The leaves of different twining-plants are arranged on the stem
(before it has twined) alternately, or oppositely, or in a spire. In
the latter case the line of insertion of the leaves and the course of
the revolutions coincide. This fact has been well shown by
Dutrochet, {14} who found different individuals of Solanum dulcamara
twining in opposite directions, and these had their leaves in each
case spirally arranged in the same direction. A dense whorl of many
leaves would apparently be incommodious for a twining plant, and some
authors assert that none have their leaves thus arranged; but a
twining Siphomeris has whorls of three leaves.
If a stick which has arrested a revolving shoot, but has not as yet
been encircled, be suddenly taken away, the shoot generally springs
forward, showing that it was pressing with some force against the
stick. After a shoot has wound round a stick, if this be withdrawn,
it retains for a time its spiral form; it then straightens itself,
and again commences to revolve. The long, much-inclined shoot of the
Ceropegia previously alluded to offered some curious peculiarities.
The lower and older internodes, which continued to revolve, were
incapable, on repeated trials, of twining round a thin stick; showing
that, although the power of movement was retained, this was not
sufficient to enable the plant to twine.
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