When the shoot follows the sun in
its revolving course, it winds round the support from right to left,
the support being supposed to stand in front of the beholder; when
the shoot revolves in an opposite direction, the line of winding is
reversed. As each internode loses from age its power of revolving,
it likewise loses its power of spirally twining. If a man swings a
rope round his head, and the end hits a stick, it will coil round the
stick according to the direction of the swinging movement; so it is
with a twining plant, a line of growth travelling round the free part
of the shoot causing it to bend towards the opposite side, and this
replaces the momentum of the free end of the rope.
All the authors, except Palm and Mohl, who have discussed the spiral
twining of plants, maintain that such plants have a natural tendency
to grow spirally. Mohl believes (p. 112) that twining stems have a
dull kind of irritability, so that they bend towards any object which
they touch; but this is denied by Palm. Even before reading Mohl's
interesting treatise, this view seemed to me so probable that I
tested it in every way that I could, but always with a negative
result.
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