The Power of Movement in Plants

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 } to ? inch above the cork. This difference in the result, which is in itself curious as showing what a very slight difference in the conditions influences the amount of injury in- flicted, may be attributed, as we believe, to the sur- rounding warmer air not circulating freely beneath the closely pinned leaves and thus slightly warming them. This conclusion is supported by some analogous facts hereafter to be given.

We will now describe in detail the experiments which were tried. These were troublesome from our not being able to predict how much cold the leaves of the several species could endure. Many plants had every leaf killed, both those which were secured in a horizontal position and those which were allowed to sleep—that is, to rise up or sink down vertically. Others again had not a single leaf in the least in- jured, and these had to be re-exposed either for a longer time or to a lower temperature.

Oxalis acetusella—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 2h. 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 (— 8° to — 4° C. on the grass), and the pot was again exposed but this time for only 30m. Hight 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 1h. 30 m., the tempera- ture on the grass being only — 2° C., and not one leaf was injured: the pinned open leaves, however, all stood from 3 to 2 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 40m. By a mis- take the thermometer was left on an adjoining sun-dial 3 feet high, instead of being placed on the grass; it recorded 25° to 26° F. (— 3:8° to — 38° 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 30m. to a clear sky, the thermometer on the grass standing at — 2° C, with some of its Jeaves 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 30m., when the temperature on the grass was only a little lower, viz, — 8° 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 hori- zontally to the zenith suffered most, This same pot was after- wards exposed for 85-40 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. corniculuta (var. Atro

purpurea) were exposed for 2h. and 3h. toa clear sky with the temp. on grass — 2° C., and none of the leaves, whether free or pinned open, were at all injured. Arachis hypogceea—Some plants in a pot were exposed at night for 80m. 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 1h. 30 m. On neither occasion was a siagle leaf, whether pinned open or frec, injured; and this surprised us much, considering its native tropical African home. Two plants were next exposed (March 16th) for 30 m. toa clear sky, the temperature of the surrounding grass being now lower, viz., between — 3° and — 4°C., and all four pinned-open leaves were killed and blackened. These two plants bore 22 other and free leaves (excluding some very young bud-like ones) and only two of these were killed and three some- what injured; that is, 23 per cent. were either killed or injured, whereas all four pinned open leaves were utterly killed. =

On another night two pots with several plants were exposed for between 35m. and 40m. to a clear sky, and perhaps to a rather lower temperature, for a thermometer on a dial, 3 feet high, close by stood at — 3:3° to — 388° 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 unburt; that is, 50 per cent. of the free leaves were either killed or much in- jured. Taking the two pots together, we may say that rather more than half of the free leaves, which were asleep, were either killed or injured, whilst all the ten horizontally extended leaves, which had been prevented from going to sleep, were either killed or much injured.

Cassia floribunda.—A bush was exposed at night for 40 m. to a clear sky, the temperature on the surrounding grass being ~— 2° C., and not a leaf was injured.* It was again exposed on to a clear sky fur 35 m., and C. calliantha (a Guiana species) for 60 m., the temperature on the surrounding grass being — 2° C., and neither were in the least was exposed for 1 h., the temp. on the surrounding grass being between — 3° and — 4° C., every leaf was killed.

another night for Lh., 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 fo'lowing morning every single leaflet on these four leaves was dead, with both their upper and lower surfaces completely blackened. Of the many free leaves on the bush, only seven were blackened, and of these only a single one (which was a younger and more tender leaf than any of the pinned ones) had both surfaces of the leaflets blackened. The contrast in this latter respect was well shown by a free leaf, which stood between two pinned-open ones; for these latter had the lower surfaces of their leaflets as black as ink, whilst the inter- mediate free leaf, though badly injured, still retained a plain tinge of green on the lower surface of the leaflets. ‘This bush exhibited in a striking manner the evil effects of the leaves not being allowed to assume at night their normal dependent posi- tion; for had they all been prevented from doing so, assuredly every single leaf on the bush would have been utterly killed by this exposure of only 30m. The leaves whilst sinking down- wards in the evening twist round, so that the upper surface is turned inwards, and is thus better protected than the outwardly turned lower surface.

Nevertheless, it was always the upper surface which was more blackened than the lower, whenever any difference could be perceived between them; but whether this was due to the cells near the upper surface being more tender, or merely to their containing more chlorophyll, we do not know. Melilutus officinalis—A large pot with many plants, which had been kept during the winter in the greenhouse, was exposed during 5h. 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 norizontally 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 ont horizontally, three with their upper and three with their lower surfaces turned to the zenith. ‘The plants were exposed for 5h. 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 swaveolens—Some plants with 8 leaves pinned out were exposed to a clear sky during 2h., 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 Taur ica.—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 aud below on the same branches which had gone to sleep. Another pot, which had likewise been kept in the greenhouse, was exposed for 85-40 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 Petitpierrcana.—The plants were exposed to a clear sky for 85-40 m.: temperature on surrounding grass — 8° to —4°C. Six leaves had been pinned out so as to stand about 4 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, 88 per cent. We estimated that there were 200 free leaves on the plants, and of these about 50 were killed and 20 badly injured, so that about 35 per cent. of the free leaves were killed or injured. Lotus aristata.—Six plants were exposed for nearly 5h. tn a clear sky; temperature on surrounding grass — 15° C. Four leaves had been pinned out horizontally, and 2 cf 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 Jacobeus, an inhabitant of so hot a country as the Cape Verde Islands, were exposed one night toa 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 1h. 385m. to a clear sky, the temperature on the surrounding ground being — 2° C., and not a single leaf wasinjured. After an interval of some days the plant was again exposed for Lh. 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 cxposed for 1h., the temperature on the ground being again — 4° GC. Six of the leaves were killed, and one which did not at first appear injured after- wards became streaked with brown. The trailing branches, which rested on the frozen ground, had one-half or three-quarters of thicit 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, tempera- ture (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 thcir 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 1h. 45m, 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.* Hence it appears, as far as these few trials tell anything, that the vertical position at night of the cotyledons of A/imosa 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 Oxalts purpurea, which naturally sink at night * We were surprised that young seedlings of so tropical a plant as Mimosa pudica were able to resi-t, as well as they did, ex- posure tor 1 br. 45 m. tu a clear pky, the temperature on the sur- ruunding ground being 29° F.

