IV Movement and Nervous Action in Plants 65 



twining" plants they show a revolution of young shoots, but 

 with a marked tendency to change the direction of circuit ; 

 they approach the tendril-bearers in having petioles or leaf- 

 tips sensitive to contact and able to clasp their support. 

 This sensitiveness is often exquisitely fine, indeed it seems 

 more delicate than the tactile sense of most animals. 

 Thus Darwin observed a petiole responding to the exces- 

 sively slight but continued pressure of a loop of soft thread 

 weighing only -^^ of a grain. The response is a bend- 

 ing towards the side which is touched, and sometimes 

 begins a few minutes after contact. After clasping has 

 been effected the leaf- stalks become stronger and more 

 woody, often acquiring a stem-like internal structure. It 

 is interesting to observe that while most species of Clematis 

 and Tropaeolum are effective leaf-climbers, there are some 

 species of more sluggish constitution, in which both the 

 mobility and the sensitiveness of the petiole are enfeebled, 

 or even lost altogether. 



Both mobility and sensitiveness reach their climax in 

 the tendril-bearers. In the common pea the tendrils 

 revolve in ellipses, taking about an hour and a half to 

 complete their orbit. "Whilst young and about an inch 

 in length, with the leaflets on the petiole only partially 

 expanded, they are highly sensitive ; a single light touch 

 with a twig on the inferior or concave surface near the tip 

 caused them to bend quickly, as did occasionally a loop 

 of thread weighing i of a grain." The long and thick 

 tendrils of the vine — modified flower-stalks in structure — 

 are much less active, but move from side to side, or in 

 narrow elliptical revolutions. In both the pea and the 

 vine, and in most tendril -bearers, the tendrils contract 

 spirally a day or two after they have clasped some object. 

 In this way they become apparently shorter and obviously 

 more elastic, not only drawing the shoot nearer its support, 



