They revolve at various rates, in most cases rather rapidly.
Some few can ascend by spirally twining round a support. Differently
from most twiners, there is a strong tendency in the same shoot to
revolve first in one and then in an opposite direction. The object
gained by the revolving movement is to bring the petioles or the tips
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.
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