One of my chief reasons
for doubting whether the curvature from a touch is the result of
growth, is the extraordinary rapidity of the movement. I have seen
the extremity of a tendril of Passiflora gracilis, after being
touched, distinctly bent in 25 seconds, and often in 30 seconds; and
so it is with the thicker tendril of Sicyos. It appears hardly
credible that their outer surfaces could have actually grown in
length, which implies a permanent modification of structure, in so
short a time. The growth, moreover, on this view must be
considerable, for if the touch has been at all rough the extremity is
coiled in two or three minutes into a spire of several turns.
When the extreme tip of the tendril of Echinocystis caught hold of a
smooth stick, it coiled itself in a few hours (as described at p.
132) twice or thrice round the stick, apparently by an undulatory
movement. At first I attributed this movement to the growth of the
outside; black marks were therefore made, and the interspaces
measured, but I could not thus detect any increase in length. Hence
it seems probable in this case and in others, that the curvature of
the tendril from a touch depends on the contraction of the cells
along the concave side.
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