] Seedlings are often protected from
radiation by a very thin covering of straw; and fruit-trees on walls by
a few fir-branches, or even by a fishing-net, suspended over them.
There is a variety of the gooseberry,[9] the flowers of which from
being produced before the leaves, are not protected by them from
radiation, and consequently often fail to yield fruit. An excellent
observer[10] has remarked
that one variety of the cherry has the petals of its flowers much
curled backwards, and after a severe frost all the stigmas were killed;
whilst at the same time, in another variety with incurved petals, the
stigmas were not in the least injured.
[3] This distinction was first pointed out (according to Pfeffer, ‘Die
Periodischen Bewegungen der Blattorgane,’ 1875, p. 161) by Dassen in
1837.
[4] ‘Flora,’ 1873, p. 433.
[5] ‘Bot. Zeitung,’ 1879, Dec. 19th, p. 830.
[6] Pfeffer, ‘Die Period. Beweg. der Blattorgane.’ 1875, p. 159.
[7] ‘Die Nat. Wagerechte Richtung von Pflanzentheilen,’ 1870, p. 52
[8] Martins in ‘Bull. Soc. Bot. de France,’ tom. xix. 1872. Wells, in
his famous ‘Essay on Dew,’ remarks that an exposed thermometer rises
as soon as even a fleecy cloud, high in the sky, passes over the
zenith.
[9] ‘Loudon’s Gardener’s Mag.,’ vol. iv. 1828, p. 112.
[10] Mr. Rivers in ‘Gardener’s Chron.,’ 1866, p. 732
This view that the sleep of leaves saves them from being chilled at
night by radiation, would no doubt have occurred to Linnæus, had the
principle of radiation been then discovered; for he suggests in many
parts of his ‘Somnus Plantarum’ that the position of the leaves at
night protects the young stems and buds, and often the young
inflorescence, against cold winds. We are far from doubting that an
additional advantage may be thus gained; and we have observed with
several plants, for instance, Desmodium gyrans, that whilst the blade
of the leaf sinks vertically down at night, the petiole rises, so that
the blade has to move through a greater angle in order to assume its
vertical position than would otherwise have been necessary; but with
the result that all the leaves on the same plant are crowded together
as if for mutual protection.
We doubted at first whether radiation would affect in any important
manner objects so thin as are many cotyledons and leaves, and more
especially affect differently their upper and lower surfaces; for
although the temperature of their upper surfaces would undoubtedly fall
when freely exposed to a clear sky, yet we thought that they would so
quickly acquire by conduction the temperature of the surrounding air,
that it could hardly make any sensible difference to them, whether they
stood horizontally and radiated into the open sky, or vertically and
radiated chiefly in a lateral direction towards neighbouring plants and
other objects. We endeavoured, therefore, to ascertain something on
this head by preventing the leaves
of several plants from going to sleep, and by exposing to a clear sky
when the temperature was beneath the freezing-point, these, as well as
the other leaves on the same plants which had already assumed their
nocturnal vertical position. Our experiments show that leaves thus
compelled to remain horizontal at night, suffered much more injury from
frost than those which were allowed to assume their normal vertical
position. It may, however, be said that conclusions drawn from such
observations are not applicable to sleeping plants, the inhabitants of
countries where frosts do not occur. But in every country, and at all
seasons, leaves must be exposed to nocturnal chills through radiation,
which might be in some degree injurious to them, and which they would
escape by assuming a vertical position.
In our experiments, leaves were prevented from assuming their
nyctitropic position, generally by being fastened with the finest
entomological pins (which did not sensibly injure them) to thin sheets
of cork supported on sticks. But in some instances they were fastened
down by narrow strips of card, and in others by their petioles being
passed through slits in the cork. The leaves were at first fastened
close to the cork, for as this is a bad conductor, and as the leaves
were not exposed for long periods, we thought that the cork, which had
been kept in the house, would very slightly warm them; so that if they
were injured by the frost in a greater degree than the free vertical
leaves, the evidence would be so much the stronger that the horizontal
position was injurious. But we found that when there was any slight
difference in the result, which could be detected only occasionally,
the leaves which had been fastened closely down suffered rather more
than those fastened with very long and
thin pins, so as to stand from ½ to 3/4 inch above the cork. This
difference in the result, which is in itself curious as showing what a
very slight difference in the conditions influences the amount of
injury inflicted, may be attributed, as we believe, to the surrounding
warmer air not circulating freely beneath the closely pinned leaves and
thus slightly warming them. This conclusion is supported by some
analogous facts hereafter to be given.
We will now describe in detail the experiments which were tried. These
were troublesome from our not being able to predict how much cold the
leaves of the several species could endure. Many plants had every leaf
killed, both those which were secured in a horizontal position and
those which were allowed to sleep—that is, to rise up or sink down
vertically. Others again had not a single leaf in the least injured,
and these had to be re-exposed either for a longer time or to a lower
temperature.
Oxalis acetosella.—A very large pot, thickly covered with between 300
and 400 leaves, had been kept all winter in the greenhouse. Seven
leaves were pinned horizontally open, and were exposed on March 16th
for 2 h. to a clear sky, the temperature on the surrounding grass being
–4° C. (24° to 25° F.). Next morning all seven leaves were found quite
killed, so were many of the free ones which had previously gone to
sleep, and about 100 of them, either dead or browned and injured were
picked off. Some leaves showed that they had been slightly injured by
not expanding during the whole of the next day, though they afterwards
recovered. As all the leaves which were pinned open were killed, and
only about a third or fourth of the others were either killed or
injured, we had some little evidence that those which were prevented
from assuming their vertically dependent position suffered most.
The following night (17th) was clear and almost equally cold (–3° to
–4° C. on the grass), and the pot was again exposed, but this time for
only 30 m. Eight leaves had been pinned out,
and in the morning two of them were dead, whilst not a single other
leaf on the many plants was even injured.
On the 23rd the pot was exposed for 1 h. 30 m., the temperature on the
grass being only –2° C., and not one leaf was injured: the pinned open
leaves, however, all stood from ½ to 3/4 of an inch above the cork.
On the 24th the pot was again placed on the ground and exposed to a
clear sky for between 35 m. and 40 m. By a mistake the thermometer was
left on an adjoining sun-dial 3 feet high, instead of being placed on
the grass; it recorded 25° to 26° F. (–3.3° to –3.8° C.), but when
looked at after 1 h. had fallen to 22° F. (–5.5° C.); so that the pot
was perhaps exposed to rather a lower temperature than on the two first
occasions. Eight leaves had been pinned out, some close to the cork and
some above it, and on the following morning five of them (i.e. 63 per
cent.) were found killed. By counting a portion of the leaves we
estimated that about 250 had been allowed to go to sleep, and of these
about 20 were killed (i.e. only 8 per cent.), and about 30 injured.
Considering these cases, there can be no doubt that the leaves of this
Oxalis, when allowed to assume their normal vertically dependent
position at night, suffer much less from frost than those (23 in
number) which had their upper surfaces exposed to the zenith.
