The advantage gained by climbing is to reach the light and free air
with as little expenditure of organic matter as possible; now, with
twining plants, the stem is much longer than is absolutely necessary;
for instance, I measured the stem of a kidney-bean, which had
ascended exactly two feet in height, and it was three feet in length:
the stem of a pea, on the other hand, which had ascended to the same
height by the aid of its tendrils, was but little longer than the
height reached. That this saving of the stem is really an advantage
to climbing plants, I infer from the species that still twine but are
aided by clasping petioles or tendrils, generally making more open
spires than those made by simple twiners. Moreover, the plants thus
aided, after taking one or two turns in one direction, generally
ascend for a space straight, and then reverse the direction of their
spire. By this means they ascend to a considerably greater height,
with the same length of stem, than would otherwise have been
possible; and they do this with safety, as they secure themselves at
intervals by their clasping petioles or tendrils.
We have seen that tendrils consist of various organs in a modified
state, namely, leaves, flower-peduncles, branches, and perhaps
stipules.
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