The tropical twiners, on the other hand, can
ascend thicker trees; I hear from Drs. Thomson and Hooker that this
is the case with the Butea parviflora, one of the Menispermaceae, and
with some Dalbergias and other Leguminosae. {19} This power would be
necessary for any species which had to ascend by twining the large
trees of a tropical forest; otherwise they would hardly ever be able
to reach the light. In our temperate countries it would be injurious
to the twining plants which die down every year if they were enabled
to twine round trunks of trees, for they could not grow tall enough
in a single season to reach the summit and gain the light.
By what means certain twining plants are adapted to ascend only thin
stems, whilst others can twine round thicker ones, I do not know. It
appeared to me probable that twining plants with very long revolving
shoots would be able to ascend thick supports; accordingly I placed
Ceropegia Gardnerii near a post 6 inches in diameter, but the shoots
entirely failed to wind round it; their great length and power of
movement merely aid them in finding a distant stem round which to
twine. The Sphaerostemma marmoratum is a vigorous tropical twiner;
and as it is a very slow revolver, I thought that this latter
circumstance might help it in ascending a thick support; but though
it was able to wind round a 6-inch post, it could do this only on the
same level or plane, and did not form a spire and thus ascend.
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