Oxalis carnosa.—A plant of this Chilian species was exposed for 30 m.
to a clear sky, the thermometer on the grass standing at –2° C., with
some of its leaves pinned open, and not one leaf on the whole bushy
plant was in the least injured. On the 16th of March another plant was
similarly exposed for 30 m., when the temperature on the grass was only
a little lower, viz., –3° to –4° C. Six of the leaves had been pinned
open, and next morning five of them were found much browned. The plant
was a large one, and none of the free leaves, which were asleep and
depended vertically, were browned, excepting four very young ones. But
three other leaves, though not browned, were in a rather flaccid
condition, and retained their nocturnal position during the whole of
the following day. In this case it was obvious that the leaves which
were exposed horizontally to the zenith suffered most. This same pot
was afterwards exposed for 35–40 m. on a slightly colder night, and
every leaf, both the pinned open and the free ones, was killed. It may
be added that two pots of O. corniculata (var.
Atro-purpurea) were exposed for 2 h. and 3 h. to a clear sky with the
temp. on grass –2° C., and none of the leaves, whether free or pinned
open, were at all injured.
Arachis hypogoea.—Some plants in a pot were exposed at night for 30 m.
to a clear sky, the temperature on the surrounding grass being –2° C.,
and on two nights afterwards they were again exposed to the same
temperature, but this time during 1 h. 30 m. On neither occasion was a
single leaf, whether pinned open or free, injured; and this surprised
us much, considering its native tropical African home. Two plants were
next exposed (March 16th) for 30 m. to a clear sky, the temperature of
the surrounding grass being now lower, viz., between –3° and –4° C.,
and all four pinned-open leaves were killed and blackened. These two
plants bore 22 other and free leaves (excluding some very young
bud-like ones) and only two of these were killed and three somewhat
injured; that is, 23 per cent. were either killed or injured, whereas
all four pinned-open leaves were utterly killed.
On another night two pots with several plants were exposed for between
35 m. and 40 m. to a clear sky, and perhaps to a rather lower
temperature, for a thermometer on a dial, 3 feet high, close by stood
at –3.3° to –3.8° C. In one pot three leaves were pinned open, and all
were badly injured; of the 44 free leaves, 26 were injured, that is, 59
per cent. In the other pot 3 leaves were pinned open and all were
killed; four other leaves were prevented from sleeping by narrow strips
of stiff paper gummed across them, and all were killed; of 24 free
leaves, 10 were killed, 2 much injured, and 12 unhurt; that is, 50 per
cent. of the free leaves were either killed or much injured. Taking the
two pots together, we may say that rather more than half of the free
leaves, which were asleep, were either killed or injured, whilst all
the ten horizontally extended leaves, which had been prevented from
going to sleep, were either killed or much injured.
Cassia floribunda.—A bush was exposed at night for 40 m. to a clear
sky, the temperature on the surrounding grass being –2° C., and not a
leaf was injured.[11] It was again exposed on
another night for 1 h., when the temperature of the grass was –4° C.;
and now all the leaves on a large bush, whether pinned flat open or
free, were killed, blackened, and shrivelled, with the exception of
those on one small branch, low down, which was very slightly protected
by the leaves on the branches above. Another tall bush, with four of
its large compound leaves pinned out horizontally, was afterwards
exposed (temp. of surrounding grass exactly the same, viz., –4° C.),
but only for 30 m. On the following morning every single leaflet on
these four leaves was dead, with both their upper and lower surfaces
completely blackened. Of the many free leaves on the bush, only seven
were blackened, and of these only a single one (which was a younger and
more tender leaf than any of the pinned ones) had both surfaces of the
leaflets blackened. The contrast in this latter respect was well shown
by a free leaf, which stood between two pinned-open ones; for these
latter had the lower surfaces of their leaflets as black as ink, whilst
the intermediate free leaf, though badly injured, still retained a
plain tinge of green on the lower surface of the leaflets. This bush
exhibited in a striking manner the evil effects of the leaves not being
allowed to assume at night their normal dependent position; for had
they all been prevented from doing so, assuredly every single leaf on
the bush would have been utterly killed by this exposure of only 30 m.
The leaves whilst sinking downwards in the evening twist round, so that
the upper surface is turned inwards, and is thus better protected than
the outwardly turned lower surface. Nevertheless, it was always the
upper surface which was more blackened than the lower, whenever any
difference could be perceived between them; but whether this was due to
the cells near the upper surface being more tender, or merely to their
containing more chlorophyll, we do not know.
[11] Cassia laevigata was exposed to a clear sky for 35 m., and C.
calliantha (a Guiana species) for 60 m., the temperature on the
surrounding grass being –2° C., and neither was in the least injured.
But when C. laevigata was exposed for 1 h., the temp. on the
surrounding grass being between –3° and –4° C., every leaf was killed.
Melilotus officinalis.—A large pot with many plants, which had been
kept during the winter in the greenhouse, was exposed during 5 h. at
night to a slight frost and clear sky. Four leaves had been pinned out,
and these died after a few days; but so did many of the free leaves.
Therefore nothing certain could be inferred from this trial, though it
indicated that the horizontally extended leaves suffered most. Another
large pot with many plants was next exposed for 1 h., the temperature
on the surrounding grass being lower, viz., -3° to –4° C. Ten leaves
had been pinned out, and the result was striking, for on the following
morning all these were found much injured or
killed, and none of the many free leaves on the several plants were at
all injured, with the doubtful exception of two or three very young
ones.
Melilotus Italica.—Six leaves were pinned out horizontally, three with
their upper and three with their lower surfaces turned to the zenith.
The plants were exposed for 5 h. to a clear sky, the temperature on
ground being about –1° C. Next morning the six pinned-open leaves
seemed more injured even than the younger and more tender free ones on
the same branches. The exposure, however, had been too long, for after
an interval of some days many of the free leaves seemed in almost as
bad a condition as the pinned-out ones. It was not possible to decide
whether the leaves with their upper or those with their lower surfaces
turned to the zenith had suffered most.
Melilotus suaveolens.—Some plants with 8 leaves pinned out were exposed
to a clear sky during 2 h., the temperature on the surrounding grass
being –2° C. Next morning 6 out of these 8 leaves were in a flaccid
condition. There were about 150 free leaves on the plant, and none of
these were injured, except 2 or 3 very young ones. But after two days,
the plants having been brought back into the greenhouse, the 6
pinned-out leaves all recovered.
Melilotus Taurica.—Several plants were exposed for 5 h. during two
nights to a clear sky and slight frost, accompanied by some wind; and 5
leaves which had been pinned out suffered more than those both above
and below on the same branches which had gone to sleep. Another pot,
which had likewise been kept in the greenhouse, was exposed for 35–40
m. to a clear sky, the temperature of the surrounding grass being
between –3° and –4° C. Nine leaves had been pinned out, and all of
these were killed. On the same plants there were 210 free leaves, which
had been allowed to go to sleep, and of these about 80 were killed,
i.e. only 38 per cent.
Melilotus Petitpierreana.—The plants were exposed to a clear sky for
35–40 m.: temperature on surrounding grass –3° to –4° C. Six leaves had
been pinned out so as to stand about ½ inch above the cork, and four
had been pinned close to it. These 10 leaves were all killed, but the
closely pinned ones suffered most, as 4 of the 6 which stood above the
cork still retained small patches of a green colour. A considerable
number, but not nearly all, of the free leaves, were killed or much
injured, whereas all the pinned out ones were killed.
Melilotus macrorrhiza.—The plants were exposed in the same manner as in
the last case. Six leaves had been pinned out horizontally, and five of
them were killed, that is, 83 percent. We estimated that there were 200
free leaves on the plants, and of these about 50 were killed and 20
badly injured, so that about 35 per cent of the free leaves were killed
or injured.
