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they are not merely areas in which disturbances in the fluid tension of the
tissues have special facility in giving rise to movements due to structural
peculiarities and to relations to other masses of tissue. The areas of special
irritability are ordinarily affirmed to lie in the under surface of the primary
pulvinus, the inner surfaces of the secondary ones, and the upper surfaces of the
pinnules and tertiary pulvini. But these are all areas in which structural
peculiarities of the tissues and the mutual relations of various parts must inevi-
tably favour the occurrence of movements as the result of fluctuations in fluid
tension incapable of giving rise to movement in other parts in which such
adjuvant agents are absent or are replaced by positively opposing ones, as they
are in the upper portions of the primary pulvinus, in the outer parts of the
secondary ones and in the under parts of the tertiary ones. Where negative
fluctuations in fluid tension take place in the so-called irritable areas, they act
in favour of the structurally stronger masses of tissue which are constantly
striving to establish the passive or nocturnal position of the various parts ; where-
as in the other areas they act in direct opposition to them. The tendency to
more certain and ready occurrence of movement as the result of influences
calculated to give rise to negative fluctuations in the fluid content of the tissues,
when acting on the so-called irritable areas than when acting elsewhere, can
be accounted for on purely physical grounds, and does not necessarily imply
the presence of any special irritability or contractility of the protoplasm
of the parts. According to the theory that the occurrence of movements
is due to mere physical causes, we can account for the distribution
of so-called irritability. According to the theory that it is due to special
functional properties in the protoplasm of the various masses of tissue, we
cannot do so save by the aid of pure assumption.
Whether all the movements which occur in Mimosa ought to be regarded
as mere mechanical results of fluctuations in the fluid tension, as the nyctitropic
ones unequivocally are, remains an open question, but there are grounds for the
belief that to a great extent, at all events, they are so. It appears probable that
to a very great extent the so-called movements of irritation differ merely in
degree, and not in kind, from common nyctitropic ones, the difference lying
merely in the degree of rapidity with which the movements are carried out.
In the case of the leaves of Mimosa pudica, just as in the case of leaves
in which all conspicuous movements are of a purely nyctitropic character, the
displacement of the various parts is due to the opposition of masses of tissue
differing from one another in their structural strength and power of absorption
of fluid, those which possess the greatest passive structural strength having
the least capacity for absorption and vice versâ. Due to this the relative
strength of the various masses differs at different times under the influence
of different conditions, and corresponding changes take place in the position of
the parts on which they work. During the night there is a general diminution in
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