They act
best when each branch seizes a few thin stems, like the culms of a
grass, which they afterwards draw together into a solid bundle by the
spiral contraction of all the branches. In Cobaea the finely-
branched tendrils alone revolve; the branches terminate in sharp,
hard, double, little hooks, with both points directed to the same
side; and these turn by well-adapted movements to any object with
which they come into contact. The tips of the branches also crawl
into dark crevices or holes. The tendrils and internodes of
Ampelopsis have little or no power of revolving; the tendrils are but
little sensitive to contact; their hooked extremities cannot seize
thin objects; they will not even clasp a stick, unless in extreme
need of a support; but they turn from the light to the dark, and,
spreading out their branches in contact with any nearly flat surface,
develop discs. These adhere by the secretion of some cement to a
wall, or even to a polished surface; and this is more than the discs
of the Bignonia capreolata can effect.
The rapid development of these adherent discs is one of the most
remarkable peculiarities possessed by any tendrils. We have seen
that such discs are formed by two species of Bignonia, by Ampelopsis,
and, according to Naudin, {37} by the Cucurbitaceous genus Peponopsis
adhaerens.
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