But this will not account for the whole
movement, for the segments actually bend or curve to the dark side
besides turning round on their axes so that their upper surfaces may
face the light.
When the Cobaea grows in the open air, the wind must aid the
extremely flexible tendrils in seizing a support, for I found that a
mere breath sufficed to cause the extreme branches to catch hold by
their hooks of twigs, which they could not have reached by the
revolving movement. It might have been thought that a tendril, thus
hooked by the extremity of a single branch, could not have fairly
grasped its support. But several times I watched cases like the
following: tendril caught a thin stick by the hooks of one of its
two extreme branches; though thus held by the tip, it still tried to
revolve, bowing itself to all sides, and by this movement the other
extreme branch soon caught the stick. The first branch then loosed
itself, and, arranging its hooks, again caught hold. After a time,
from the continued movement of the tendril, the hooks of a third
branch caught hold. No other branches, as the tendril then stood,
could possibly have touched the stick. But before long the upper
part of the main stem began to contract into an open spire.
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