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;* and since Linnaeus 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
 
* Pfeffer has given a clear and interesting sketch of the history of this subject in his 'Die Periodischen Bewegungen der Blattorgane,' 1875, P. 163. [page 281]
 
between the sleep of animals and that of plants,* whether of leaves or flowers. It seems therefore, advisable to give a distinct name to the so-called sleep-movements of plants. These have also generally been confounded, under the term "periodic," with the slight daily rise and fall of leaves, as described in the fourth chapter; and this makes it all the more 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.
 
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 com-
 
* Ch. Royer must, however, be excepted; see 'Annales des Sc. Nat.' (5th series), Bot. vol. ix. 1868, p. 378.
 
[page 282] pound leaves, forwards, that is, towards the apex of the leaf, or backwards, that is, towards its base; or, again, they may rotate on their own axes without moving either upwards or downwards. But in almost every case the plane of the blade is so placed as to stand nearly or quite vertically at night. Therefore the apex, or the base, or either lateral edge, may be directed towards the zenith. Moreover, the upper surface of each leaf, and more especially of each leaflet, is often brought into close contact with that of the opposite one; and this is sometimes effected by singularly complicated movements. This fact suggests that the upper surface requires more protection than the lower one. For instance, the terminal leaflet in Trifolium, after turning up at night so as to stand vertically, often continues to bend over until the upper surface is directed downwards whilst the lower surface is fully exposed to the sky; and an arched roof is thus formed over the two lateral leaflets, which have their upper surfaces pressed closely together. Here we have the unusual case of one of the leaflets not standing vertically, or almost vertically, at night.
 
Considering that leaves in assuming their nyctitropic positions often move through an angle of 90o; 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 [page 283] great advantage from such remarkable powers of movement.
 
The nyctitropic movements of leaves and cotyledons are effected in two ways,* firstly, by means of pulvini which become, as Pfeffer has shown, alternately more turgescent on opposite sides; and secondly, by increased growth along one side of the petiole or midrib, and then on the opposite side, as was first proved by Batalin.** But as it has been shown by De Vries*** that in these latter cases increased growth is preceded by the increased turgescence of the cells, the difference between the above two means of 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 import-
 
* This distinction was first pointed out (according to Pfeffer, 'Die Periodischen Bewegungen der Blattorgane,' 1875, p. 161) by Dassen in 1837.
 
** 'Flora,' 1873, p. 433.
 
*** 'Bot. Zeitung,' 1879, Dec. 19th, p. 830.
 
[page 284] ance 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;* but this statement applies only to periodic and nyctitropic movements as may be inferred from other cases given by Frank.** 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.*** Seedlings are often protected from radiation by a very thin covering of straw; and fruit-trees on walls by a few fir-branches, or even by a fishing-net, suspended over them. There is a variety of the gooseberry,**** the flowers of which from being produced before the leaves, are not protected by them from radiation, and consequently often fail to yield fruit. An excellent observer***** has remarked
 
* Pfeffer, 'Die Period. Beweg. der Blattorgane.' 1875, p. 159.
 
** 'Die Nat. Wagerechte Richtung von Pflanzentheilen,' 1870, p. 52
 
*** 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.
 
**** 'Loudon's Gardener's Mag.,' vol. iv. 1828, p. 112.
 
[page 285]
 
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.
 
This view that the sleep of leaves saves them from being chilled at night by radiation, would no doubt have occurred to Linnaeus, 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 [page 286] 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 [page 287] thin pins, so as to stand from ? 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 -4o C. (24o to 25o 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 (-3o to -4o C. on the grass), and the pot was again exposed, but this time for only 30 m. Eight leaves had been pinned out, [page 288] 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 -2o C., and not one leaf was injured: the pinned open leaves, however, all stood from ? 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 25o to 26o F. (-3.3o to -3.8o C.), but when looked at after 1 h. had fallen to 22o F. (-5.5o 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 -2o 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., -3o to -4o 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- [page 289] purpurea) were exposed for 2 h. and 3 h. to a clear sky with the temp. on grass -2o 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 -2o 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 -3o and -4o 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 -3.3o to -3.8o 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 -2o C., and not a leaf was injured.* It was again exposed on
 
* 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 -2o 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 -3o and -4o C., every leaf was killed. [page 290]
 
another night for 1 h., when the temperature of the grass was -4o 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., -4o 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.
 
