LEAF-CHMBEUS. 



47 



the leaf by its subsequent growth became quite slack, though the 

 terminal stalk remained coiled round the stick ; hence it would 

 appear that the 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 -climbers, — Plants belonging to eight families 

 are known to have clasping petioles, and plants belonging to four 

 families climb by the tips of their leaves. With all the plants 

 observed by me, the young internodes revolved more or less 

 regularly, in some cases as regularly as does any twining plant, 

 and at various rates, but generally rather rapidly. Some few can 

 ascend by twining spirally round a support. Differently from 

 most twiners, there is a strong tendency in the same shoot to 

 revolve first in one and then in. the opposite direction. The ob- 

 ject gained by the revolving movement, as could be plainly seen, 

 was to bring the petioles or the tips of the leaves into contact 

 with surrounding objects ; without this aid there would be a poor 

 chance of success. With rare exceptions, the petioles are sensitive 

 only whilst young ; they are sensitive on all sides, but in differ- 

 ent degrees in different plants, and in some species of Clematis in 

 very different degrees in different parts of the same petiole. The 

 hooked tips of the leaves of the Gloriosa are sensitive only on their 

 inner or inferior surface. The petioles are sensitive to a touch and 

 to excessively slight continued pressure, even from a loop of soft 

 thread weighing only the one-sixteenth of a grain ; and there is 

 reason to believe that the rather thick and stiff petioles of Clematis 

 fiammula are sensitive to even a less weight when spread over a 

 wider surface. The petioles always bend towards the touched 

 or pressed side, at different rates in different plants, sometimes 

 within a few minutes, but generally after a much longer period. 

 After temporary contact with any object, the petiole continues to 

 bend for a considerable time ; afterwards it slowly becomes straight 

 again, and can then re-act. A petiole excited by an extremely 

 slight weight sometimes bends a little, and then becomes habi- 

 tuated to the stimulus, and either bends no more or becomes 

 straight again, the weight still remaining suspended. Petioles 

 which have elapsed any object for some little time cannot recover 

 their original position. After remaining clasped for two or three 

 days, they generally increase much in thickness, either through- 

 out or on one side alone ; they subsequently become, sometimes 

 in a wonderful degree, stronger and more woody; and in some 



cases they acquire an internal structure like that of the stem or 

 axis. 



