108 ME. DARWIN ON CLIMBTNa PLANTS. 



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wonderfully little expenditure of organized matter, in comparison 

 witli 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 : — Krstly, hook-climbers, which are, at least in our 

 temperate countries, the least eflScient 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 mucli in 

 the shade ; they cannot pass from branch to branch, and thus cover 

 the wbole summit of a tree, for their rootlets can adliere 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 re- 

 volving. 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 ex- 

 ception, revolve in exactly the same manner as twiners ; and some 

 few can still twine well, and many others in a more or less imper- 

 fect 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 inter- 

 nodes, 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 



