Page:Darwin - On the movements and habits of climbing plants.djvu/48

 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.

.—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 object 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 different 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 flammula 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 habituated 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 throughout 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.