HOW PLANTS CLIMB. 203 



We pass now to our second Class. This consists of plants 

 climbing by means of sensitive or irritable, organs, which, touching 

 any object, clasp it in some way or other. These are — 



II. — Climbers. We divide this class into two groups for 

 convenience' sake : — 



A. — Leaf-Climbers. 



B. — Tendril-Bearers. 



In some cases these shade the one into the other. 



A. — Leaf-Climbers. 



These are intermediate in some respects between twiners and 

 tendril-bearers. They climb in two ways ; some by means of 

 \}ci€\x petioles^ and others by their produced mid-ribs^ or tips. 



In nearly all the species examined by Darwin, the young 

 internodes showed a revolving power, in some cases quite as 

 regular as in a twining-plant ; in most cases the revolutions were 

 rapid. 



The purpose of the revolving in these leaf-climbers is not to 

 climb around a support, but to enable the leaf-stalks, or the leaf- 

 tips, as the case may be, to get near to some object which they 

 can clasp. Of course this power of revolution greatly assists the 

 plants in making use of their sensitive organs. 



As in true twiners, the first internodes do not revolve, nor 

 do the petioles or tips of the earliest formed leaves appear to be 

 sensitive. 



There are some eight orders in which we find leaves with clasp- 

 ing petioles. Prominent among these are sundry species of 

 Clematis.^ Tropceolum^ Solannni, and Fwnaria. In many of these 

 there is a tendency to revolve in opposite directions, thus differing 

 from true twiners. They are, of course, inferior twiners. 



The petioles are enabled to clasp any object in virtue of an 

 extreme sensitiveness to toitcJi. On being touched, or rubbed, they 

 bend towards the irritating object, or towards the point of irrita- 

 tion. If they find a twig or stalk of any kind, they grasp it, 

 sometimes taking two or three turns round it. If they find 

 nothing to hold by, they gradually uncoil and straighten them- 

 selves again ; in this position they remain permanently. 



In some species, the young leaves spontaneously shift their 

 position — /.£?., without any external stimulus ; their petioles 



