Page:Darwin - On the movements and habits of climbing plants.djvu/116

 struck every one who has noticed them with surprise, for they are quite unlike those of all common papilionaceous plants, and resemble those of a grass. In L. aphaca the tendril, which is not highly developed (for it is unbranched, and has no spontaneous revolving-power), replaces the leaves, the latter in function being replaced by the large stipules. Now if we suppose the tendrils of L. aphaca to become flattened and foliaceous, like the little-rudimentary tendrils of the Bean, and the large stipules, not being any longer wanted, to become at the same time reduced in size, we should have the exact counterpart of L. nissolia, and its curious leaves are at once rendered intelligible to us.

It may be added, as it will serve to sum up the foregoing views on the origin of tendril-bearing plants, that if these views be correct, L. nissolia must be descended from a primordial spirally-twining plant; that this became a leaf-climber; that first part of the leaf and then the whole leaf became converted into a tendril, with the stipules by compensation greatly increased in size ; that this tendril lost its branches and became simple, then lost its revolving-power (in which state it would resemble the tendril of the existing L. aphaca), and afterwards losing its prehensile power and becoming foliaceous would no longer be called a tendril. In this last stage (that of the existing L. nissolia) the former tendril would reassume its original function of a leaf, and its lately largely developed stipules, being no longer wanted, would decrease in size. If it be true that species become modified in the course of ages, we may conclude that L. nissolia is the result of a long series of changes, in some degree like those just traced.

The most interesting point in the natural history of climbing plants is their diverse powers of movement; and this led me on to their study. The most different organs—the stem, flower-peduncle, petiole, mid-ribs of the leaf or leaflets, and apparently aërial roots—all possess this power.

In the first place, the tendrils place themselves in the proper position for action, standing, for instance in the Cobæa, vertically upwards, with their branches divergent and their hooks turned outwards, and with the young terminal shoot thrown on one side; or, as in Clematis, the young leaves temporarily curve themselves downwards, so as to serve as grapnels.