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. As in many widely separated
families of plants, single species and single genera possess the power
of revolving, and have thus become twiners, they must have independently
acquired it, and cannot have inherited it from a common progenitor.
Hence, I was led to predict that some slight tendency to a movement of
this kind would be found to be far from uncommon with plants which did
not climb; and that this had afforded the basis for natural selection
to work on and improve. When I made this prediction, I knew of only one
imperfect case, namely, of the young flower-peduncles of a Maurandia
which revolved slightly and irregularly, like the stems of twining
plants, but without making any use of this habit. Soon afterwards
Fritz Muller discovered that the young stems of an Alisma and of a
Linum--plants which do not climb and are widely separated in the natural
system--revolved plainly, though irregularly, and he states that he has
reason to suspect that this occurs with some other plants. These slight
movements appear to be of no service to the plants in question; anyhow,
they are not of the least use in the way of climbing, which is the point
that concerns us. Nevertheless we can see that i