Lotus aristata.—Six plants were exposed for nearly 5 h. to a clear sky;
temperature on surrounding grass –1.5° C. Four leaves had been pinned
out horizontally, and 2 of these suffered more than those above or
below on the same branches, which had been allowed to go to sleep. It
is rather a remarkable fact that some plants of Lotus Jacoboeus, an
inhabitant of so hot a country as the Cape Verde Islands, were exposed
one night to a clear sky, with the temperature of the surrounding grass
–2° C., and on a second night for 30 m. with the temperature of the
grass between –3° and –4° C., and not a single leaf, either the
pinned-out or free ones, was in the least injured.
Marsilea quadrifoliata.—A large plant of this species—the only
Cryptogamic plant known to sleep—with some leaves pinned open, was
exposed for 1 h. 35 m. to a clear sky, the temperature on the
surrounding ground being –2° C., and not a single leaf was injured.
After an interval of some days the plant was again exposed for 1 h. to
a clear sky, with the temperature on the surrounding ground lower,
viz., –4° C. Six leaves had been pinned out horizontally, and all of
them were utterly killed. The plant had emitted long trailing stems,
and these had been wrapped round with a blanket, so as to protect them
from the frozen ground and from radiation; but a very large number of
leaves were left freely exposed, which had gone to sleep, and of these
only 12 were killed. After another interval, the plant, with 9 leaves
pinned out, was again exposed for 1 h., the temperature on the ground
being again –4° C. Six of the leaves were killed, and one which did not
at first appear injured afterwards became streaked with brown. The
trailing branches, which rested on the frozen ground, had one-half or
three-quarters of their leaves killed, but of the many other leaves on
the plant, which alone could be fairly compared with the pinned-out
ones, none appeared at first sight to have been killed, but on careful
search 12 were found in this state. After another interval, the plant
with 9 leaves pinned out, was exposed for 35–40 m. to a clear sky and
to nearly the same, or perhaps a rather lower, temperature (for the
thermometer by an accident had been left on a
sun-dial close by), and 8 of these leaves were killed. Of the free
leaves (those on the trailing branches not being considered), a good
many were killed, but their number, compared with the uninjured ones,
was small. Finally, taking the three trials together, 24 leaves,
extended horizontally, were exposed to the zenith and to unobstructed
radiation, and of these 20 were killed and 1 injured; whilst a
relatively very small proportion of the leaves, which had been allowed
to go to sleep with their leaflets vertically dependent, were killed or
injured.
The cotyledons of several plants were prepared for trial, but the
weather was mild and we succeeded only in a single instance in having
seedlings of the proper age on nights which were clear and cold. The
cotyledons of 6 seedlings of Mimosa pudica were fastened open on cork
and were thus exposed for 1 h. 45 m. to a clear sky, with the
temperature on the surrounding ground at 29° F.; of these, 3 were
killed. Two other seedlings, after their cotyledons had risen up and
had closed together, were bent over and fastened so that they stood
horizontally, with the lower surface of one cotyledon fully exposed to
the zenith, and both were killed. Therefore of the 8 seedlings thus
tried 5, or more than half, were killed. Seven other seedlings with
their cotyledons in their normal nocturnal position, viz., vertical and
closed, were exposed at the same time, and of these only 2 were
killed.[12] Hence it appears, as far as these few trials tell anything,
that the vertical position at night of the cotyledons of Mimosa pudica
protects them to a certain degree from the evil effects of radiation
and cold.
[12] We were surprised that young seedlings of so tropical a plant as
Mimosa pudica were able to resist, as well as they did, exposure for 1
hr. 45 m. to a clear sky, the temperature on the surrounding ground
being 29° F. It may be added that seedlings of the Indian ‘Cassia
pubescens’ were exposed for 1 h. 30 m. to a clear sky, with the temp.
on the surrounding ground at –2° C., and they were not in the least
injured.
Concluding Remarks on the Radiation from Leaves at Night.—We exposed on
two occasions during the summer to a clear sky several pinned-open
leaflets of Trifolium pratense, which naturally rise at night, and of
Oxalis purpurea, which naturally sink at night (the plants growing out
of doors), and looked at
them early on several successive mornings, after they had assumed their
diurnal positions. The difference in the amount of dew on the
pinned-open leaflets and on those which had gone to sleep was generally
conspicuous; the latter being sometimes absolutely dry, whilst the
leaflets which had been horizontal were coated with large beads of dew.
This shows how much cooler the leaflets fully exposed to the zenith
must have become, than those which stood almost vertically, either
upwards or downwards, during the night.
From the several cases above given, there can be no doubt that the
position of the leaves at night affects their temperature through
radiation to such a degree, that when exposed to a clear sky during a
frost, it is a question of life and death. We may therefore admit as
highly probable, seeing that their nocturnal position is so well
adapted to lessen radiation, that the object gained by their often
complicated sleep movements, is to lessen the degree to which they are
chilled at night. It should be kept in mind that it is especially the
upper surface which is thus protected, as it is never directed towards
the zenith, and is often brought into close contact with the upper
surface of an opposite leaf or leaflet.
We failed to obtain sufficient evidence, whether the better protection
of the upper surface has been gained from its being more easily injured
than the lower surface, or from its injury being a greater evil to the
plant. That there is some difference in constitution between the two
surfaces is shown by the following cases. Cassia floribunda was exposed
to a clear sky on a sharp frosty night, and several leaflets which had
assumed their nocturnal dependent position with their lower surfaces
turned outwards so as to be
exposed obliquely to the zenith, nevertheless had these lower surfaces
less blackened than the upper surfaces which were turned inwards and
were in close contact with those of the opposite leaflets. Again, a pot
full of plants of Trifolium resupinatum, which had been kept in a warm
room for three days, was turned out of doors (Sept. 21st) on a clear
and almost frosty night. Next morning ten of the terminal leaflets were
examined as opaque objects under the microscope. These leaflets, in
going to sleep, either turn vertically upwards, or more commonly bend a
little over the lateral leaflets, so that their lower surfaces are more
exposed to the zenith than their upper surfaces. Nevertheless, six of
these ten leaflets were distinctly yellower on the upper than on the
lower and more exposed surface. In the remaining four, the result was
not so plain, but certainly whatever difference there was leaned to the
side of the upper surface having suffered most.
It has been stated that some of the leaflets experimented on were
fastened close to the cork, and others at a height of from ½ to 3/4 of
an inch above it; and that whenever, after exposure to a frost, any
difference could be detected in their states, the closely pinned ones
had suffered most. We attributed this difference to the air, not cooled
by radiation, having been prevented from circulating freely beneath the
closely pinned leaflets. That there was really a difference in the
temperature of leaves treated in these two different methods, was
plainly shown on one occasion; for after the exposure of a pot with
plants of Melilotus dentata for 2 h. to a clear sky (the temperature on
the surrounding grass being –2° C.), it was manifest that more dew had
congealed into hoar-frost on the closely pinned leaflets, than on those
which stood horizontally
a little above the cork. Again, the tips of some few leaflets, which
had been pinned close to the cork, projected a little beyond the edge,
so that the air could circulate freely round them. This occurred with
six leaflets of Oxalis acetosella, and their tips certainly suffered
rather less then the rest of the same leaflets; for on the following
morning they were still slightly green. The same result followed, even
still more clearly, in two cases with leaflets of Melilotus officinalis
which projected a little beyond the cork; and in two other cases some
leaflets which were pinned close to the cork were injured, whilst other
free leaflets on the same leaves, which had not space to rotate and
assume their proper vertical position, were not at all injured.
