Several leaf-climbing genera
are closely allied to other genera which are simple twiners. It
should also be observed, that the possession of leaves with sensitive
petioles, and with the consequent power of clasping an object, would
be of comparatively little use to a plant, unless associated with
revolving internodes, by which the leaves are brought into contact
with a support; although no doubt a scrambling plant would be apt, as
Professor Jaeger has remarked, to rest on other plants by its leaves.
On the other hand, revolving internodes, without any other aid,
suffice to give the power of climbing; so that it seems probable that
leaf-climbers were in most cases at first twiners, and subsequently
became capable of grasping a support; and this, as we shall presently
see, is a great additional advantage.
From analogous reasons, it is probable that all tendril-bearers were
primordially twiners, that is, are the descendants of plants having
this power and habit. For the internodes of the majority revolve;
and, in a few species, the flexible stem still retains the capacity
of spirally twining round an upright stick. Tendril-bearers have
undergone much more modification than leaf-climbers; hence it is not
surprising that their supposed primordial habits of revolving and
twining have been more frequently lost or modified than in the case
of leaf-climbers.
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