Page:Popular Science Monthly Volume 17.djvu/659

Rh If a leaf of a clematis succeed by any means in hooking on to a neighboring object, the special characteristic of leaf-climbing plants comes into play. The stalk of the leaf curls strongly over toward the object touching it, and clasps it firmly. It is obvious how great is the advantage thus gained over a mere hook. A leaf such as that shown



in Fig. 2 might be made to catch on to a neighboring twig by its bent stalk, in such a way that, although it managed to stay where it was, it could bear none of the weight of the plant, and would be liable to be displaced by a strong wind or other disturbance. But, when the stalk of the leaf had curled close round the twig, nothing could displace it, and it could take its share in the work of supporting the plant.

The extreme sensitiveness of the leaf-stalk to slight and gentle touches gives a curious idea of the alertness of the plant in its search for supporting objects. A leaf may be excited to bend by a loop of string weighing only one-sixteenth of a grain. It is an interesting fact that, in such a hook-like leaf as that of Clematis viticella (Fig. 1), the hooked end of the leaf, which has the best chance of coming into contact with obstacles, is the most sensitive part. This has been made out by hanging small weights on different parts of the leaf, and it is found that the terminal leaflet bends in a few hours after a loop of string weighing less than a grain is hung on it, and which produced no effect in twenty-four hours on the other petioles. One may see proof of the sensitiveness of the leaf-stalks of the wild English clematis, which sometimes catches withered leaves or delicate stalks of the quaking-grass. The same thing is shown by a leaf after having been touched with a little water-color, the delicate crust of dry paint being mistaken for something touching the plant. In such cases, or when the leaf has been merely rubbed with a twig, which is taken away before the leaf seizes