Theories of Twining 389 
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 move- 
ment 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, ie. with the sun, twines in 
the same direction, and vice versd. 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 the same time say with Pfeffer that “the 
causes of twining are...unknown1.” 
Let us leave this difficult question and consider some other 
points made out in the progress of the work on climbing plants. 
One result of what he called his “niggling?” work on tendrils was 
the discovery of the delicacy of their sense of touch, and the rapidity 
of their movement. Thus in a passion-flower tendril, a bit of platinum 
wire weighing 1°2 mg. produced curvature’, as did a loop of cotton 
weighing 2mg. Pfeffer*, however, subsequently found much greater 
sensitiveness: thus the tendril of Sicyos angulatus reacted to 
0°00025 mg., but this only occurred when the delicate rider of cotton- 
1 The Physiology of Plants, Eng. Tr. (Oxford, 1906), mr. p. 37. 
2 Life and Letters, 111. p. 312. 3 Climbing Plants, p. 171. 
4 Untersuchungen a. d. Bot. Inst. z. Tiibingen, Bd. 1. 1881—85, p. 506. 
