4l6 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION 



a diagrammatic representation is given (fig. 227). The 

 following tabular statement gives a synopsis of results with 

 four specimens, two of Porana and the other two of 

 Thunbergia. 



Table LVl. — Effect of Unilateral Geotropic Stimulation. 

 ''T = auto- torsion ; G = induced torsion under gravity.) 



The results of the experiments on the effect of unilateral 

 stimulation by gravity show that there is nothing to support 

 the assumption of three types of geotropic reaction in twining 

 stems, negative, dia-geotropic, and lateral. The results, on 

 the other hand, demonstrate the characteristic effects of 

 diffuse and of unilateral stimulation on anisotropic organs. 

 In the vertical and erect position, the natural rate of torsion 

 is slightly retarded by the diffuse action of geotropic stimu- 

 lation, the statolithic particles pressing on the basal end 

 of each statocyst. In the vertical but inverted position, 

 the effective stimulation is intensified on account, pre- 

 sumably, of the greater excitability of the apical ends of the 

 cells. The result is an increased retardation which some- 

 times culminates in an actual reversal of torsion from 

 positive to negative. 



The share of geotropic reaction in the movement of a 

 young stem about to twine, of which the lower portion is 

 erect whilst the upper portion overhangs, requires careful 

 analysis. The erect portion twists round its own axis, 

 a movement not affected by gravity, which causes the 

 circumnutation of the overhanging portion. Of this the 

 terminal internode twists but slightly : it tends to curve 

 upwards in response to gravity. In the succeeding inter- 

 nodes torsion is most active, and it is affected by gravity 



