of the leaves into contact with surrounding objects; and without this
aid the plant would be much less successful in climbing. 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 the several parts of the same petiole
differ much in sensitiveness. The hooked tips of the leaves of the
Gloriosa are sensitive only on their inner or inferior surfaces. 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 (4.05 mg.); and there is reason to believe that
the rather thick and stiff petioles of Clematis flammula are
sensitive to even much less weight if spread over a wide surface.
The petioles always bend towards the side which is pressed or
touched, at different rates in different species, 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 accustomed to the
stimulus, and either bends no more or becomes straight again, the
weight still remaining suspended. Petioles which have clasped an
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 their whole diameter or
on one side alone; they subsequently become stronger and more woody,
sometimes to a wonderful degree; and in some cases they acquire an
internal structure like that of the stem or axis.

The young internodes of the Lophospermum as well as the petioles are
sensitive to a touch, and by their combined movement seize an object.