Page:Darwin - On the movements and habits of climbing plants.djvu/103

 the Echinocystis they retained their sensitiveness for some time after they had ceased revolving and had drooped downwards; in this position, even if they should seize any object, they could be of little or no use in supporting the stem. It is a rare circumstance thus to be able to detect any imperfection or superfluity in tendrils—organs which are so admirably adapted for the functions which they have to perform; but we see that they are not always absolutely perfect, and it would be rash to assume that any existing tendril has reached the utmost limit of perfection.

Some tendrils have their revolving motion accelerated and retarded in moving to and from the light; others, as with the Pea, seem indifferent to its action; others move from the light to the dark, and this aids them in an important manner in finding a support. In Bignonia capreolata the tendrils bend from the light to the dark, like a banner from the wind. In the Cobæa and Eccremocarpus the extremities alone twist and turn about, so as to bring their finer branches and hooks into close contact with any surface, or into dark crevices and holes. This latter movement is one of the best adapted exhibited by tendrils.

A short time after a tendril (with some rare exceptions) has caught a support, it contracts spirally; but the manner of contraction and the several important advantages thus gained have been so lately discussed, that nothing need be here said on the subject. Again, tendrils soon after catching a support grow much stronger and thicker, and sometimes in a wonderful degree durable; and all this shows how much their internal tissues must change. Tendrils which have caught nothing soon shrink and wither; in some species of Bignonia they disarticulate and fall off like leaves in autumn.

Any one who did not closely study tendrils of various kinds would probably infer that their action would always be uniform. This is the case with most kinds of tendrils, of which the extremities simply curl round objects of any moderate degree of thickness, and of various shapes or natures. But Bignonia shows us what diversity of action there may be in the tendrils of even closely allied species. In all the nine species of this genus observed by me the young internodes revolved vigorously; as did the petioles of nearly all, but in very unequal degrees; in three of the species the petioles were sensitive to contact; the tendrils of all are sensitive to contact, and likewise revolve, but in some of the species in a very feeble manner. In the first-described unnamed species, the tendrils, in shape like a bird's foot, are of no service