334 DARWINIANA. 



of tlie compass, through an action which circulates 

 around the stem in the direction of the sweep), and of 

 the consequent twining, i. e., " with the sun," or with 

 the movement of the hands of a watch, in the hop, 

 or in the opposite direction in pole-beans and most 

 twiners. 



Twining plants, therefore, ascend trees or other 

 stems by an action and a movement of their own, from 

 which they derive advantage. To plants liable to be 

 overshadowed by more robust companions, climbing is 

 an economical method of obtaining a freer exposure to 

 light and air with the smallest possible expenditure of 

 material. But twiners have one disadvantage : to rise 

 ten feet they must produce fifteen feet of stem or 

 thereabouts, according to the diameter of the sup- 

 port, and the openness or closeness of the coil. A root- 

 let-climber saves much in this respect, but has a re- 

 stricted range of action, and other disadvantages. 



There are two other modes, which combine the ut- 

 most economy of material with freer range of action. 

 There are, in the first place, leaf -climbers of various 

 sorts, agreeing only in this, that the duty of laying 

 hold is transferred to the leaves, so that the stem may 

 rise in a direct line. Sometimes the blade or leaflets, 

 or some of them, but more commonly their slender 

 stalks, undertake the work, and the plant rises as a boy 

 ascends a tree, grasping first with one hand or arm, 

 then with the other. Indeed, the comparison, like the 

 leaf-stalk, holds better than would be supposed ; for 

 the grasping of the latter is not the result of a blind 

 groping in all directions by a continuous movement, 

 but of a definite sensitiveness which acts only upon the 



