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.
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