Page:Darwin - On the movements and habits of climbing plants.djvu/100

 than the internodes. In all cases the conditions of life must be favourable for the perfect action of the tendrils. Generally both internodes and tendrils revolve together; in other cases, as in Cissus, Cobæa, and most Passifloræ, the tendrils alone revolve; in other cases, as with Lathyrus aphaca, the internodes alone move, carrying with them the motionless tendrils; and, lastly (and this is the fourth possible case), neither internodes nor tendrils spontaneously revolve, as with Lathyrus grandiflorus and the Ampelopsis. In most Bignonias, iu the Eccremocarpus, Mutisia, and the Fumariaceæ, the petioles as well as the tendrils, together with the internodes, all spontaneously move together.

The tendrils revolve by the curvature of their whole length, excepting the extremity and excepting the base, which parts do not move, or move but little. The movement is of the same nature as that of the revolving internodes. Hence, if a line be painted along that surface which at the time happens to be convex, the line becomes first lateral and then concave, and ultimately again convex. This experiment can be tried only on the thicker tendrils, which are not affected by a thin crust of dried paint. The extremities, however, of the tendrils, which so often are slightly curved or hooked, never reverse their curvature; and in this respect they differ from the extremities of the shoots of twining plants, which not only reverse their curvature, or at least become periodically straight, but curve in a greater degree than the lower portions. But, in fact, the tendril answers to the upper internode of the several revolving internodes of a twining plant; and in the former part of this paper it was explained how the several internodes move together by the whole successively curving to all points of the compass. There is, however, in many cases this unimportant difference, that the curving tendril is separated from the curving internode by a rigid petiole. There is also another difference, namely, that the summit of the shoot, which in itself has no power of revolving, projects above the point from which the tendril arises; but the summit of the shoot is generally thrown on one side, so as to be out of the way of the revolutions swept by the tendril. In those plants in which the terminal shoot is not sufficiently out of the way, the tendril, as we have seen with the Echinocystis, as soon as it comes in its revolving course to this point, stiffens and straightens itself, and, rising up vertically, passes over the obstacle.

All tendrils are sensitive, but in very various degrees, to contact with any object, and curve towards the touched side. With