much twisted. The influence of the roughness of the support in
causing axial twisting was well seen in the stems which had twined up
the glass rods; for these rods were fixed into split sticks below,
and were secured above to cross sticks, and the stems in passing
these places became much twisted. As soon as the stems which had
ascended the iron rods reached the summit and became free, they also
became twisted; and this apparently occurred more quickly during
windy than during calm weather. Several other facts could be given,
showing that the axial twisting stands in some relation to
inequalities in the support, and likewise to the shoot revolving
freely without any support. Many plants, which are not twiners,
become in some degree twisted round their own axes; {7} but this
occurs so much more generally and strongly with twining-plants than
with other plants, that there must be some connexion between the
capacity for twining and axial twisting. The stem probably gains
rigidity by being twisted (on the same principle that a much twisted
rope is stiffer than a slackly twisted one), and is thus indirectly
benefited so as to be enabled to pass over inequalities in its spiral
ascent, and to carry its own weight when allowed to revolve freely.
{8}

I have alluded to the twisting which necessarily follows on
mechanical principles from the spiral ascent of a stem, namely, one
twist for each spire completed. This was well shown by painting
straight lines on living stems, and then allowing them to twine; but,
as I shall have to recur to this subject under Tendrils, it may be
here passed over.

The revolving movement of a twining plant has been compared with that
of the tip of a sapling, moved round and round by the hand held some