As a pulvinus is
formed by the arrestment of the growth of its cells, movements dependent on
their action may be long-continued without any increase in length of the
part thus provided; and such long-continued movements seem to be one chief
end gained by the development of a pulvinus. Long-continued movement would
be impossible in any part, without an inordinate increase in its length, if
the turgescence of the cells was always followed by growth.
Disturbance of the Periodic Movements of Cotyledons by Light.--The
hypocotyls and cotyledons of most seedling plants are, as is well known,
extremely heliotropic; but cotyledons, besides being heliotropic, are
affected paratonically (to use Sachs' expression) by light; that is, their
daily periodic movements are greatly and quickly disturbed by changes in
its intensity or by its absence. It is not that they cease to circumnutate
in darkness, for in all the many cases observed by us they continued to do
so; but the normal order of their movements in relation to the alternations
of day and night is much disturbed or quite annulled.
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