The three great tendril-bearing families in which
this loss has occurred in the most marked manner, are the
Cucurbitaceae, Passifloraceae, and Vitaceae. In the first, the
internodes revolve; but I have heard of no twining form, with the
exception (according to Palm, p. 29. 52) of Momordica balsamina, and
this is only an imperfect twiner. In the two other families I can
hear of no twiners; and the internodes rarely have the power of
revolving, this power being confined to the tendrils. The
internodes, however, of Passiflora gracilis have the power in a
perfect manner, and those of the common Vine in an imperfect degree:
so that at least a trace of the supposed primordial habit has been
retained by some members of all the larger tendril-bearing groups.
On the view here given, it may be asked, Why have the species which
were aboriginally twiners been converted in so many groups into leaf-
climbers or tendril-bearers? Of what advantage has this been to
them? Why did they not remain simple twiners? We can see several
reasons. It might be an advantage to a plant to acquire a thicker
stem, with short internodes bearing many or large leaves; and such
stems are ill fitted for twining.
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