f the stems of these
plants had been flexible, and if under the conditions to which they are
exposed it had profited them to ascend to a height, then the habit
of slightly and irregularly revolving might have been increased and
utilised through natural selection, until they had become converted into
well-developed twining species.

With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in
the case of the revolving movements of twining plants. As a vast number
of species, belonging to widely distinct groups, are endowed with this
kind of sensitiveness, it ought to be found in a nascent condition in
many plants which have not become climbers. This is the case: I observed
that the young flower-peduncles of the above Maurandia curved themselves
a little towards the side which was touched. Morren found in several
species of Oxalis that the leaves and their foot-stalks moved,
especially after exposure to a hot sun, when they were gently and
repeatedly touched, or when the plant was shaken. I repeated these
observations on some other species of Oxalis with the same result; in
some of them the movement was distinct, but was best seen in the young
leaves; in others it was extremely slight. It is a more important fact
that according to the high authority of Hofmeister, the young shoots and
leaves of all plants move after being shaken; and with climbing plants
it is, as we know, only during the early stages of growth that the
foot-stalks and tendrils are sensitive.

It is scarcely possible that the above slight movements, due to a touch
or shake, in the young and growing organs of plants, can be of any
functional importance to them. But plants possess, in obedience to
various stimuli, powers of movement, which are of manifest importance
to them; for instance, towards and more rarely from the light--in
opposition to, and more rarely in the direction of, the attraction
of gravity. When the nerves and muscles of an animal are excited by
galvanism or by the absorption of strychnine, the consequent movements
may be called an incidental result, for the nerves and muscles have not
been rendered specially sensitive to these stimuli. So with plants it
appears that, from having the power of movement in obedience to certain
stimuli, they are excited in an incidental manner by a touch, or by
being shaken. Hence there is no great difficulty in admitting that i