The tendril, when in full action, stands vertically
up, with the projecting extremity of the young stem thrown a little
on one side, so as to be out of the way; but the tendril bears on the
inner side, near its base, a short rigid branch, which projects out
at right angles like a spur, with the terminal half bowed a little
downwards. Hence, as the main vertical branch revolves, the spur,
from its position and rigidity, cannot pass over the extremity of the
shoot, in the same curious manner as do the three branches of the
tendril of the Echinocystis, namely, by stiffening themselves at the
proper point. The spur is therefore pressed laterally against the
young stem in one part of the revolving course, and thus the sweep of
the lower part of the main branch is much restricted. A nice case of
co-adaptation here comes into play: in all the other tendrils
observed by me, the several branches become sensitive at the same
period: had this been the case with the Hanburya, the inwardly
directed, spur-like branch, from being pressed, during the revolving
movement, against the projecting end of the shoot, would infallibly
have seized it in a useless or injurious manner.
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