He who will read my memoir on these plants will, I think, admit
that all the many gradations in function and structure between simple
twiners and tendril-bearers are in each case beneficial in a high degree to
the species. For instance, it is clearly a great advantage to a twining
plant to become a leaf-climber; and it is probable that every twiner which
possessed leaves with long foot-stalks would have been developed into a
leaf-climber, if the foot-stalks had possessed in any slight degree the
requisite sensitiveness to a touch.
As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire this
power in an incipient degree, afterwards to be improved and increased
through natural selection. The power of twining depends, firstly, on the
stems while young being extremely flexible (but this is a character common
to many plants which are not climbers); and, secondly, on their continually
bending to all points of the compass, one after the other in succession, in
the same order. By this movement the stems are inclined to all sides, and
are made to move round and round. As soon as the lower part of a stem
strikes against any object and is stopped, the upper part still goes on
bending and revolving, and thus necessarily twines round and up the
support. The revolving movement ceases after the early growth of each
shoot.
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