Another analogous fact deserves notice: we observed on several
occasions that a greater number of free leaves were injured on the
branches which had been kept motionless by some of their leaves having
been pinned to the corks, than on the other branches. This was
conspicuously the case with those of Melilotus Petitpierreana, but the
injured leaves in this instance were not actually counted. With Arachis
hypogaea, a young plant with 7 stems bore 22 free leaves, and of these
5 were injured by the frost, all of which were on two stems, bearing
four leaves pinned to the cork-supports. With Oxalis carnosa, 7 free
leaves were injured, and every one of them belonged to a cluster of
leaves, some of which had been pinned to the cork. We could account for
these cases only by supposing that the branches which were quite free
had been slightly waved about by the wind, and that their leaves had
thus been a little warmed by the surrounding warmer air. If we hold our
hands motionless before a hot fire, and then wave them about, we
immediately feel relief; and this is evidently an analogous, though
reversed, case. These several facts—in relation to leaves pinned close
to or a little above the cork-supports—to their tips projecting beyond
it—and to the leaves on branches kept motionless—seem to us curious, as
showing how a difference, apparently trifling, may determine the
greater or less injury of the leaves. We may even infer as probable
that the less or greater destruction during a frost of the leaves on a
plant which does not sleep, may often depend on the greater or less
degree of flexibility of their petioles and of the branches which bear
them.
NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.
We now come to the descriptive part of our work, and will begin with
cotyledons, passing on to leaves in the next chapter. We have met with
only two brief notices of cotyledons sleeping. Hofmeister,[13] after
stating that the cotyledons of all the observed seedlings of the
Caryophylleae (Alsineae and Sileneae) bend upwards at night (but to
what angle he does not state), remarks that those of Stellaria media
rise up so as to touch one another; they may therefore safely be said
to sleep. Secondly, according to Ramey,[14] the cotyledons of Mimosa
pudica and of Clianthus Dampieri rise up almost vertically at night and
approach each other closely. It has been shown in a previous chapter
that the cotyledons of a large number of plants bend a little upwards
at night, and we here have to meet the difficult question at what
inclination may they be said to sleep? According to the view which we
maintain, no movement deserves to be called
nyctitropic, unless it has been acquired for the sake of lessening
radiation; but this could be discovered only by a long series of
experiments, showing that the leaves of each species suffered from this
cause, if prevented from sleeping. We must therefore take an arbitrary
limit. If a cotyledon or leaf is inclined at 60° above or beneath the
horizon, it exposes to the zenith about one-half of its area;
consequently the intensity of its radiation will be lessened by about
half, compared with what it would have been if the cotyledon or leaf
had remained horizontal. This degree of diminution certainly would make
a great difference to a plant having a tender constitution. We will
therefore speak of a cotyledon and hereafter of a leaf as sleeping,
only when it rises at night to an angle of about 60°, or to a still
higher angle, above the horizon, or sinks beneath it to the same
amount. Not but that a lesser diminution of radiation may be
advantageous to a plant, as in the case of Datura stramonium, the
cotyledons of which rose from 31° at noon to 55° at night above the
horizon. The Swedish turnip may profit by the area of its leaves being
reduced at night by about 30 per cent., as estimated by Mr. A. S.
Wilson; though in this case the angle through which the leaves rose was
not observed. On the other hand, when the angular rise of cotyledons or
of leaves is small, such as less than 30°, the diminution of radiation
is so slight that it probably is of no significance to the plant in
relation to radiation. For instance, the cotyledons of Geranium
Ibericum rose at night to 27° above the horizon, and this would lessen
radiation by only 11 per cent.: those of Linum Berendieri rose to 33°,
and this would lessen radiation by 16 per cent.
[13] ‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327.
[14] ‘Adansonia,’ March 10th, 1869.
There are, however, some other sources of doubt with
respect to the sleep of cotyledons. In certain cases, the cotyledons
whilst young diverge during the day to only a very moderate extent, so
that a small rise at night, which we know occurs with the cotyledons of
many plants, would necessarily cause them to assume a vertical or
nearly vertical position at night; and in this case it would be rash to
infer that the movement was effected for any special purpose. On this
account we hesitated long whether we should introduce several
Cucurbitaceous plants into the following list; but from reasons,
presently to be given, we thought that they had better be at least
temporarily included. This same source of doubt applies in some few
other cases; for at the commencement of our observations we did not
always attend sufficiently to whether the cotyledons stood nearly
horizontally in the middle of the day. With several seedlings, the
cotyledons assume a highly inclined position at night during so short a
period of their life, that a doubt naturally arises whether this can be
of any service to the plant. Nevertheless, in most of the cases given
in the following list, the cotyledons may be as certainly said to sleep
as may the leaves of any plant. In two cases, namely with the cabbage
and radish, the cotyledons of which rise almost vertically during the
few first nights of their life, it was ascertained by placing young
seedlings in the klinostat, that the upward movement was not due to
apogeotropism.
The names of the plants, the cotyledons of which stand at night at an
angle of at least 60° with the horizon, are arranged in the appended
list on the same system as previously followed. The numbers of the
Families, and with the Leguminosae the numbers of the Tribes, have been
added to show how widely the plants in question are distributed
throughout the
dicotyledonous series. A few remarks will have to be made about many of
the plants in the list. In doing so, it will be convenient not to
follow strictly any systematic order, but to treat of the Oxalidæ and
the Leguminosae at the close; for in these two Families the cotyledons
are generally provided with a pulvinus, and their movements endure for
a much longer time than those of the other plants in the list.
List of Seedling Plants, the cotyledons of which rise or sink at night
to an angle of at least 60° above or beneath the horizon.
Brassica oleracea. Cruciferae (Fam. 14). — napus (as we are informed by
Prof. Pfeffer). Raphanus sativus. Cruciferae. Githago segetum.
Caryophylleae (Fam. 26). Stellaria media (according to Hofmeister, as
quoted). Caryophylleae. Anoda Wrightii. Malvaceae (Fam. 36). Gossypium
(var. Nankin cotton). Malvaceae. Oxalis rosea. Oxalidæ (Fam. 41). —
floribunda. — articulata. — Valdiviana. — sensitiva. Geranium
rotundifolium. Geraniaceae (Fam. 47). Trifolium subterraneum.
Leguminosae (Fam. 75, Tribe 3). — strictum. — leucanthemum. Lotus
ornithopopoides. Leguminosae (Tribe 4). — peregrinus. — Jacobæus.
Clianthus Dampieri. Leguminosae (Tribe 5)—according to M. Ramey.
Smithia sensitiva. Leguminosae (Tribe 6). Haematoxylon Campechianum.
Leguminosae (Tribe 13)—according to Mr. R. I. Lynch. Cassia mimosoides.
Leguminosae (Tribe 14). — glauca. — florida. — corymbosa. — pubescens.
— tora. — neglecta. — 3 other Brazilian unnamed species. Bauhinia
(sp.?. Leguminosae (Tribe 15). Neptunia oleracea. Leguminosae (Tribe
20). Mimosa pudica. Leguminosae (Tribe 21). — albida. Cucurbita
ovifera. Cucurbitaceæ (Fam. 106). — aurantia. Lagenaria vulgaris.
Cucurbitaceæ. Cucumis dudaim. Cucurbitaceæ. Apium petroselinum.
