That this power is innate, and is not
excited by any external agencies, beyond those necessary for growth and
vigour, is obvious. No one doubts that this power has been gained for the
sake of enabling climbing plants to ascend to a height, and thus to reach
the light. This is effected by two very different methods; first, by
twining spirally round a support, but to do so their stems must be long and
flexible; and, secondly, in the case of leaf-climbers and tendril-bearers,
by bringing these organs into contact with a support, which is then seized
by the aid of their sensitiveness. It may be here remarked that these
latter movements have no relation, as far as we can judge, with
circumnutation. In other cases the tips of tendrils, after having been
brought into contact with a support, become developed into little discs
which adhere firmly to it.
We have said that the circumnutation of climbing plants differs from that
of ordinary plants chiefly by its greater amplitude. But most leaves
circumnutate
[page 266]
in an almost vertical plane, and therefore describe very narrow ellipses,
whereas the many kinds of tendrils which consist of metamorphosed leaves,
make much broader ellipses or nearly circular figures; and thus they have a
far better chance of catching hold of a support on any side.
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