They continue night and day for some months, and
are displayed by young unexpanded leaves, and by old ones which have lost
their sensibility to a touch, but which, after absorbing animal matter,
close their lobes. We shall hereafter meet with the same kind of movement
in the joints of certain Gramineae, and it is probably common to many
plants while circumnutating. It is, therefore, a strange fact that no such
movement could be detected in the tentacles of Drosera rotundifolia, though
a member of the same family with Dionaea; yet the tentacle which was
observed was so sensitive, that it began to curl inwards in 23 seconds
after being touched by a bit of raw meat.
One of the most interesting facts with respect to the circumnutation of
leaves is the periodicity of their movements; for they often, or even
generally, rise a little in the evening and early part of the night, and
sink again on the following morning. Exactly the same phenomenon was
observed in the case of cotyledons. The leaves in 16 genera out of the 33
which were observed behaved in this manner, as did probably 2 others.
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