An
unattached tendril rolls itself up into a flat helix, as in the case
of Cardiospermum, if the contraction commences at the extremity and
is quite regular; but if the continued growth of the outer surface is
a little lateral, or if the process begins near the base, the
terminal portion cannot be rolled up within the basal portion, and
the tendril then forms a more or less open spire. A similar result
follows if the extremity has caught some object, and is thus held
fast.
The tendrils of many kinds of plants, if they catch nothing, contract
after an interval of several days or weeks into a spire; but in these
cases the movement takes place after the tendril has lost its
revolving power and hangs down; it has also then partly or wholly
lost its sensibility; so that this movement can be of no use. The
spiral contraction of unattached tendrils is a much slower process
than that of attached ones. Young tendrils which have caught a
support and are spirally contracted, may constantly be seen on the
same stem with the much older unattached and uncontracted tendrils.
In the Echinocystis I have seen a tendril with the two lateral
branches encircling twigs and contracted into beautiful spires,
whilst the main branch which had caught nothing remained for many
days straight.
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