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. On the whole we may at present conclude that
in-

* Sachs first showed ('Lehrbuch,' etc., 4th edit. p. 452) the intimate
connection between turgescence and growth. For De Vries' interesting essay,
'Wachsthumskrümmungen mehrzelliger Organe,' see 'Bot. Zeitung,' Dec. 19,
1879, p. 830.

** 'Die Periodischen Bewegungen der Blattorgane,' 1875.

*** 'Untersuchungen über den Heliotropismus,' Sitzb. der K. Akad. der
Wissenschaft. (Vienna), Jan. 1880.

[page 3]
creased growth, first on one side and then on another, is a secondary
effect, and that the increased turgescence of the cells, together with the
extensibility of their walls, is the primary cause of the movement of
circumnutation.*