In the case of leaves the ellipses are
generally narrow.
Until recently the cause of all such bending movements was believed to be
due to the increased growth of the side which becomes for a time convex;
that this side does temporarily grow more quickly than the concave side has
been well established; but De Vries has lately shown that such increased
growth follows a previously increased state of turgescence on the convex
side.* In the case of parts provided with a so-called joint, cushion or
pulvinus, which consists of an aggregate of small cells that have ceased to
increase in size from a very early age, we meet with similar movements; and
here, as Pfeffer has shown** and as we shall see in the course of this
work, the increased turgescence of the cells on opposite sides is not
followed by increased growth. Wiesner denies in certain cases the accuracy
of De Vries' conclusion about turgescence, and maintains*** that the
increased extensibility of the cell-walls is the more important element.
That such extensibility must accompany increased turgescence in order that
the part may bend is manifest, and this has been insisted on by several
botanists; but in the case of unicellular plants it can hardly fail to be
the more important element.
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