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Taking the data in the separate tables, we find additional evidence of this
in the following phenomena. In Table I there is evidence to show that the
extension of movement is related to exposure to sunshine or shade, the tend-
ency to extension being, under otherwise like circumstances, much greater under
the former than the latter condition. But exposure to direct sunshine is cer-
tainly exposure to increased evaporative loss of fluid. This, of course, needs no
demonstration, but if it did, an examination of the condition of the leaves of
any mass of the plant, before and after it has been exposed to direct sunshine,
ought to be sufficient to meet all requirements. In a vigorous mass of the
plant growing under favourable circumstances, we find that it is in the early
morning, and whilst the leaves are still moist with dew, that the condition of
expansion is at a maximum. As the sunshine strikes on it, and as the leaves
gradually lose their adherent moisture, we find that the expansion tends to
diminish, and, under continued exposure, in many cases becomes very con-
spicuously less than it was. Not only do we find slow, insensibly progressive
movements taking place under the influence of the sunshine, but, under certain
circumstances, rapid, large movements occur similar to those resulting from
so-called stimulation of the tissues. If a mass of the plant be carefully
watched just as it is struck by the sunshine, such movements can frequently
be observed to take place in large numbers. Especially conspicuous are those
taking place in the case of young, terminal and only partially-expanded leaves
situated at the tips of axes. These are, as a rule, so long as the plants remain
in the shade, in a condition of extreme erection, but after a brief exposure to
the sunshine in many instances, quite apart from any other appreciable exciting
cause, become suddenly depressed, due to action of their primary pulvini and
depression of the petioles. Similar movements may also be seen to occur,
under like circumstances, in other portions of the leaves. They appear often
very conspicuously in the case of leaves in a condition of partial recovery from
the effects of application of heat to distal pinnules. Recovery in such cases
usually commences with elevation of the petiole, followed by divergence of
the pinnæ, and gradual centrifugal expansion of the pinnules. In leaves in which
recovery has advanced so far in the shade that the basal pinnules have begun
to diverge, the access of direct sunshine is often followed almost immediately
by movements of relapse in varying degree, in some cases confined to the
pinnules or secondary pulvini, but in other cases complete and involving the
primary pulvinus and leading to renewed petiolar depression. Similar phenomena
present themselves in the case of the leaves on detached branches kept in a
saturated atmosphere beneath bell glasses, if the latter be carefully removed
and direct access of sunshine permitted.
Exposure to direct sunshine implies increased facility for evaporative
loss, and we find from the experiments that it also implies increased tend-
ency to extension of movement, The theory that extension of movement is
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