Page:Darwin - On the movements and habits of climbing plants.djvu/43

 regularly at an average rate of 3 h. 26 m. The shoots, however, sometimes stand still. It is considered a greenhouse plant; but when kept there, the petioles took several days to clasp a stick: in the hothouse a stick was clasped in 7 h. In the greenhouse a

petiole was not affected by a loop of string, suspended during several days and weighing 2½ grains; in the hothouse one was made to curve by a loop weighing 1.64 (and, on the removal of the string, became straight again), but was not at all affected by another loop weighing .82 of a grain. We have seen that the petioles of some other leaf-climbing plants were affected by one-thirteenth of this latter weight. In this plant, and in no other leaf-climber seen by me, a leaf grown to its full size was capable of clasping a stick; but the movement was so extraordinarily slow that in the greenhouse the act required several weeks; but on each succeeding week it was clear that the petiole became more and more curved, until finally it firmly clasped the stick.

When the flexible petiole of a half- or a quarter-grown leaf has clasped any object, in three or four days it increases much in thickness, and after several weeks becomes wonderfully hard and rigid; so that I could hardly remove one from its support. On comparing a thin transverse slice of this petiole with one from the next or older leaf beneath, which had not clasped anything, its diameter was found to be fully doubled, and its structure greatly changed. In two other petioles similarly compared, and