Chap. 1L LEAF-CLIMBEKS. 81 



chief use of the coiling, at least whilst the plant is 

 young, is to support the pitcher with its load of 

 secreted fluid. 



Summary on Leaf-elimhers. — ^Plants belonging to 

 eight families are known to have clasping petioles, and 

 plants belonging to four families climb by the tips of 

 their leaves. In all the species observed by me, 

 with one exception, the young intemodes revolve more 

 or less regularly, in some cases as regularly as those 

 of a twining plant. They revolve at various rates, 

 in most cases rather rapidly. Some few can ascend 

 by spirally twining round a support. Differently from 

 most twiners, there is a strong tendency in the same 

 shoot to revolve first in one and then in an opposite 

 direction. The object gained by the revolving move- 

 ment is to bring the petioles or the tips of the leaves 

 into contact with surrounding objects ; and without this 

 aid the plant would be much less successful in climb- 

 ing. With rare exceptions, the petioles are sensitive 

 only whilst young. They are sensitive on all sides, 

 but in different degrees in different plants; and in 

 some species of Clematis the several parts of the same 

 petiole differ much in sensitiveness. The hooked 

 tips of the leaves of the Gloriosa are sensitive only on 

 their inner or inferior surfaces. The petioles are sen- 

 sitive to a touch and to excessively slight continued 

 pressure, even from a loop of soft thread weighing 

 only the one-sixteenth of a grain (4-05 mg.); and 

 there is reason to believe that the rather thick and 



