The Power of Movement in Plants · Charles Darwin
EPINASTY—HYPONASTY.
Concluding Remarks on the Radiation from Leaves at Night.—We exposed on two occasions during the summer to a clear sky several pinned-open leaflets of Trifolium pratense, which naturally rise at night, and of Oxalis purpurea, which naturally sink at night (the plants growing out of doors), and looked at them early on several successive mornings, after they had assumed their diurnal positions. The difference in the amount of dew on the pinned-open leaflets and on those which had gone to sleep was generally conspicuous; the latter being sometimes absolutely dry, whilst the leaflets which had been horizontal were coated with large beads of dew. This shows how much cooler the leaflets fully exposed to the zenith must have become, than those which stood almost vertically, either upwards or downwards, during the night. From the several cases above given, there can be no doubt that the position of the leaves at night affects their temperature through radiation to such a degree, that when exposed to a clear sky during a frost, it is a question of life and death. We may therefore admit as highly probable, seeing that their nocturnal position is so well adapted to lessen radiation, that the object gained by their often complicated sleep movements, is to lessen the degree to which they are chilled at night. It should be kept in mind that it is especially the upper surface which is thus protected, as it is never directed towards the zenith, and is often brought into close contact with the upper surface of an opposite leaf or leaflet.
We failed to obtain sufficient evidence, whether the better protection of the upper surface has been gained from its being more easily injured than the lower surface, or from its injury being a greater evil to the plant. That there is some difference in constitution between the two surfaces is shown by the following cases. Cassia floribunda was exposed to a clear sky on a sharp frosty night, and several leaflets which had assumed their nocturnal dependent position with their lower surfaces turned outwards so as to be exposed obliquely to the zenith, nevertheless had these lower surfaces less blackened than the upper surfaces which were turned inwards and were in close contact with those of the opposite leaflets. Again, a pot full of plants of Trifolium resupinatum, which had been kept in a warm room for three days, was turned out of doors (Sept. 21st) on a clear and almost frosty night. Next morning ten of the terminal leaflets were examined as opaque objects under the microscope. These leaflets, in going to sleep, either turn vertically upwards, or more commonly bend a little over the lateral leaflets, so that their lower surfaces are more exposed to the zenith than their upper surfaces. Nevertheless, six of these ten leaflets were distinctly yellower on the upper than on the lower and more exposed surface. In the remaining four, the result was not so plain, but certainly whatever difference there was leaned to the side of the upper surface having suffered most.
It has been stated that some of the leaflets experimented on were fastened close to the cork, and others at a height of from ½ to 3/4 of an inch above it; and that whenever, after exposure to a frost, any difference could be detected in their states, the closely pinned ones had suffered most. We attributed this difference to the air, not cooled by radiation, having been prevented from circulating freely beneath the closely pinned leaflets. That there was really a difference in the temperature of leaves treated in these two different methods, was plainly shown on one occasion; for after the exposure of a pot with plants of Melilotus dentata for 2 h. to a clear sky (the temperature on the surrounding grass being –2° C.), it was manifest that more dew had congealed into hoar-frost on the closely pinned leaflets, than on those which stood horizontally a little above the cork. Again, the tips of some few leaflets, which had been pinned close to the cork, projected a little beyond the edge, so that the air could circulate freely round them. This occurred with six leaflets of Oxalis acetosella, and their tips certainly suffered rather less then the rest of the same leaflets; for on the following morning they were still slightly green. The same result followed, even still more clearly, in two cases with leaflets of Melilotus officinalis which projected a little beyond the cork; and in two other cases some leaflets which were pinned close to the cork were injured, whilst other free leaflets on the same leaves, which had not space to rotate and assume their proper vertical position, were not at all injured.
Another analogous fact deserves notice: we observed on several occasions that a greater number of free leaves were injured on the branches which had been kept motionless by some of their leaves having been pinned to the corks, than on the other branches. This was conspicuously the case with those of Melilotus Petitpierreana, but the injured leaves in this instance were not actually counted. With Arachis hypogaea, a young plant with 7 stems bore 22 free leaves, and of these 5 were injured by the frost, all of which were on two stems, bearing four leaves pinned to the cork-supports. With Oxalis carnosa, 7 free leaves were injured, and every one of them belonged to a cluster of leaves, some of which had been pinned to the cork. We could account for these cases only by supposing that the branches which were quite free had been slightly waved about by the wind, and that their leaves had thus been a little warmed by the surrounding warmer air. If we hold our hands motionless before a hot fire, and then wave them about, we immediately feel relief; and this is evidently an analogous, though reversed, case. These several facts—in relation to leaves pinned close to or a little above the cork-supports—to their tips projecting beyond it—and to the leaves on branches kept motionless—seem to us curious, as showing how a difference, apparently trifling, may determine the greater or less injury of the leaves. We may even infer as probable that the less or greater destruction during a frost of the leaves on a plant which does not sleep, may often depend on the greater or less degree of flexibility of their petioles and of the branches which bear them.
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