The presence of light or its absence cannot
be supposed to be the direct cause of the movements, for these are
wonderfully diversified even with the leaflets of the same leaf, although
all have of course been similarly exposed. The movements depend on innate
causes, and are of an adaptive nature. The alternations of light and
darkness merely give notice to the leaves that the period has arrived for
them to move in a certain manner. We may infer from the fact of several
plants (Tropaeolum, Lupinus, etc.) not sleeping unless they have been well
illuminated during the day, that it is not the actual decrease of light in
the evening, but the contrast between the amount at this hour and during
the early part of the day, which excites the leaves to modify their
ordinary mode of circumnutation.
As the leaves of most plants assume their proper diurnal position in the
morning, although light be excluded, and as the leaves of some plants
continue to move in the normal manner in darkness during at least a whole
day, we may conclude that the periodicity of their movements is to a
certain extent inherited.
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