Page:Darwin - On the movements and habits of climbing plants.djvu/113

 species of the Antirrhineæ, of Solanum, of Cocculus, of Gloriosa are related to other genera in the same family, or even to other species in the same genus, which are true twiners. On the other hand, the leaf-climbing species of Clematis are very closely allied to the tendril-bearing Naravelia: the Fumariaceæ include closely allied genera which are leaf-climbers and tendril-hearers. Lastly, one species of Bignonia is both a leaf-climber and a tendril-bearer, and other closely allied species are twiners.

Tendrils of the second great division consist of modified flower-peduncles. In this case likewise we have many interesting transitional states. The common Vine (not to mention the Cardiospermum) gives us every possible grade from finely developed tendrils to a bunch of flower-buds, bearing the single usual lateral flower-tendril. And when the latter itself bears some flowers, as we know is not rarely the case, and yet retains the power of clasping a support, we see the primordial state of all those tendrils which have been formed by the modification of flower-peduncles.

According to Mohl and others, some tendrils consist of modified branches: I have seen no such case, and therefore of course know nothing of any transitional states, if such occur. But Lophospermum at least shows us that such a transition is possible; for its branches spontaneously revolve, and are sensitive to contact. Hence, if the leaves of some of the branches were to abort, they would be converted into true tendrils. Nor is it so improbable as it may at first appear that certain branches alone should become modified, the others remaining unaltered; for we have seen with certain varieties of Phaseolus that some of the branches are thin and flexible and twine, whilst other branches on the same plant are stiff and have no such power.

If we inquire how the petiole of a leaf, or the peduncle of a flower, or a branch first becomes sensitive and acquires the power of bending towards the touched side, we get no certain answer. Nevertheless an observation by Hofmeister well deserves attention, namely, that the shoots and leaves of all plants, whilst young, move after being shaken; and it is almost invariably young petioles and young tendrils, whether formed of modified leaves or flower-peduncles, which move on being touched; so that it would appear as if these plants had utilized and perfected a widely distributed and incipient capacity, which capacity, as far as we can see, is of no service to ordinary plants. If we