Page:Darwin - On the movements and habits of climbing plants.djvu/109

 wonderfully little expenditure of organized matter, in comparison with trees, which have to support a load of heavy branches by a massive trunk. Hence, no doubt, it arises that there are in all quarters of the world so many climbing plants belonging to so many different orders. These plants have been here classed under three heads:—Firstly, hook-climbers, which are, at least in our temperate countries, the least efficient of all, and can climb only in the midst of an entangled mass of vegetation. Secondly, root-climbers, which are excellently adapted to ascend naked faces of rock: when they climb trees, they are compelled to keep much in the shade; they cannot pass from branch to branch, and thus cover the whole summit of a tree, for their rootlets can adhere only by long-continued and close contact with a steady surface. Thirdly, the great class of spiral-twiners, with the subordinate divisions of leaf-climbers and tendril-bearers, which together far exceed in number and in perfection of mechanism the climbers of the two previous classes. These plants, by their power of spontaneously revolving and of grasping objects with which they come in contact, can easily pass from branch to branch, and securely ramble over a wide and sun-lit surface.

I have ranked twiners, leaf- and tendril-climbers as subdivisions of one class, because they graduate into each other, and because nearly all have the same remarkable power of spontaneously revolving. Does this gradation, it may be asked, indicate that plants belonging to one subdivision have passed, during the lapse of ages, or can pass, from one state to the other; has, for instance, a tendril-bearing plant assumed its present structure without having previously existed as either a leaf-climber or a twiner? If we consider leaf-climbers alone, the idea that they were primordially twiners is forcibly suggested. The internodes of all, without exception, revolve in exactly the same manner as twiners; and some few can still twine well, and many others in a more or less imperfect manner. Several leaf-climbing genera are closely allied to other genera which are simple twiners. It should be observed, that the possession by a plant of leaves with their petioles or tips sensitive, and with the consequent power of clasping any object, would be of very little use, unless associated with revolving internodes, by which the leaves could be brought into contact with surrounding objects. On the other hand, revolving internodes, without other aid, suffice to give the power of climbing; so that, unless we suppose that leaf-climbers simultaneously acquired both capacities, it seems probable that they were at first twiners, and