The Power of Movement in Plants | Page 3

here briefly to describe its nature. If we observe a
circumnutating stem, which happens at the time to be bent, we will say
towards the north, it will be found gradually to bend more and more
easterly, until it faces the east; and so onwards to the south, then to the
west, and back again to the north. If the movement had been quite
regular, the apex would have described a circle, or rather, as the stem is
always growing upwards, a circular spiral. But it generally describes

irregular elliptical or oval figures; for the apex, after pointing in any
one direction, commonly moves back to the opposite side, not, however,
returning along the same line. Afterwards other irregular ellipses or
ovals are successively described, with their longer [page 2] axes
directed to different points of the compass. Whilst describing such
figures, the apex often travels in a zigzag line, or makes small
subordinate loops or triangles. 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. 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.*
In the course of the present volume it will be shown that apparently
every growing part of every plant is continually circumnutating, though
often on a small scale. Even the stems of seedlings before they have
broken through the ground, as well as their buried radicles,
circumnutate, as far as the pressure of the surrounding earth permits. In
this universally present movement we have the basis or groundwork for
the acquirement, according to the requirements of the plant, of the most
diversified movements. Thus, the great sweeps made by the stems of
twining plants, and by the tendrils of other climbers, result from a mere
increase in the amplitude of the ordinary movement of circumnutation.
The position which young leaves and other organs ultimately assume is
acquired by the circumnutating movement being increased in some one
direction. the leaves of various plants are said to sleep at night, and it
will be seen that their blades then assume a vertical position through
modified circumnutation, in order to protect their upper surfaces from
being chilled through radiation. The movements of various organs to
the light, which are so general throughout the vegetable kingdom, and
occasionally from the light, or transversely with respect to it, are all
modified
* See Mr. Vines' excellent discussion ('Arbeiten des Bot. Instituts in
Würzburg,' B. II. pp. 142, 143, 1878) on this intricate subject.
Hofmeister's observations ('Jahreschrifte des Vereins für Vaterl.
Naturkunde in Würtemberg,' 1874, p. 211) on the curious movements
of Spirogyra, a plant consisting of a single row of cells, are valuable in
relation to this subject.
[page 4] forms of circumnutation; as again are the equally prevalent
movements of stems, etc., towards the zenith, and of roots towards the
centre of the earth. In accordance with these conclusions, a
considerable difficulty in the way of evolution is in part removed, for it
might have been asked, how did all these diversified movements for the

most different purposes first arise? As the case