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. 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