Umbelliferae (Fam. 113). — graveolens. Lactuca scariola. Compositæ
(Fam. 122). Helianthus annuus (?). Compositæ. Ipomœa caerulea.
Convolvulaceae (Fam. 151). — purpurea. — bona-nox. — coccinea.
Solanum lycopersicum. Solaneae (Fam. 157.) Mimulus, (sp. ?)
Scrophularineae (Fam. 159)—from information given us by Prof. Pfeffer.
Mirabilis jalapa. Nyctagineae (Fam. 177). Mirabilis longiflora. Beta
vulgaris. Polygoneae (Fam. 179). Amaranthus caudatus. Amaranthaceae
(Fam. 180). Cannabis sativa (?). Cannabineae (Fam. 195).
Brassica oleracea (Cruciferae).—It was shown in the first chapter that
the cotyledons of the common cabbage rise in the evening and stand
vertically up at night with their petioles in contact. But as the two
cotyledons are of unequal height, they frequently interfere a little
with each other’s movements, the shorter one often not standing quite
vertically. They awake early in the morning; thus at 6.45 A.M. on Nov.
27th, whilst if was still dark, the cotyledons, which had been vertical
and in contact on the previous evening, were reflexed, and thus
presented a very different appearance. It should be borne in mind that
seedlings in germinating at the proper season, would not be subjected
to darkness at this hour in the morning. The above amount of movement
of the cotyledons is only temporary, lasting with plants kept in a warm
greenhouse from four to six days; how long it would last with seedlings
growing out of doors we do not know.
Raphanus sativus.—In the middle of the day the blades of the cotyledons
of 10 seedlings stood at right angles to their hypocotyls, with their
petioles a little divergent; at night the blades stood vertically, with
their bases in contact and with their petioles parallel. Next morning,
at 6.45 A.M., whilst it was still dark, the blades were horizontal. On
the following night they were much raised, but hardly stood
sufficiently vertical to be said to be asleep, and so it was in a still
less degree on the third night. Therefore the cotyledons of this plant
(kept in the greenhouse) go to sleep for even a shorter time than those
of the cabbage. Similar observations were made, but only during a
single day and night, on 13 other seedlings likewise raised in the
greenhouse, with the same result.
The petioles of the cotyledons of 11 young seedlings of Sinapis nigra
were slightly divergent at noon, and the blades stood at right angles
to the hypocotyls; at night the petioles were in close contact, and the
blades considerably raised, with their bases in contact, but only a few
stood sufficiently upright to be called asleep. On the following
morning,
the petioles diverged before it was light. The hypocotyl is slightly
sensitive, so that if rubbed with a needle it bends towards the rubbed
side. In the case of Lepidium sativum, the petioles of the cotyledons
of young seedlings diverge during the day and converge so as to touch
each other during the night, by which means the bases of the tripartite
blades are brought into contact; but the blades are so little raised
that they cannot be said to sleep. The cotyledons of several other
cruciferous plants were observed, but they did not rise sufficiently
during the night to be said to sleep.
Githago segetum (Caryophylleae).—On the first day after the cotyledons
had burst through the seed-coats, they stood at noon at an angle of 75°
above the horizon; at night they moved upwards, each through an angle
of 15° so as to stand quite vertical and in contact with one another.
On the second day they stood at noon at 59° above the horizon, and
again at night were completely closed, each having risen 31°. On the
fourth day the cotyledons did not quite close at night. The first and
succeeding pairs of young true leaves behaved in exactly the same
manner. We think that the movement in this case may be called
nyctitropic, though the angle passed through was small. The cotyledons
are very sensitive to light and will not expand if exposed to an
extremely dim one.
Anoda Wrightii (Malvaceae).—The cotyledons whilst moderately young, and
only from .2 to .3 inch in diameter, sink in the evening from their
mid-day horizontal position to about 35° beneath the horizon. But when
the same seedlings were older and had produced small true leaves, the
almost orbicular cotyledons, now .55 inch in diameter, moved vertically
downwards at night. This fact made us suspect that their sinking might
be due merely to their weight; but they were not in the least flaccid,
and when lifted up sprang back through elasticity into their former
dependent position. A pot with some old seedlings was turned upside
down in the afternoon, before the nocturnal fall had commenced, and at
night they assumed in opposition to their own weight (and to any
geotropic action) an upwardly directed vertical position. When pots
were thus reversed, after the evening fall had already commenced, the
sinking movement appeared to be somewhat disturbed; but all their
movements were occasionally variable without any apparent cause. This
latter fact, as well as that of the young cotyledons not sinking nearly
so much as the older ones, deserves notice.
Although the movement of the cotyledons endured for a long time, no
pulvinus was exteriorly visible; but their growth continued for a long
time. The cotyledons appear to be only slightly heliotropic, though the
hypocotyl is strongly so.
Gossypium arboreum (?) (var. Nankin cotton) (Malvaceae).—The cotyledons
behave in nearly the same manner as those of the Anoda. On June 15th
the cotyledons of two seedlings were .65 inch in length (measured along
the midrib) and stood horizontally at noon; at 10 P.M. they occupied
the same position and had not fallen at all. On June 23rd, the
cotyledons of one of these seedlings were 1.1 inch in length, and by 10
P.M. they had fallen from a horizontal position to 62° beneath the
horizon. The cotyledons of the other seedling were 1.3 inch in length,
and a minute true leaf had been formed; they had fallen at 10 P.M. to
70° beneath the horizon. On June 25th, the true leaf of this latter
seedling was .9 inch in length, and the cotyledons occupied nearly the
same position at night. By July 9th the cotyledons appeared very old
and showed signs of withering; but they stood at noon almost
horizontally, and at 10 P.M. hung down vertically.
Gossypium herbaceum.—It is remarkable that the cotyledons of this
species behave differently from those of the last. They were observed
during 6 weeks from their first development until they had grown to a
very large size (still appearing fresh and green), viz. 2½ inches in
breadth. At this age a true leaf had been formed, which with its
petiole was 2 inches long. During the whole of these 6 weeks the
cotyledons did not sink at night; yet when old their weight was
considerable and they were borne by much elongated petioles. Seedlings
raised from some seed sent us from Naples, behaved in the same manner;
as did those of a kind cultivated in Alabama and of the Sea-island
cotton. To what species these three latter forms belong we do not know.
We could not make out in the case of the Naples cotton, that the
position of the cotyledons at night was influenced by the soil being
more or less dry; care being taken that they were not rendered flaccid
by being too dry. The weight of the large cotyledons of the Alabama and
Sea-island kinds caused them to hang somewhat downwards, when the pots
in which they grew were left for a time upside down. It should,
however, be observed that these three kinds were raised in the middle
of the winter, which sometimes greatly interferes with the proper
nyctitropic movements of leaves and cotyledons.
Cucurbitaceæ.—The cotyledons of Cucurbita aurantia and ovifera, and of
Lagenaria vulgaris, stand from the 1st to the 3rd day of their life at
about 60° above the horizon, and at night rise up so as to become
vertical and in close contact with one another. With Cucumis dudaim
they stood at noon at 45° above the horizon, and closed at night. The
tips of the cotyledons of all these species are, however, reflexed, so
that this part is fully exposed to the zenith at night; and this fact
is opposed to the belief that the movement is of the same nature as
that of sleeping plants. After the first two or three days the
cotyledons diverge more during the day and cease to close at night.
