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Show 294 MODIFIED CIRCUl\1NUTATION. CHAP. VI. 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. vVe may therefore adm~t as highly probable, seeing that their nocturnal posr" tion 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 t~us protected as it is never directed towards the zemth, and is oft~n 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 t~e lower surface, or from its injury being a gr~ater ev~l to the plant. That there is some difference m constr· tution between the two surfaces is shown by the following cases. Cassia floribwnda was exposed to a clear s.ky on a sharp frosty night, and several leaflet~ wh~ch had assumed their nocturnal dependent positwn wr~ their lower surfaces turned outwards so as to e CHAP. VI. USE OF SLEEP MOVEMENTS. 29G exposed obliquely to the zenith, nevertheless had these lower surfaces less blackened than the upper surfaces which were turned inward and were in elose contact with those of the opposite leaflets. Again, a pot full of plants of T1·ijoli~tm 1·esupinatum, which had been kept in a warm room for three days, was tnn1ed out of doors (Sept. 21st) on a el ar and almost frosty night. Next morning ten of the terminal leaflets wer~ examined as ~paqu~ objects uncler the microscope. These leaflets, m gomg 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 th ir upper sur±~tees. Nevertheless, six of these ten leaflets wer 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 the~e was leaned to the side of the upper surfuc havmg suffered most. It has been stated that some of the leaflets experimented on were fastened close to the cork an(l other· at a height of from ! to i of an inch ab~ve it· and that whenever, after exposure to a frost, any cliff;renc could be detected in their states, the closely pinned ones had suffered most. We attributed this differenc to the air, not cooled by radiation, havino· been prev~ nted from circulating freely beneath ~he cJo ely pmned leaflets. That there was really a difference in the temperature of leaves treated. in these two dif~ erent methods, was plainly shown on one occasion; t after the exposure of a pot with plants of Melilotus entata f?r 2 h. to a clear sky (the temperature on the surroundmg· gr·a ss b c·m g - 2o 0 .) , I. t was mam.f est that m.o re dew h a d congea1 e d I. nto hoar-frost on the closely Ptnned leaflet s, th an on t h ose wh w· h stood horizontally i' ·:.·t .... . •' ·~I I ,..•. ,.,. |
