In Tropaeolum tricolorum we have another
kind of passage; for the leaves which are first formed on the young
stems are entirely destitute of laminae, and must be called tendrils,
whilst the later formed leaves have well-developed laminae. In all
cases the acquirement of sensitiveness by the mid-ribs of the leaves
appears to stand in some close relation with the abortion of their
laminae or blades.
On the view here given, leaf-climbers were primordially twiners, and
tendril-bearers (when formed of modified leaves) were primordially
leaf-climbers. The latter, therefore, are intermediate in nature
between twiners and tendril-bearers, and ought to be related to both.
This is the case: thus the several leaf-climbing species of the
Antirrhineae, of Solanum, Cocculus, and Gloriosa, have within the
same family and even within the same genus, relatives which are
twiners. In the genus Mikania, there are leaf-climbing and twining
species. The leaf-climbing species of Clematis are very closely
allied to the tendril-bearing Naravelia. The Fumariaceae include
closely allied genera which are leaf-climbers and tendril-bearers.
Lastly, a species of Bignonia is at the same time both a leaf-climber
and a tendril-bearer; and other closely allied species are twiners.
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