Page:Darwin - On the movements and habits of climbing plants.djvu/102

 to prolonged contact. Hence we see that the sensitiveness of tendrils is a special and localized capacity, quite independent of the power of spontaneously moving; for the curling of the terminal portion from a touch does not in the least interrupt the spontaneous revolving movement of the lower part. In Bignonia unguis and its close allies the petioles of the leaves, as well as the tendrils, are sensitive to a touch.

Twining plants when they come into contact with a stick, curl round it invariably in the direction of their revolving movement; but tendrils curl indifferently to either side, in accordance with the position of the stick and the side which is first touched. The clasping-movement of the extremity apparently is not steady, but vermicular in its nature, as may be inferred from the manner in which the tendrils of the Echinocystis slowly crawled round a smooth stick.

As with a few exceptions tendrils spontaneously revolve, it may be asked, Why are they endowed with sensitiveness?—why, when they come into contact with a stick, do they not, like a twining plant, spirally wind round it? One reason may be that in most cases they are so flexible and thin that, when brought into contact with a stick, they would yield, and their revolving movement would not be arrested; they would thus be dragged onwards and away from the stick. Moreover the sensitive extremities have no revolving power, and could not by this means curl round any object. With twining plants, on the other hand, the extremity of the shoot spontaneously bends more than any other part; and this is of high importance to the ascending power of the plant, as may be seen on a windy day. It is, however, possible that the slow movement of the basal and stiffer parts of certain tendrils, which wind round sticks placed in their course, may be analogous to that of twining plants. I doubt this; but I hardly attended sufficiently to this point, and it would be difficult to distinguish between a movement due to extremely dull sensitiveness and that resulting from the arrestment of the lower part together with the continued movement of the terminal part of a tendril.

Tendrils which are only three-fourths grown, and perhaps even when younger, but not whilst extremely young, have the power of revolving and of grasping any object which they may touch. These two capacities generally commence at about the same period, and fail when the tendril is full grown. But in the Cobæa and Passiflora punctata the tendrils began revolving in a quite useless manner, before they became sensitive. In