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The belief that twiners have a natural tendency to grow spirally,
probably arose from their assuming a spiral form when wound round a
support, and from the extremity, even whilst remaining free,
sometimes assuming this form. The free internodes of vigorously
growing plants, when they cease to revolve, become straight, and show
no tendency to be spiral; but when a shoot has nearly ceased to grow,
or when the plant is unhealthy, the extremity does occasionally
become spiral. I have seen this in a remarkable manner with the ends
of the shoots of the Stauntonia and of the allied Akebia, which
became wound up into a close spire, just like a tendril; and this was
apt to occur after some small, ill-formed leaves had perished. The
explanation, I believe, is, that in such cases the lower parts of the
terminal internodes very gradually and successively lose their power
of movement, whilst the portions just above move onwards and in their
turn become motionless; and this ends in forming an irregular spire.
When a revolving shoot strikes a stick, it winds round it rather more
slowly than it revolves. For instance, a shoot of the Ceropegia,
revolved in 6 hrs.


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