Tt may be added that seedlings of the Indian Cass/u pubescens were exposed fur 1 b. 30 m, to a clear sky, with the temp. on the sur- rounding ground at — 2° C., and they were not in the least injured them early on several successive mornings, after they had assumed their diurnal positions. ‘The difference in the amount of dew on the pinned-open leaflets and on those which had gone to sleep was generally conspicuous; the latter being sometimes absolutely dry, whilst the leaflets which had been horizontal were coated with large beads of dew. This shows how much cooler the leaflets fully exposed to the zenith must have become, than those which stood almost vertically, either upwards or downwards, during the night.

From the several cases above given, there can be no doubt that the position of the leaves at night affects their temperature through radiation to such a degree, that when exposed to a clear sky during a frost, it is a question of life and death. We may therefore admit as highly probable, seeing that their nocturnal posi- tion is so well adapted to lessen radiation, that the object gained by their often complicated sleep move- ments, is to lessen the degree to which they are chilled at night. It should be kept in mind that it is especially the upper surface which is thus pro- tected, as it is never directed towards the zenith, and is often 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 consti- tution between the two surfaces is shown by the follow- ing cases. Cassia floribunda was exposed toa 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 resupinatwm, which had been kept in a warm room for three days, was turned out of doors (Sept. 21st) on a clear and almost frosty night. Next morning ten of the terminal leaflets were examined as opaque objects under the microscope. These leaflets, in going to sleep, either turn vertically upwards, or more commonly bend a little over the lateral leaflets, so that their lower surfaces are more exposed to the zenith than their upper surfaces. Nevertheless, six of these ten leaflets were distinctly yellower on the upper than on the lower and more exposed surface. In the remaining four, the result was not so plain, but certainly whatever difference there was leaned to the side of the upper surface having suffered most.

It has been stated that some of the leaflets experi- mented on were fastened close to the cork, and others at a height of from 3 to ? of an inch above it; and that whenever, after exposure to a frost, any difference could be detected in their states, the closely pinned ones had suffered most. We attributed this difference to the air, not cooled by radiation, having been pre- vented from circulating freely beneath the closely pinned leaflets. That there was really a difference in the temperature of leaves treated in these two dif- ferent methods, was plainly shown on one occasion; for after the exposure of a pot with plants of Melzlotus dentata for 2h. 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, pro- ected a little beyond the edge, so that the air could circulate freely round them. ‘his occurred with six leaflets of Oxulis acetosellu, and their tips certainly suffered rather less than the rest of the same leaflets ; for on the following morning they were still slightly green. ‘The same result followed, even still more clearly, in two cases with leaflets of Melilotus officinalis which projected a little beyond the cork; and in two other cases some leaflets which were pinned close to the cork were injured, whilst other free leaflets on the sume 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 Melzlotus Petitpierreana, but the injured leaves in this instance were not actually counted. With Arachis hypogea, 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 Ozalis carnosa, 7 free leaves were injured, and every one of them belonged to a cluster of leaves, some of which had been pinned to the cork. We could account for these cases only by supposing that the branches which were quite free had been slightly waved about by the wind, and that their leaves had thus been a little warmed by the sur- rounding warmer air. If we hold our hands motion less before a hot fire, and then wave them about, we

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— und 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.

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 Caryophyllee (Alsinee and Silene) bend upwards at night (but to what angle he does not state), remarks that those of Stellarta media rise up so as to touch one another; they may therefore safely be said to sleep. Secondly, according to Ramey,f{ the cotyledons of Mimosa pudica and of Clianthus Dam- piert 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 pre- vented from sleeping. We must therefore take an arbitrary limit. If a cotyledon or leaf is inclined at 60° above or beneath the horizon, it exposes to the zenith about one-half of its area; consequently the intensity of its radiation will be lessened by about half, compared with what it would have been if the cotyledon or leaf had remained horizontal. This degree of diminution certainly would make a great difference to a plant having a tender constitution. We will therefore speak of a cotyledon and hereafter of a leaf as sleeping, only when it rises at night to an angle of about 60°, or to a still higher angle, above the horizon, or sinks beneath it to the same amount. Not but that a lesser diminution of radiation may be advantageous to a plant, as in the case of Datura stramonium, the cotyledons of which rose from 31° at noon to 55° at night above the horizon. The Swedish turnip may profit by the area of its leaves being reduced at night by about 30 per cent., as estimated by Mr. A. 8. 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 smal], such as less than 30°, the diminution of radiation is so slight that it probably is of no sig- nificance to the plant in relation to radiation. For instance, the cotyledons of Geranium Ibericum rose at night to 27° above the horizon, and this would lessen radiation by only 11 per cent.: those of Linum Beren- diert rose to 83°, and this would lessen radiation by 16 per cent.