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., -3o to -4o C. Ten leaves had been pinned out, and the result was striking, for on the following morning all these were found much injured or [page 291] killed, and none of the many free leaves on the several plants were at all injured, with the doubtful exception of two or three very young ones.
 
Melilotus 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 -1o 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 -2o 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 -3o and -4o 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 -3o to -4o C. Six leaves had been pinned out so as to stand about ? 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. [page 292]
 
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 -1.5o 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 -2o C., and on a second night for 30 m. with the temperature of the grass between -3o and -4o 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 -2o 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., -4o 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 -4o 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 [page 293] 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 29o 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.* Hence it appears, as far as these few trials tell anything, that the vertical position at night of the cotyledons of Mimosa pudica protects them to a certain degree from the evil effects of radiation and cold.]
 
Concluding Remarks on the Radiation from Leaves at 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
 
* 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 29o 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 -2o C., and they were not in the least injured. [page 294]
 
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 [page 295] exposed obliquely to the zenith, nevertheless had these lower surfaces less blackened than the upper surfaces which were turned inwards and were in close contact with those of the opposite leaflets. Again, a pot full of plants of Trifolium resupinatum, which had been kept in a warm room for three days, was turned out of doors (Sept. 21st) on a clear and almost frosty night. Next morning ten of the terminal leaflets were examined as opaque objects under the microscope. These leaflets, in going to sleep, either turn vertically upwards, or more commonly bend a little over the lateral leaflets, so that their lower surfaces are more exposed to the zenith than their upper surfaces. Nevertheless, six of these ten leaflets were distinctly yellower on the upper than on the lower and more exposed surface. In the remaining four, the result was not so plain, but certainly whatever difference there was leaned to the side of the upper surface having suffered most.
 
It has been stated that some of the leaflets experimented on were fastened close to the cork, and others at a height of from ? 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 -2o C.), it was manifest that more dew had congealed into hoar-frost on the closely pinned leaflets, than on those which stood horizontally [page 296] 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 [page 297] immediately feel relief; and this is evidently an analogous, though reversed, case. These several facts—in relation to leaves pinned close to or a little above the cork-supports—to their tips projecting beyond it— and to the leaves on branches kept motionless—seem to us curious, as showing how a difference, apparently trifling, may determine the greater or less injury of the leaves. We may even infer as probable that the less or greater destruction during a frost of the leaves on a plant which does not sleep, may often depend on the greater or less degree of flexibility of their petioles and of the branches which bear them.
 
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,* 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**, 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
 
* 'Die Lehre von der Pflanzenzelle,' 1867, p. 327.
 
** 'Adansonia,' March 10th, 1869.
 
[page 298] 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 60o 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 60o, 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 31o at noon to 55o 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 30o, 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 27o above the horizon, and this would lessen radiation by only 11 per cent.: those of Linum Berendieri rose to 33o, and this would lessen radiation by 16 per cent.
 
There are, however, some other sources of doubt with [page 299] respect to the sleep of cotyledons. In certain cases, the cotyledons whilst young diverge during the day to only a very moderate extent, so that a small rise at night, which we know occurs with the cotyledons of many plants, would necessarily cause them to assume a vertical or nearly vertical position at night; and in this case it would be rash to infer that the movement was effected for any special purpose. On this account we hesitated long whether we should introduce several Cucurbitaceous plants into the following list; but from reasons, presently to be given, we thought that they had better be at least temporarily included. This same source of doubt applies in some few other cases; for at the commencement of our observations we did not always attend sufficiently to whether the 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 60o 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 [page 300] 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 Oxalidae 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.
 
