After
temporary contact with any object, the petiole continues to bend for
a considerable time; afterwards it slowly becomes straight again, and
can then re-act. A petiole excited by an extremely slight weight
sometimes bends a little, and then becomes accustomed to the
stimulus, and either bends no more or becomes straight again, the
weight still remaining suspended. Petioles which have clasped an
object for some little time cannot recover their original position.
After remaining clasped for two or three days, they generally
increase much in thickness either throughout their whole diameter or
on one side alone; they subsequently become stronger and more woody,
sometimes to a wonderful degree; and in some cases they acquire an
internal structure like that of the stem or axis.
The young internodes of the Lophospermum as well as the petioles are
sensitive to a touch, and by their combined movement seize an object.
The flower-peduncles of the Maurandia semperflorens revolve
spontaneously and are sensitive to a touch, yet are not used for
climbing. The leaves of at least two, and probably of most, of the
species of Clematis, of Fumaria and Adlumia, spontaneously curve from
side to side, like the internodes, and are thus better adapted to
seize distant objects.
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