Page:Darwin - On the movements and habits of climbing plants.djvu/68

 about two hours, and was ready to act again. As soon as the tendrils begin to grow old their extremities become hooked, and they then appear, with their two or three pairs of branches, an admirable grappling instrument; but this is not really the case, for at this period the tips have generally quite lost their sensitiveness; when hooked on to twigs some were not at all affected, and others required from 18 h. to 24 h. to clasp the twigs. Ultimately the lateral branches of the tendril, but not the middle or main stem, contract spirally.

Lathyrus aphaca.—As the tendril here replaces the whole leaf (except occasionally in very young plants), the leaf itself being replaced in function by the large stipules, it might have been expected that the tendrils would have been highly organized; this, however, is not so. They are moderately long, thin, and unbranched, with their tips slightly curved: they are sensitive whilst young on all sides, but chiefly on the concave side of the extremity. They have no spontaneous revolving power, but are at first inclined upwards at an angle of about 45°, then move into a horizontal position, and ultimately bend downwards. The young internodes, on the other hand, revolve in ellipses, and carry with them the tendrils: two ellipses were completed, each in nearly 5 h.; the longer axes of these two, and of some subsequently formed ellipses, were directed at about an angle of 45° from the line of the axis of the previous ellipse.

Lathyrus grandiflorus.—The plants observed were young, and not growing vigorously, yet sufficiently so, I think, for my observations to be trusted. Here we have the rare case of neither internodes nor tendrils having any spontaneous revolving power. The tendrils in vigorous plants are above 4 inches in length, and are often twice divided into three branches; the tips are curved and are sensitive on the concave side; the lower part of the central stem is hardly at all sensitive. Hence this plant climbs simply by its tendrils being brought, through the growth of the stem, or the more efficient aid of the wind, into contact with surrounding objects, which are then effectually clasped. I may add that the tendrils, or the internodes, or both, of Vicia sativa spontaneously revolve.

.—Mutisia clematis.—The enormous family of Compositæ is well known to include very few climbing plants. We have seen in the Table in the first Part that Mikania is a regular twiner, and Mutisia is the only genus, as far as I can learn, which bears tendrils: it is therefore interesting to discover that these tendrils, though rather less metamorphosed from their primordial