Sachs himself admits {39} that "if the
growth which takes place in the entire tendril at the time of contact
with a support is small, a considerable acceleration occurs on the
convex surface, but in general there is no elongation on the concave
surface, or there may even be a contraction; in the case of a tendril
of Cucurbita this contraction amounted to nearly one-third of the
original length." In a subsequent passage Sachs seems to feel some
difficulty in accounting for this kind of contraction. It must not
however be supposed from the foregoing remarks that I entertain any
doubt, after reading De Vries' observations, about the outer and
stretched surfaces of attached tendrils afterwards increasing in
length by growth. Such increase seems to me quite compatible with
the first movement being independent of growth. Why a delicate touch
should cause one side of a tendril to contract we know as little as
why, on the view held by Sachs, it should lead to extraordinarily
rapid growth of the opposite side. The chief or sole reason for the
belief that the curvature of a tendril when touched is due to rapid
growth, seems to be that tendrils lose their sensitiveness and power
of movement after they have grown to their full length; but this fact
is intelligible, if we bear in mind that all the functions of a
tendril are adapted to drag up the terminal growing shoot towards the
light.
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