376 chap; XXXIV. THE MECHANISM OF THE TWINING STEM 



the weight of the free portion of the apex, but this has been 

 shown to be untrue by various investigators. The causes 

 of twining are therefore unknown.' ^ There has been much 

 discussion in regard to the question whether circumnutation 

 is autonomous or whether it results from the action of some 

 external stimulus. 



Characteristic Response of Anisotropic Organs 



The torsion, or twisting growth, characteristic of the 

 twining stem is the result of the unequal growth of two 

 opposite sides. The stem is, in fact, anisotropic ; it may 

 be regarded as consisting, at any given moment, of two 

 diverse longitudinal halves, which differ not only in their 

 rate of growth but also in their degree of excitability so 

 that they are differently affected by an identical change in 

 the environment. The differentially growing stem may be 

 compared with the anisotropic pulvinus of Mimosa. The 

 upper and the low^er halves of the pulvinus are excitable, but 

 in a different degree. Local stimulation of the upper half 

 produces a contraction of that half, causing an up-movement, 

 while local stimulation of the lower causes a more vigorous 

 down-movement. Diffuse stimulation causes antagonistic 

 reactions in the two halves, but since the contraction 

 of the lower half is greater, the predominant effect is a 

 resultant down-movement. For the sake of simplicity the 

 antagonistic action of the upper half may be ignored, and 

 the responsive movement attributed mainly to the action 

 of the more active half of the organ. The essential difference 

 between the anisotropic pulvinus of Mimosa and the aniso- 

 tropic stem of twining plants is that the plane which de- 

 marcates the two diverse halves in the former is fixed ; 

 whereas in the latter it regularly travels from segment to 

 segment of the circumference in a slow revolving motion. 



Whilst the action of the less active half of a twisting 

 organ may, in general, be ignored, some complications may 



* Pfeffer, ihid. vol. iii. p. 37. 



