In most other
respects a tendril acts as if it were one of several revolving
internodes, which all move together by successively bending to each
point of the compass. There is, however, in many cases this
unimportant difference, that the curving tendril is separated from
the curving internode by a rigid petiole. With most tendril-bearers
the summit of the stem or shoot projects above the point from which
the tendril arises; and it is generally bent to one side, so as to be
out of the way of the revolutions swept by the tendril. In those
plants in which the terminal shoot is not sufficiently out of the
way, as we have seen with the Echinocystis, as soon as the tendril
comes in its revolving course to this point, it stiffens and
straightens itself, and thus rising vertically up passes over the
obstacle in an admirable manner.
All tendrils are sensitive, but in various degrees, to contact with
an object, and curve towards the touched side. With several plants a
single touch, so slight as only just to move the highly flexible
tendril, is enough to induce curvature. Passiflora gracilis
possesses the most sensitive tendrils which I have observed: a bit
of platina wire 0.
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