With twining plants, on the other hand,
the extremity spontaneously bends more than any other part; and this
is of high importance for the ascent of the plant, as may be seen on
a windy day. It is, however, possible that the slow movement of the
basal and stiffer parts of certain tendrils, which wind round sticks
placed in their path, may be analogous to that of twining plants.
But I hardly attended sufficiently to this point, and it would have
been difficult to distinguish between a movement due to extremely
dull irritability, from the arrestment of the lower part, whilst the
upper part continued to move onwards.
Tendrils which are only three-fourths grown, and perhaps even at an
earlier age, but not whilst extremely young, have the power of
revolving and of grasping any object which they touch. These two
capacities are generally acquired at about the same period, and both
fail when the tendril is full grown. But in Cobaea and Passiflora
punctata the tendrils begin to revolve in a useless manner, before
they have become sensitive. In the Echinocystis they retain their
sensitiveness for some time after they have ceased to revolve and
after they have sunk downwards; in this position, even if they were
able to seize an object, such power would be of no service in
supporting the stem.
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