Those of Trichosanthes anguina are somewhat thick and fleshy, and did
not rise at night; and they could perhaps hardly be expected to do so.
On the other hand, those of _Acanthosicyos horrida_[15] present nothing
in their appearance opposed to their moving at night in the same manner
as the preceding species; yet they did not rise up in any plain manner.
This fact leads to the belief that the nocturnal movements of the
above-named species has been acquired for some special purpose, which
may be to protect the young plumule from radiation, by the close
contact of the whole basal portion of the two cotyledons.
[15] This plant, from Dammara Land in S. Africa, is remarkable from
being the one known member of the Family which is not a climber; it
has been described in ‘Transact. Linn. Soc.,’ xxvii. p. 30.
Geranium rotundifolium (Geraniaceae).—A single seedling came up
accidentally in a pot, and its cotyledons were observed to bend
perpendicularly downwards during several successive nights, having been
horizontal at noon. It grew into a fine plant but died before
flowering: it was sent to Kew and pronounced to be certainly a
Geranium, and in all probability the above-named species. This case is
remarkable because the cotyledons of G. cinereum, Endressii, Ibericum,
Richardsoni, and subcaulescens were observed during some weeks in the
winter, and they did not sink, whilst those of G. Ibericum rose 27° at
night.
Apium petroselinum (Umbelliferae).—A seedling had its cotyledons (Nov.
22nd) almost fully expanded during the day; by 8.30 P.M. they had risen
considerably, and at 10.30 P.M. were almost closed, their tips being
only 8/100 of an inch apart. On the following morning (23rd) the tips
were 58/100 of an inch apart,
or more than seven times as much. On the next night the cotyledons
occupied nearly the same position as before. On the morning of the 24th
they stood horizontally, and at night were 60° above the horizon; and
so it was on the night of the 25th. But four days afterwards (on the
29th), when the seedlings were a week old, the cotyledons had ceased to
rise at night to any plain degree.
Apium graveolens.—The cotyledons at noon were horizontal, and at 10
P.M. stood at an angle of 61° above the horizon.
Lactuca scariola (Compositæ).—The cotyledons whilst young stood
sub-horizontally during the day, and at night rose so as to be almost
vertical, and some were quite vertical and closed; but this movement
ceased when they had grown old and large, after an interval of 11 days.
Helianthus annuus (Compositæ).—This case is rather doubtful; the
cotyledons rise at night, and on one occasion they stood at 73° above
the horizon, so that they might then be said to have been asleep.
Ipomœa caerulea vel Pharbitis nil (Convolvulaceae).—The cotyledons
behave in nearly the same manner as those of the Anoda and Nankin
cotton, and like them grow to a large size. Whilst young and small, so
that their blades were from .5 to .6 of an inch in length, measured
along the middle to the base of the central notch, they remained
horizontal both during the middle of the day and at night. As they
increased in size they began to sink more and more in the evening and
early night; and when they had grown to a length (measured in the above
manner) of from 1 to 1.25 inch, they sank between 55° and 70° beneath
the horizon. They acted, however, in this manner only when they had
been well illuminated during the day. Nevertheless, the cotyledons have
little or no power of bending towards a lateral light, although the
hypocotyl is strongly heliotropic. They are not provided with a
pulvinus, but continue to grow for a long time.
Ipomœa purpurea (vel Pharbitis hispida).—The cotyledons behave in all
respects like those of I. caerulea. A seedling with cotyledons .75 inch
in length (measured as before) and 1.65 inch in breadth, having a small
true leaf developed, was placed at 5.30 P.M. on a klinostat in a
darkened box, so that neither weight nor geotropism could act on them.
At 10 P.M. one cotyledon stood at 77° and the other at 82° beneath the
horizon. Before being placed in the klinostat they stood at 15° and 29°
beneath the horizon. The nocturnal position depends chiefly on the
curvature of the petiole close to the blade, but the whole petiole
becomes slightly curved downwards. It deserves notice that seedlings of
this and the last-named species were raised at the end of February and
another lot in the middle of March, and the cotyledons in neither case
exhibited any nyctitropic movement.
Ipomœa bona-nox.—The cotyledons after a few days grow to an enormous
size, those on a young seedling being 3 1/4 inches in breadth. They
were extended horizontally at noon, and at 10 P.M. stood at 63° beneath
the horizon. five days afterwards they were 4½ inches in breadth, and
at night one stood at 64° and the other 48° beneath the horizon. Though
the blades are thin, yet from their great size and from the petioles
being long, we imagined that their depression at night might be
determined by their weight; but when the pot was laid horizontally,
they became curved towards the hypocotyl, which movement could not have
been in the least aided by their weight, at the same time they were
somewhat twisted upwards through apogeotropism. Nevertheless, the
weight of the cotyledons is so far influential, that when on another
night the pot was turned upside down, they were unable to rise and thus
to assume their proper nocturnal position.
Ipomœa coccinea.—The cotyledons whilst young do not sink at night, but
when grown a little older, but still only .4 inch in length (measured
as before) and .82 in breadth, they became greatly depressed. In one
case they were horizontal at noon, and at 10 P.M. one of them stood at
64° and the other at 47° beneath the horizon. The blades are thin, and
the petioles, which become much curved down at night, are short, so
that here weight can hardly have produced any effect. With all the
above species of Ipomœa, when the two cotyledons on the same seedling
were unequally depressed at night, this seemed to depend on the
position which they had held during the day with reference to the
light.
Solanum lycopersicum (Solaneae).—The cotyledons rise so much at night
as to come nearly in contact. Those of ‘S. palinacanthum’ were
horizontal at noon, and by 10 P.M. had risen only 27° 30 minutes; but
on the following morning before it was light they stood at 59° above
the horizon, and in the afternoon of the same day were again
horizontal. The behaviour of the cotyledons of this latter species
seems, therefore, to be anomalous.
Mirabilis jalapa and longiflora (Nyctagineae).—The cotyledons, which
are of unequal size, stand horizontally during the middle of the day,
and at night rise up vertically and come into close contact with one
another. But this movement with M. longiflora lasted for only the three
first nights.
Beta vulgaris (Polygoneae).—A large number of seedlings were observed
on three occasions. During the day the cotyledons sometimes stood
sub-horizontally, but more commonly at an angle of about 50° above the
horizon, and for the first two or three nights they rose up vertically
so as to be completely closed. During the succeeding one or two nights
they rose only a little, and afterwards hardly at all.
Amaranthus caudatus (Amaranthaceae).—At noon the cotyledons of many
seedlings, which had just germinated, stood at about 45° above the
horizon, and at 10.15 P.M. some were nearly and the others quite
closed. On the following morning they were again well expanded or open.
Cannabis sativa (Cannabineae).—We are very doubtful whether this plant
ought to be here included. The cotyledons of a large number of
seedlings, after being well illuminated during the day, were curved
downwards at night, so that the tips of some pointed directly to the
ground, but the basal part did not appear to be at all depressed. On
the following morning they were again flat and horizontal. the
cotyledons of many other seedlings were at the same time not in any way
affected. Therefore this case seems very different from that of
ordinary sleep, and probably comes under the head of epinasty, as is
the case with the leaves of this plant according to Kraus. The
cotyledons are heliotropic, and so is the hypocotyl in a still stronger
degree.