A List of Seedling Plants, the cotyledons of which rise or sink at night to an angle of at least 60o 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. Oxalidae (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. — Jacobaeus. 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. Cucurbitaceae (Fam. 106). — aurantia. Lagenaria vulgaris. Cucurbitaceae. Cucumis dudaim. Cucurbitaceae. Apium petroselinum. Umbelliferae (Fam. 113). — graveolens. Lactuca scariola. Compositae (Fam. 122). Helianthus annuus (?). Compositae. Ipomoea caerulea. Convolvulaceae (Fam. 151). — purpurea. — bona-nox. — coccinea. [page 301] A List of Seedling Plants (continued). 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, [page 302] 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 75o above the horizon; at night they moved upwards, each through an angle of 15o so as to stand quite vertical and in contact with one another. On the second day they stood at noon at 59o above the horizon, and again at night were completely closed, each having risen 31o. 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 35o 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. [page 303] Although the movement of the cotyledons endured for a long time, no pulvinus was exteriorly visible; but their growth continued for a long time. The cotyledons appear to be only slightly heliotropic, though the hypocotyl is strongly so.
 
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 62o 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 70o 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. 2 ? 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. [page 304]
 
Cucurbitaceae.—The cotyledons of Cucurbita aurantia and ovifera, and of Lagenaria vulgaris, stand from the 1st to the 3rd day of their life at about 60o 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 45o 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* 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.
 
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 27o 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,
 
* 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. [page 305]
 
or more than seven times as much. On the next night the cotyledons occupied nearly the same position as before. On the morning of the 24th they stood horizontally, and at night were 60o 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 61o above the horizon.
 
Lactuca scariola (Compositae).—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 (Compositae).—This case is rather doubtful; the cotyledons rise at night, and on one occasion they stood at 73o above the horizon, so that they might then be said to have been asleep.
 
Ipomoea 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 55o and 70o 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.
 
Ipomoea 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 77o and the other at 82o beneath the horizon. Before being placed in the klinostat they stood at 15o and 29o [page 306] 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.
 
Ipomoea 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 63o beneath the horizon. five days afterwards they were 4 ? inches in breadth, and at night one stood at 64o and the other 48o 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.
 
Ipomoea 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 64o and the other at 47o beneath the horizon. The blades are thin, and the petioles, which become much curved down at night, are short, so that here weight can hardly have produced any effect. With all the above species of Ipomoea, when the two cotyledons on the same seedling were unequally depressed at night, this seemed to depend on the position which they had held during the day with reference to the light.
 
Solanum lycopersicum (Solaneae).—The cotyledons rise so much at night as to come nearly in contact. Those of 'S. palinacanthum' were horizontal at noon, and by 10 P.M. had risen only 27o 30 minutes; but on the following morning before it was light they stood at 59o 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. [page 307]
 
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 50o 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 45o 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 90o. Their complicated circumnutating movements during the day have [page 308] 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 50o above the horizon, and at night rose to 77o; 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. [page 309]
 
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 45o to 55o 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 35o above the horizon; at night they rose to about 75o. 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 10o; so that they had passed through an angle of 100o. 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 45o 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 [page 310] 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,
 
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.
 
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 [page 311] 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.]
 
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 60o. 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. [page 312]
 
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 60o 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 90o 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 Ipomoea, 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, Ipomoea 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 [page 313] 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 Jacobaeus, 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 [page 314] Raphanus for only a single night. There are, however, some strong exceptions to this rule, as the cotyledons of Gossypium, Anoda and Ipomoea 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 Oxalidae, 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 [page 315] hand, the cotyledons of some plants sleep and not the leaves, as with the species of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis, named in our list. Still more striking is the fact that, in the same genus, the leaves of several or of all the species may sleep, but the cotyledons of only some of them, as occurs with Trifolium, Lotus, Gossypium, and partially with Oxalis. Again, when both the cotyledons and the leaves of the same plant sleep, their movements may be of a widely dissimilar nature: thus with Cassia the cotyledons rise vertically up at night, whilst their leaves sink down and twist round so as to turn their lower surfaces outwards. With seedlings of Oxalis Valdiviana, having 2 or 3 well-developed leaves, it was a curious spectacle to behold at night each leaflet folded inwards and hanging perpendicularly 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 coty- [page 316] ledon 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. [page 317]