In Lonicera brachypoda the hook only
straightens itself periodically, and never becomes reversed. I will
not assert that the tips of all twining plants when hooked, either
reverse themselves or become periodically straight, in the manner
just described; for the hooked form may in some cases be permanent,
and be due to the manner of growth of the species, as with the tips
of the shoots of the common vine, and more plainly with those of
Cissus discolor--plants which are not spiral twiners.
The first purpose of the spontaneous revolving movement, or, more
strictly speaking, of the continuous bowing movement directed
successively to all points of the compass, is, as Mohl has remarked,
to favour the shoot finding a support. This is admirably effected by
the revolutions carried on night and day, a wider and wider circle
being swept as the shoot increases in length. This movement likewise
explains how the plants twine; for when a revolving shoot meets with
a support, its motion is necessarily arrested at the point of
contact, but the free projecting part goes on revolving. As this
continues, higher and higher points are brought into contact with the
support and are arrested; and so onwards to the extremity; and thus
the shoot winds round its support.
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