Speech, gesture, and every
other form of human action are, in the long run, resolvable into
muscular contraction, and muscular contraction is but a transitory
change in the relative positions of the parts of a muscle. But the
scheme which is large enough to embrace the activities of the highest
form of life, covers all those of the lower creatures. The lowest plant,
or animalcule, feeds, grows, and reproduces its kind. In addition, all
animals manifest those transitory changes of form which we class under
irritability and contractility; and it is more than probable that when
the vegetable world is thoroughly explored, we shall find all plants in
possession of the same powers, at one time or other of their existence.
I am not now alluding to such phenomena, at once rare and conspicuous,
as those exhibited by the leaflets of the sensitive plants, or the
stamens of the barberry, but to much more widely spread, and at the same
time, more subtle and hidden, manifestions of vegetable contractility.
You are doubtless aware that the common nettle owes its stinging
property to the innumerable stiff and needle-like, though exquisitely
delicate, hairs which cover its surface.
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