CLIMBING PLANTS. 



641 



If a leaf of a clematis succeed by any means in hooking on to 

 a neighboring object, the special characteristic of leaf-climbing plants 

 comes into play. The stalk of the leaf curls strongly over toward the 

 object touching it, and clasps it firmly. It is obvious how great is the 

 advantage thus gained over a mere hook. A leaf such as that shown 



Fig. 1.* A Young Leaf of Clematis 

 viticella. 



Fig. 2. Clematis glandtjlosa, With 

 two youn^r leaves clasping two twigs, 

 with the clasping portions thickened. 



in Fig. 2 might be made to catch on to a neighboring twig by its bent 

 stalk, in such a way that, although it managed to stay where it was, it 

 could bear none of the weight of the plant, and would be liable to be 

 displaced by a strong wind or other disturbance. But, when the stalk 

 of the leaf had curled close round the twig, nothing could displace it, 

 and it could take its share in the work of supporting the plant. 



The extreme sensitiveness of the leaf-stalk to slight and gentle 

 touches gives a curious idea of the alertness of the plant in its search for 

 supporting objects. A leaf may be excited to bend by a loop of string 

 weighing only one-sixteenth of a grain. It is an interesting fact that, 

 in such a hook-like leaf as that of Clematis viticella (Fig. 1), the hooked 

 end of the leaf, which has the best chance of coming into contact with 

 obstacles, is the most sensitive part. This has been made out by hang- 

 ing small weights on different parts of the leaf, and it is found that the 

 terminal leaflet bends in a few hours after a loop of string weighing 

 less than a grain is hung on it, and which produced no effect in twenty- 

 four hours on the other petioles. One may see proof of the sensitive- 

 ness of the leaf-stalks of the wild English clematis, which sometimes 

 catches withered leaves or delicate stalks of the quaking-grass. The 

 same thing is shown by a leaf after having been touched with a little 

 water-color, the delicate crust of dry paint being mistaken for some- 

 thing touching the plant. In such cases, or when the leaf has been 

 merely rubbed with a twig, which is taken away before the leaf seizes 



* For the loan of this and the other woodcuts illustrating this article, we are indebted 

 to the kindness of Mr. Charles Darwin and Mr. Murray. 

 vol. xvii. 41 



