50 IjEAF-CLIMBEES. Chap. It 



Nevertheless, tlj^e main service of tlie revolving move- 

 ment is to bring them into contact with surrounding 

 objects, which are slowly but securely seized. The 

 young petioles, which alone are sensitive, have their 

 ends bowed a little downwards, so as to be in a slight 

 degree hooked ; ultimately the whole leaf, if it catches 

 nothing, becomes level. I gently rubbed with a thin 

 twig the lower surfaces of two young petioles ; and in 

 2 hrs. 30 m. they were slightly curved downwards ; in 

 5 hrs., after being rubbed, the end of one was bent 

 completely back, parallel to the basal portion ; in 4hrs. 

 subsequently it became nearly straight again. To 

 show how sensitive the young petioles are, I may 

 mention that I just touched the under sides of two 

 with a little water-colour, which when dry formed 

 an excessively thin and minute crust; but this 

 suflSced in 24 hrs. to cause both to bend downwards. 

 Whilst the plant is young, each leaf consists of three 

 divided leaflets, which barely have distinct petioles, 

 and these are not sensitive ; but when the plant is 

 well grown, the petioles of the two lateral and terminal 

 leaflets are of considerable length, and become sensi- 

 tive so as to be capable of clasping an object in any 

 direction. 



When a petiole has clasped a twig, it undergoes 

 some remarkable changes, which may be observed 

 with the other species, but in a less strongly marked 

 manner, and will here be described once for all. The 

 clasped petiole in the course of two or three days 

 swells greatly, and ultimately becomes nearly twice as 



