STIMULATION BY GRAVITY 415 



water in which the lower end of the stem is fixed. This 

 w^ater-vessel is shown to pass through the fixed circular 

 scale, the angle of rotation being measured by means of 

 the index attached to the vessel (fig. 226). 



The torsional response to unilateral stimulation by 

 gravity is, as in the case of light, not the same for all the 

 sides, but shows characteristic differences, as will be seen 

 from the results of the following experiment. 



Experiment 227. — The stem of Clitoria was rotated till 

 the maximum rate of positive torsion was obtained, indi- 

 cating the A position. Representing the natural rate by T, 



Fig. 227. Torsional response of the stem to stimulation of 

 its different sides by gravity in four positions, n, a, n', r. 



No effect at n, n'; effect at a, additive; at r, in opposition. 

 T, normal autonomous torsion; g, gravity. 



and by G the torsion induced by geotropic stimulation, the 

 result is the summated effects T + G, which in the present 

 case was 42. The plant was next rotated through 90"", 

 bringing it to N position, in which the torsional response 

 induced by G declines to zero. The response was now 7. 

 Further rotation through 90° brought the plant to the 

 R position ; now the induced torsion was negative, and it 

 overpowered and reversed the natural torsion, the resultant 

 T — G being — 24. Further rotation through 90° brought 

 the stem to the second neutral position N', the rate being 5, 

 which is not very different from 7, which was the rate at 

 the first neutral position N. 



A large number of other experiments carried out with 

 different species of plants gave very similar results, of which 



