70 THE STORY OF PLANT LIFE 



**The latter are about as thick as the hair of a man's 

 beard, but they were completely surrounded and 

 clasped. The petioles of a leaf, so young that none 

 of the leaflets were expanded, had partially seized a 

 twig. Those of almost all the old leaves, even when 

 unattached to any object, are much convoluted, but 

 this is owing to their having come, whilst young, 

 into contact during several hours with some object 

 subsequently removed. With none of the above- 

 described species, cultivated forms of Clematis — there 

 are some 170 species, cosmopolitan in range — culti- 

 vated in pots and carefully observed, was there any 

 permanent bending of the petioles without the 

 stimulus of contact. In winter the blades of the leaves 

 of C. vitalba drop off, but the petioles (as was observed 

 by Mohl) remain attached to the branches, some- 

 times during two seasons ; and, being convoluted, 

 they curiously resemble true tendrils, such as those 

 possessed by the allied genus Naravelia. The 

 petioles which have clasped some object become 

 much more stiff, hard and polished than those which 

 have failed in this, their proper function." 



This plant is one of the leaf-stalk climbers. The 

 leaf-stalk bends once round the support, and then 

 thickens and becomes woody. 



Darwin divided climbing-plants into four groups : 



(i) Twining plants, \wh\ch climb by twining round 

 their supports, as in the case of Black Bryony, the 

 Hop, etc. 



(2) Climbers with sensitive organs, tendrils which 



