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. In
two other petioles similarly compared, and here represented, the
increase in diameter was not quite so great. In the section of the
petiole in its ordinary state (A), we see a semilunar band of
cellular tissue (not well shown in the woodcut) differing slightly in
appearance from that outside it, and including three closely
approximate groups of dark vessels. Near the upper surface of the
petiole, beneath two exterior ridges, there are two other small
circular groups of vessels. In the section of the petiole (B) which
had clasped during several weeks a stick, the two exterior ridges
have become much less prominent, and the two groups of woody vessels
beneath them much increased in diameter. The semilunar band has been
converted into a complete ring of very hard, white, woody tissue,
with lines radiating from the centre. The three groups of vessels,
which, though near together, were before distinct, are now completely
blended. The upper part of this ring of woody vessels, formed by the
prolongation of the horns of the original semilunar band, is narrower
than the lower part, and slightly less compact. This petiole after
clasping the stick had actually become thicker than the stem from
which it arose; and this was chiefly due to the increased thickness