.
Also Jost, "Vorlesungen uber Pflanzenphysiologie", page 562, Jena,
1904.)

Their criticism originated in observations made on a revolving shoot
which is removed from the action of gravity by keeping the plant slowly
rotating about a horizontal axis by means of the instrument known as a
klinostat. Under these conditions circumnutation becomes irregular or
ceases altogether. When the same experiment is made with a plant which
has twined spirally up a stick, the process of climbing is checked and
the last few turns become loosened or actually untwisted. From this
it has been argued that Darwin was wrong in his description of
circumnutation as an automatic change in the region of quickest growth.
When the free end of a revolving shoot points towards the north there
is no doubt that the south side has been elongating more than the north;
after a time it is plain from the shoot hanging over to the east that
the west side of the plant has grown most, and so on. This rhythmic
change of the position of the region of greatest growth Darwin ascribes
to an unknown internal regulating power. Some modern physiologists,
however, attempt to explain the revolving movement as due to a
particular form of sensitiveness to gravitation which it is not
necessary to discuss in detail in this place. It is sufficient for my
purpose to point out that Darwin's explanation of circumnutation is
not universally accepted. Personally I believe that circumnutation is
automatic--is primarily due to internal stimuli. It is however in some
way connected with gravitational sensitiveness, since the movement
normally occurs round a vertical line. It is not unnatural that,
when the plant has no external stimulus by which the vertical can be
recognised, the revolving movement should be upset.

Very much the same may be said of the act of twining, namely that most
physiologists refuse to accept Darwin's view (above referred to) that
twining is the direct result of circumnutation. Everyone must allow
that the two phenomena are in some way connected, since a plant
which circumnutates clockwise, i.e. with the sun, twines in the same
direction, and vice versa. It must also be granted that geotropism has a
bearing on the problem, since all plants twine upwards, and cannot twine
along a horizontal support. But how these two factors are combined, and
whether any (and if so what) other factors contribute, we cannot say.
If we give up Darwin's explanation, we must at