Medical Officers of the Army of India.
cases of heating of the pinnules elevation of the petiole is the primary
phenomenon of recovery ; in cases of section expansion of the pinnules occurs
first, and the pinnæ are more or less fully expanded long before the petiole has
regained its erect condition, and the same order is followed in recovery after pure
mechanically induced movement of the leaves. The precise causation of these
differences is a subject calling for further investigation, and I would merely, in
the meantime, suggest that it is probably to be found in variations in the amount
of fluid discharged from the tissues in the different classes of cases, and in
variations in rate of discharge, the phenomena in cases of section corre-
sponding with the occurrence of rapid escape of a limited amount of fluid, those
in cases of heating with slow, protracted escape of larger quantities.
The theses originally propounded having now all been considered in detail,
and the evidence appearing to support them having been adduced, I have
merely to add a few words in regard to a question which has more and more
forced itself upon me in the course of working at the subject of the propagation
of movement in Mimosa pudica. This question is—how far are the movements
themselves due to anything more than purely mechanical agency ; how far does
any active contraction of protoplasm come into play to give rise to them ; and
how far are they not mere results of fluctuations in the fluid tension of the
tissues ? The data contained in Table XIII are very suggestive of questions of
this nature. In the experiments detailed in this table the sun's rays were carefully
focussed on a series of primary pulvini, the application being, as far as possible,
confined to the upper surfaces. The 15 experiments registered were conducted
on two successive mornings, but numerous other experiments at other periods
have demonstrated that the results then obtained were not exceptional. In
8 cases of the 15, the primary phenomenon following the application was
elevation of the petiole ; in one case it was a rotatory movement ; in only 6 was
it depression. In the majority of cases in which elevation occurred it was
followed by depression, but in two cases the elevation was not followed by
depression, although it was followed by action in the pinnæ.
These phenomena of elevation are clear indices of the occurrence of loss
of fluid so localised, at the outset at all events, as to cause relative weakening
in the tissues of the upper half of the thickness of the primary pulvinus ; that is
weakening in the tissue directly affected, and, as this was the tissue which
makes for depression, to lead to increased elevation of the petiole. The eleva-
tion was, as a rule, transitory and soon replaced by depression, because, as the
loss of fluid continued, it extended to the tissues on the underside of the
pulvinus and thereby re-established the normal potential relations of the
opposed masses of tissue, the inferior and structurally weaker becoming natur-
ally absolutely the weaker when loss of fluid had extended to it also. Such a
phenomenon is, at all events, well calculated to raise a doubt as to whether the
so-called irritable areas in the leaves are really specially irritable, or whether
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