CHAPTER XXXIV 



ft 



THE MECHANISM OF THE TWINING STEM 



An unsupported twining stem bends over and its apex 

 circumnutates in a path more or less circular. When in 

 the course of circumnutation it comes across a support, it 

 twines round it. The direction of movement is in a large 

 number of cases against the hands of the clock, which I will 

 distinguish by a plus sign ; in a few cases it is negative or 

 clockwise. It is, however, often difficult to say what the 

 sign of normal movement actually is ; for the same plant is 

 found sometimes to move in an anti-clockwise and at other 

 times in a clockwise direction. 



No satisfactory explanation has yet been given of these 

 movements. To quote Pfeffer : ' The factors which deter- 

 mine the permanent homodromous curvature of the apex 

 are uncertain. . . . The homodromous curvature of the 

 apex is probably due to autonomic variations of tone, in 

 which the external world and the progress of twining act 

 as directive stimuH. Baranetzky and Noll on insufficient 

 grounds assume the existence of a dia-geotropic irritability 

 in the apex inducing paranasty. Ambron ascribes the 

 homodromous curvature to the conjoint action of circum- 

 nutation and negative geotropism, a conclusion which 

 Schwendener disputes. The latter erroneously regards 

 circumnutation and geotropism as factors of constant 

 magnitude, and forgets that the circumnutation and the 

 klinotropic position of the shoot caused by it are themselves 

 the result of regulated geotropic reactions. De Vries sup- 

 posed the curvature to be due to the torsion produced by 



