Page:Darwin - On the movements and habits of climbing plants.djvu/114

 further inquire how the stems, petioles, tendrils, and flower-peduncles of climbing plants first acquired their power of spontaneously revolving, or, to speak more accurately, of successively bending to all points of the compass, we are again silenced, or at most can only remark, that the power of movement, both spontaneous and from various stimuli, is far more common with plants, as we shall presently see, than is generally supposed to be the case by those who have not attended to the subject. There is, however, the one remarkable case of the Maurandia semperflorens, in which the young flower-peduncles spontaneously revolve in very small circles, and bend themselves, when gently rubbed, to the touched side; yet this plant certainly profits in no way by these two feebly developed powers. A rigorous examination of other young plants would probably show some slight spontaneous movements in the peduncles and petioles, as well as that sensitiveness to shaking observed by Hofmeister. We see at least in the Maurandia a plant which might, by a little augmentation of qualities which it already possesses, come first to grasp a support by its flower-peduncles (as with Vitis or Cardiospermum) and then, by the abortion of some of its flowers, acquire perfect tendrils.

There is one interesting point which deserves notice. We have seen that some tendrils have originated from modified leaves, and others from modified flower-peduncles; so that some are foliar and others axial in their homological nature. Hence it might have been expected that they would have presented some difference in function. This is not the case. On the contrary, they present the most perfect identity in their several remarkable characteristics. Tendrils of both kinds spontaneously revolve at about the same rate. Both, when touched, bend quickly to the touched side, and afterwards recover themselves and are able to act again. In both the sensitiveness is either confined to one side or extends all round the tendril. They are either attracted or repelled by the light. The latter case is seen in the foliar tendrils of Bignonia capreolata and in the axial tendrils of the Ampelopsis, both of which move from the light. The tips of the tendrils in these two plants become, after contact, enlarged into disks, which are at first adhesive by the secretion of some cement. Tendrils of both kinds, soon after grasping a support, contract spirally; they then increase greatly in thickness and strength. When we add to these several points of identity the fact of the petiole of the Solanum jasminoides assuming the most characteristic feature of the axis, namely, a closed ring of woody vessels, we can hardly