But twiners have one disadvantage: to rise ten feet they must produce fifteen feet of stem or thereabouts, according to the diameter of the support, and the openness or closeness of the coil. A rootlet-climber saves much in this respect, but has a restricted range of action, and other disadvantages. There are two other modes, which combine the utmost economy of material with freer range of action. There are, in the first place, leaf-climbers of various sorts, agreeing only in this, that the duty of laying hold is transferred to the leaves, so that the stem may rise in a direct line. Sometimes the blade or leaflets, or some of them, but more commonly their slender stalks, undertake the work, and the plant rises as a boy ascends a tree, grasping first with one hand or arm, then with the other. Indeed, the comparison, like the leaf-stalk, holds better than would be supposed; for the grasping of the latter is not the result of a blind groping in all directions by a continuous movement, but of a definite sensitiveness which acts only upon the occasion. Most leaves make no regular sweeps; but when the stalks of a leaf-climbing species come into prolonged contact with any fitting extraneous body, they slowly incurve and make a turn around it, and then commonly thicken and harden until they attain a strength which may equal that of the stem itself. Here we have the faculty of movement to a definite end, upon external irritation, of the same nature with that displayed by Dionaea and Drosera, although slower for the most part than even in the latter. But the movement of the hour-hand of the clock is not different in nature or cause from that of the second-hand.