When an unattached tendril contracts spirally, the spire always runs
in the same direction from tip to base. A tendril, on the other
hand, which has caught a support by its extremity, although the same
side is concave from end to end, invariably becomes twisted in one
part in one direction, and in another part in the opposite direction;
the oppositely turned spires being separated by a short straight
portion. This curious and symmetrical structure has been noticed by
several botanists, but has not been sufficiently explained. {35} It
occurs without exception with all tendrils which after catching an
object contract spirally, but is of course most conspicuous in the
longer tendrils. It never occurs with uncaught tendrils; and when
this appears to have occurred, it will be found that the tendril had
originally seized some object and had afterwards been torn free.
Commonly, all the spires at one end of an attached tendril run in one
direction, and all those at the other end in the opposite direction,
with a single short straight portion in the middle; but I have seen a
tendril with the spires alternately turning five times in opposite
directions, with straight pieces between them; and M.
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