In Bignonia unguis and its close
allies, the petioles of the leaves, as well as the tendrils, are
sensitive to a touch.
Twining plants when they come into contact with a stick, curl round
it invariably in the direction of their revolving movement; but
tendrils curl indifferently to either side, in accordance with the
position of the stick and the side which is first touched. The
clasping movement of the extremity is apparently not steady, but
undulatory or vermicular in its nature, as may be inferred from the
curious manner in which the tendrils of the Echinocystis slowly
crawled round a smooth stick.
As with a few exceptions tendrils spontaneously revolve, it may be
asked,--why have they been endowed with sensitiveness?--why, when
they come into contact with a stick, do they not, like twining
plants, spirally wind round it? One reason may be that they are in
most cases so flexible and thin, that when brought into contact with
any object, they would almost certainly yield and be dragged onwards
by the revolving movement. Moreover, the sensitive extremities have
no revolving power as far as I have observed, and could not by this
means curl round a support.
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