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fluctuations will be positive ; where the latter prevail, negative, and, consequently,
the effect on the leaves will not be constant as in the previous case, but in one
case there will be an increased tendency to the maintenance of the diurnal
position, in the latter a tendency to the assumption of the nocturnal one.
According to this the tendency to centrifugal propagation of movement
ought to be greater than that to centripetal propagation, and this is precisely
what we find to be the case as a matter of experiment. In the case of section
of the tips of axes the conditions are different. Here we have to deal solely
with conditions favouring abnormal escape of fluid,—conditions under the in-
fluence of which negative fluctuations are liable to be propagated centripetally
much more constantly and in much greater degree than when conditions of
obstruction, too, are present, and, in accordance with this, we find that much
more conspicuous and extensive centripetal propagation of movement occurs in
many instances in cases of section of the tips of axes than ever present them-
selves as the result of heating them in their course. There can, I believe, then,
be little doubt that the theory which ascribes the propagation of movement to
fluctuations in fluid tension is here, again, more in accordance with fact than that
which regards it as due to a propagation of stimulation along the course of con-
tinuous protoplasm.
We have now to enquire into the evidence for the final proposition regard-
ing the order of events manifested in many cases of propagation of movement.
This is contained in Table XII, showing the results following the section of
distal pinnules of young heavy leaves heavily loaded with adherent moisture.
It shows that in 8 out of the 10 cases dealt with the primary pulvinus was
either the very first part of the leaf to act or acted after the injured pinnules
alone had moved. The number of experiments included in the table is small,
but the results may be taken as typical for those in experiments conducted under
like conditions. That this is so is rendered clear by the following data regard-
ing the results of experiments carried out in regard to this point on three distinct
mornings. On the first occasion, out of 204 leaves, 135 manifested such
premature action in the primary pulvinus ; on the second, out of 139 leaves 100
did so ; on the third, out of 126 leaves 100 did so. The phenomenon to be
accounted for by the theories of the causation of propagation is the discontinuity
of action in such cases. According to the protoplasmic theory an explanation
can only be arrived at by the assumption of special irritability or contractility in
the protoplasm of the primary pulvinus, as compared with that in the other contrac-
tile organs. This special irritability or contractility must, moreover, be present
in certain cases only, being very frequently present in young and moist leaves,
and comparatively rare in old and dry ones, for the differences in the prevalence
of the phenomenon in leaves differing in age and amount of adherent moisture
is very conspicuous. On the same mornings on which premature pulvinar
action occurred in 135 of 204 and in 100 of 139 young leaves loaded with
adherent moisture, it only occurred in 11 out of 44 and 7 out of 61 old and
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