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Medical Officers of the Army of India.

101

the fluid content of the tissues and hence the structural stronger masses,
although absolutely weaker then than during the day become relatively stronger,
and therefore the nocturnal position of parts is always more or less that which
they make for. Under the influence of light the structurally weaker masses
attain their maximum of relative strength, because of the greater absorption
of fluid which they are capable of due to the larger amount of active elements,
and specially of chlorophyll containing elements which is present in them than in
the opposed masses, and which determines excessive absorption either as a direct
result of light stimulation, or as the result of the development of specially
absorptive matters by the protoplasm under the influence of such stimulation.
The position of the various parts is at any time one due to equilibrium between
the opposing masses of tissue, and movements result whenever this equilibrium
is disturbed by alterations in their fluid tension. In ordinary nyctitropic leaves
facilities are not provided for the occurrence of rapid fluctuations of fluid tension
in the tissues, and hence the movements are normally of a slow and insensible
character only manifesting themselves under the prolonged influence of con-
dition determining general changes in amount of fluid content of the entire
plant. In the case of Mimosa the structural peculiarities in the opposing masses
of tissue determining the position of the various parts are such as to permit
of very rapid redistribution of large masses of fluid, and hence a possibility arises
for the occurrence, not merely of movements of the ordinary nyctitropic type, but
of rapid movements connected with fluctuations in fluid-content too slight
and transitory to be capable of producing appreciable effects apart from such
facilities. The equilibrium between the opposing masses of tissue in Mimosa is
much more unstable than it is in common nyctitropic tissues due to the
presence of structural peculiarities, and in connection with this we find corre-
sponding differences in the character of the movements. The instability of
diurnal equilibrium in Mimosa is further increased by the exceptionally high
fluid tension, throughout the plant, which is maintained, in spite of the con-
stant loss of large quantities of fluid, by the absorption of equally large
quantities by the roots. In other words, the diurnal condition of the leaves
in Mimosa pudica is determined by the equilibrium of opposing masses of tissue.
But this equilibrium is extremely unstable, as it is only maintained as the result
of extreme fluid tension in masses of tissue of very different structural power, in
a system from which constant losses of fluid are taking place and in which the
structural peculiarities of the parts directly concerned in maintaining the
expansion of the leaves afford special facilities for the occurrence of rapid local
redistribution of fluid. Under such circumstances we should expect to find that
fluctuations in fluid tension incapable of giving rise to conspicuous movements
in common nyctitropic leaves are capable of doing so here, and this we find to
be actually the case. In fine, the position of expansion of the leaves of Mimosa

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