5 grains (163 mg.); but in the hothouse
one was made to curve by a loop weighing 1.64 gr. (106.27 mg.); and,
on the removal of the string, it became straight again. Another
petiole was not at all acted on by a loop weighing only 0.82 of a
grain (53.14 mg.) We have seen that the petioles of some other leaf-
climbing plants are affected by one-thirteenth of this latter weight.
In this species, and in no other leaf-climber seen by me, a full-
grown leaf is capable of clasping a stick; but in the greenhouse the
movement was so extraordinarily slow that the act required several
weeks; on each succeeding week it was clear that the petiole had
become more and more curved, until at last it firmly clasped the
stick.
The flexible petiole of a half or a quarter grown leaf which has
clasped an object for three or four days increases much in thickness,
and after several weeks becomes so wonderfully hard and rigid that it
can hardly be removed from its support. On comparing a thin
transverse slice of such a petiole with one from an older leaf
growing close beneath, which had not clasped anything, its diameter
was found to be fully doubled, and its structure greatly changed.
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