Whether the advantage thus gained by twining plants accounts for
their summits being so frequently hooked, we do not know, as this structure
is not very rare with plants which do not climb, and with some climbers
(for instance, Vitis, Ampelopsis, Cissus, etc.) to whom it does not afford
any assistance in climbing.
With respect to those cases in which the tip remains always bent or hooked
towards the same side, as in the genera just named, the most obvious
explanation is that the bending is due to continued growth in excess along
the convex side. Wiesner, however, maintains* that in all cases the hooking
of the tip is the result of its plasticity and weight,--a conclusion which
from what we have already seen with several climbing plants is certainly
erroneous. Nevertheless, we fully admit that the weight of the part, as
well as geotropism, etc., sometimes come into play.
Ampelopsis tricuspidata.--This plant climbs by the aid of adhesive
tendrils, and the hooked tips of the shoots do not appear to be of any
service to it. The hooking depends chiefly, as far as we could ascertain,
on the tip being affected by epinasty and geotropism; the lower and older
parts continually straightening themselves through hyponasty and
apogeotropism.
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