Oxalis.—We now come to cotyledons provided with a pulvinus, all of
which are remarkable from the continuance of the nocturnal movements
during several days or even weeks, and apparently after growth has
ceased. The cotyledons of O. rosea, floribunda and articulata sink
vertically down at night and clasp the upper part of the hypocotyl.
Those of O. Valdiviana and sensitiva, on the contrary, rise vertically
up, so that their upper surfaces come into close contact; and after the
young leaves are developed these are clasped by the cotyledons. As in
the daytime they stand horizontally, or are even a little deflected
beneath the horizon, they move in the evening through an angle of at
least 90°. Their complicated circumnutating movements during the day
have
been described in the first chapter. The experiment was a superfluous
one, but pots with seedlings of O. rosea and floribunda were turned
upside down, as soon as the cotyledons began to show any signs of
sleep, and this made no difference in their movements.
Leguminosae.—It may be seen in our list that the cotyledons of several
species in nine genera, widely distributed throughout the Family, sleep
at night; and this probably is the case with many others. The
cotyledons of all these species are provided with a pulvinus; and the
movement in all is continued during many days or weeks. In Cassia the
cotyledons of the ten species in the list rise up vertically at night
and come into close contact with one another. We observed that those of
C. florida opened in the morning rather later than those of C. glauca
and pubescens. The movement is exactly the same in C. mimosoides as in
the other species, though its subsequently developed leaves sleep in a
different manner. The cotyledons of an eleventh species, namely, C.
nodosa, are thick and fleshy, and do not rise up at night. The
circumnutation of the cotyledons during the day of C. tora has been
described in the first chapter. Although the cotyledons of Smithia
sensitiva rose from a horizontal position in the middle of the day to a
vertical one at night, those of S. Pfundii, which are thick and fleshy,
did not sleep. When Mimosa pudica and albida have been kept at a
sufficiently high temperature during the day, the cotyledons come into
close contact at night; otherwise they merely rise up almost
vertically. The circumnutation of those of M. pudica has been
described. The cotyledons of a Bauhinia from St. Catharina in Brazil
stood during the day at an angle of about 50° above the horizon, and at
night rose to 77°; but it is probable that they would have closed
completely, if the seedlings had been kept in a warmer place.
Lotus.—In three species of Lotus the cotyledons were observed to sleep.
Those of L. Jacoboeus present the singular case of not rising at night
in any conspicuous manner for the first 5 or 6 days of their life, and
the pulvinus is not well developed at this period. Afterwards the
sleeping movement is well displayed, though to a variable degree, and
is long continued. We shall hereafter meet with a nearly parallel case
with the leaves of Sida rhombifolia. The cotyledons of L. Gebelii are
only slightly raised at night, and differ much in this respect from the
three species in our list.
Trifolium.—The germination of 21 species was observed. In most of them
the cotyledons rise hardly at all, or only slightly, at night; but
those of T. glomeratum, striatum and incarnactum rose from 45° to 55°
above the horizon. With T. subterraneum, leucanthemum and strictum,
they stood up vertically; and with T. strictum the rising movement is
accompanied, as we shall see, by another movement, which makes us
believe that the rising is truly nyctitropic. We did not carefully
examine the cotyledons of all the species for a pulvinus, but this
organ was distinctly present in those of T. subterraneum and strictum;
whilst there was no trace of a pulvinus in some species, for instance,
in T. resupinatum, the cotyledons of which do not rise at night.
Trifolium subterraneum.—The blades of the cotyledons on the first day
after germination (Nov. 21st) were not fully expanded, being inclined
at about 35° above the horizon; at night they rose to about 75°. Two
days afterwards the blades at noon were horizontal, with the petioles
highly inclined upwards; and it is remarkable that the nocturnal
movement is almost wholly confined to the blades, being effected by the
pulvinus at their bases; whilst the petioles retain day and night
nearly the same inclination. On this night (Nov. 23rd), and for some
few succeeding nights, the blades rose from a horizontal into a
vertical position, and then became bowed inwards at about an average
angle of 10°; so that they had passed through an angle of 100°. Their
tips now almost touched one another, their bases being slightly
divergent. The two blades thus formed a highly inclined roof over the
axis of the seedling. This movement is the same as that of the terminal
leaflet of the tripartite leaves of many species of Trifolium. After an
interval of 8 days (Nov. 29th) the blades were horizontal during the
day, and vertical at night, and now they were no longer bowed inwards.
They continued to move in the same manner for the following two months,
by which time they had increased greatly in size, their petioles being
no less than .8 of an inch in length, and two true leaves had by this
time been developed.
Trifolium strictum.—On the first day after germination the cotyledons,
which are provided with a pulvinus, stood at noon horizontally, and at
night rose to only about 45° above the horizon. Four days afterwards
the seedlings were again observed at night, and now the blades stood
vertically and were in contact, excepting the tips, which were much
deflexed, so that they faced the zenith. At this age the petioles are
curved
upwards, and at night, when the bases of the blades are in contact, the
two petioles together form a vertical ring surrounding the plumule. The
cotyledons continued to act in nearly the same manner for 8 or 10 days
from the period of germination; but the petioles had by this time
become straight and had increased much in length. After from 12 to 14
days the first simple true leaf was formed, and during the ensuing
fortnight a remarkable movement was repeatedly observed. At I. (Fig.
125) we have a sketch, made in the middle of the day, of a seedling
about a fortnight old. The two cotyledons, of which Rc is the right and
Lc the left one, stand directly opposite one another, and the first
true leaf (F) projects at right angles to them. At night (see II. and
III.) the right cotyledon (Rc) is greatly raised, but is not otherwise
changed in position. The left cotyledon (Lc) is likewise raised, but it
is also twisted so that its blade, instead of exactly facing the
opposite one, now stands at nearly right angles to it. This nocturnal
twisting movement is effected not by means of the pulvinus, but by the
twisting of the whole length of the petiole, as could be seen by the
curved line of its upper concave surface. At the same time the true
leaf (F) rises up, so as to stand vertically, or it even passes the
vertical and is inclined a little inwards. It also twists a little, by
which means the upper surface of its blade fronts, and almost comes
into contact with, the upper surface of the twisted
left cotyledon. This seems to be the object gained by these singular
movements. Altogether 20 seedlings were examined on successive nights,
and in 19 of them it was the left cotyledon alone which became twisted,
with the true leaf always so twisted that its upper surface approached
closely and fronted that of the left cotyledon. In only one instance
was the right cotyledon twisted, with the true leaf twisted towards it;
but this seedling was in an abnormal condition, as the left cotyledon
did not rise up properly at night. This whole case is remarkable, as
with the cotyledons of no other plant have we seen any nocturnal
movement except vertically upwards or downwards. It is the more
remarkable, because we shall meet with an analogous case in the leaves
of the allied genus Melilotus, in which the terminal leaflet rotates at
night so as to present one edge to the zenith and at the same time
bends to one side, so that its upper surface comes into contact with
that of one of the two now vertical lateral leaflets.
Fig. 125. Trifolium strictum: diurnal and nocturnal positions of the
two cotyledons and of the first leaf. I. Seedling viewed obliquely from
above, during the day: Rc, right cotyledon; Lc, left cotyledon; F,
first true leaf. II. A rather younger seedling, viewed at night: Rc,
right cotyledon raised, but its position not otherwise changed; Lc,
left cotyledon raised and laterally twisted; F, first leaf raised and
twisted so as to face the left twisted cotyledon. III. Same seedling
viewed at night from the opposite side. The back of the first leaf, F,
is here shown instead of the front, as in II.
