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, 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 the same time say with Pfeffer that "the 

 causes of twining are... unknown 1 ." 



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 2 " 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 3 , as did a loop of cotton 

 weighing 2mg. Pfeffer 4 , 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), in. p. 37. 



2 Life and Letters, in. p. 312. 3 Climbing Plants, p. 171. 

 4 Untersuchungen a. d. Bot. Inst. z. Tubingen, Bd. i. 1881—85, p. 506. 