Concluding Remarks on the Nyctitropic Movements of Cotyledons.—The
sleep of cotyledons (though this is a subject which has been little
attended to), seems to be a more common phenomenon than that of leaves.
We observed the position of the cotyledons during the day and night in
153 genera, widely distributed throughout the dicotyledonous series,
but otherwise selected almost by hazard; and one or more species in 26
of these genera placed their cotyledons at night so as to stand
vertically or almost vertically, having generally moved through an
angle of at least 60°. If we lay on one side the Leguminosae, the
cotyledons of which are particularly liable to sleep, 140 genera
remain; and out of these, the cotyledons of at least one species in 19
genera slept. Now if we were to select by hazard 140 genera, excluding
the Leguminosae, and observed their leaves at night, assuredly not
nearly so many as 19 would be found to include sleeping species. We
here refer exclusively to the plants observed by ourselves.
In our entire list of seedlings, there are 30 genera, belonging to 16
Families, the cotyledons of which in some of the species rise or sink
in the evening or early night, so as to stand at least 60° above or
beneath the horizon. In a large majority of the genera, namely, 24, the
movement is a rising one; so that the same direction prevails in these
nyctitropic movements as in the lesser periodic ones described in the
second chapter. The cotyledons move downwards during the early part of
the night in only 6 of the genera; and in one of them, Cannabis, the
curving down of the tip is probably due to epinasty, as Kraus believes
to be the case with the leaves. The downward movement to the amount of
90° is very decided in Oxalis Valdiviana and sensitiva, and in Geranium
rotundifolium. It is a remarkable fact that with Anoda Wrightii, one
species of Gossypium and at least 3 species of Ipomœa, the cotyledons
whilst young and light sink at night very little or not at all;
although this movement becomes well pronounced as soon as they have
grown large and heavy. Although the downward movement cannot be
attributed to the weight of the cotyledons in the several cases which
were investigated, namely, in those of the Anoda, Ipomœa purpurea and
bona-nox, nor in that of I. coccinea, yet bearing in mind that
cotyledons are continually circumnutating, a slight cause might at
first have determined whether the great nocturnal movement should be
upwards or downwards. We may therefore suspect that in some aboriginal
member of the groups in question, the weight of the cotyledons first
determined the downward direction. The fact of the cotyledons of these
species not sinking down much whilst they are young and tender, seems
opposed to the belief that the greater movement when they are
grown older, has been acquired for the sake of protecting them from
radiation at night; but then we should remember that there are many
plants, the leaves of which sleep, whilst the cotyledons do not; and if
in some cases the leaves are protected from cold at night whilst the
cotyledons are not protected, so in other cases it may be of more
importance to the species that the nearly full-grown cotyledons should
be better protected than the young ones.
In all the species of Oxalis observed by us, the cotyledons are
provided with pulvini; but this organ has become more or less
rudimentary in O. corniculata, and the amount of upward movement of its
cotyledons at night is very variable, but is never enough to be called
sleep. We omitted to ascertain whether the cotyledons of Geranium
rotundifolium possess pulvini. In the Leguminosae all the cotyledons
which sleep, as far as we have seen, are provided with pulvini. But
with Lotus Jacobæus, these are not fully developed during the first few
days of the life of the seedling, and the cotyledons do not then rise
much at night. With Trifolium strictum the blades of the cotyledons
rise at night by the aid of their pulvini; whilst the petiole of one
cotyledon twists half-round at the same time, independently of its
pulvinus.
As a general rule, cotyledons which are provided with pulvini continue
to rise or sink at night during a much longer period than those
destitute of this organ. In this latter case the movement no doubt
depends on alternately greater growth on the upper and lower side of
the petiole, or of the blade, or of both, preceded probably by the
increased turgescence of the growing cells. Such movements generally
last for a very short period—for instance, with Brassica and Githago
for 4 or 5 nights, with Beta for 2 or 3, and with
Raphanus for only a single night. There are, however, some strong
exceptions to this rule, as the cotyledons of Gossypium, Anoda and
Ipomœa do not possess pulvini, yet continue to move and to grow for a
long time. We thought at first that when the movement lasted for only 2
or 3 nights, it could hardly be of any service to the plant, and hardly
deserved to be called sleep; but as many quickly-growing leaves sleep
for only a few nights, and as cotyledons are rapidly developed and soon
complete their growth, this doubt now seems to us not well-founded,
more especially as these movements are in many instances so strongly
pronounced. We may here mention another point of similarity between
sleeping leaves and cotyledons, namely, that some of the latter (for
instance, those of Cassia and Githago) are easily affected by the
absence of light; and they then either close, or if closed do not open;
whereas others (as with the cotyledons of Oxalis) are very little
affected by light. In the next chapter it will be shown that the
nyctitropic movements both of cotyledons and leaves consist of a
modified form of circumnutation.
As in the Leguminosae and Oxalidæ, the leaves and the cotyledons of the
same species generally sleep, the idea at first naturally occurred to
us, that the sleep of the cotyledons was merely an early development of
a habit proper to a more advanced stage of life. But no such
explanation can be admitted, although there seems to be some
connection, as might have been expected, between the two sets of cases.
For the leaves of many plants sleep, whilst their cotyledons do not do
so—of which fact Desmodium gyrans offers a good instance, as likewise
do three species of Nicotiana observed by us; also Sida rhombifolia,
Abutilon Darwinii, and Chenopodium album. On the other
hand, the cotyledons of some plants sleep and not the leaves, as with
the species of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis,
named in our list. Still more striking is the fact that, in the same
genus, the leaves of several or of all the species may sleep, but the
cotyledons of only some of them, as occurs with Trifolium, Lotus,
Gossypium, and partially with Oxalis. Again, when both the cotyledons
and the leaves of the same plant sleep, their movements may be of a
widely dissimilar nature: thus with Cassia the cotyledons rise
vertically up at night, whilst their leaves sink down and twist round
so as to turn their lower surfaces outwards. With seedlings of Oxalis
Valdiviana, having 2 or 3 well-developed leaves, it was a curious
spectacle to behold at night each leaflet folded inwards and hanging
perpendicularly downwards, whilst at the same time and on the same
plant the cotyledons stood vertically upwards.
These several facts, showing the independence of the nocturnal
movements of the leaves and cotyledons on the same plant, and on plants
belonging to the same genus, lead to the belief that the cotyledons
have acquired their power of movement for some special purpose. Other
facts lead to the same conclusion, such as the presence of pulvini, by
the aid of which the nocturnal movement is continued during some weeks.
In Oxalis the cotyledons of some species move vertically upwards, and
of others vertically downwards at night; but this great difference
within the same natural genus is not so surprising as it may at first
appear, seeing that the cotyledons of all the species are continually
oscillating up and down during the day, so that a small cause might
determine whether they should rise or sink at night. Again, the
peculiar nocturnal movement of the left-hand
cotyledon of Trifolium strictum, in combination with that of the first
true leaf. Lastly, the wide distribution in the dicotyledonous series
of plants with cotyledons which sleep. Reflecting on these several
facts, our conclusion seems justified, that the nyctitropic movements
of cotyledons, by which the blade is made to stand either vertically or
almost vertically upwards or downwards at night, has been acquired, at
least in most cases, for some special purpose; nor can we doubt that
this purpose is the protection of the upper surface of the blade, and
perhaps of the central bud or plumule, from radiation at night.