4l8 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION 



with that of T + G, the combined action of T + G + L 

 giving rise to a large positive or anti-clockwise torsion. In 

 the second case, light acts in opposition, the resultant action 

 being T + G — L. Now, if the light be sufficiently intense, 

 it will overpower T + G, and the resultant will be a negative 

 or clockwise torsion. The theoretical result is diagram- 

 matically represented in fig. 228, where, in the A position, 



Fig. 228. Combined effects of simultaneous geotropic and 

 photic stimulation on autonomous torsion t. 



Light, L, alternately applied above or below (straight arrow). 

 Induced torsion (curved arrow) under light l and gravity g. 



the unshaded half to the right is the less excitable. In the 

 figure to the left, light L and gravity G act from above ; 

 their additive effect co-operates with autonomous torsion T. 

 In the figure to the right, G acts from above on the flank A, 

 while light acts from below on the opposite flank R. The 

 torsion induced by L is therefore in the opposite direction 

 to that induced by G. 



Experiment 228. — Thunbergia placed in A position. 



1. Rate of torsion without light, T + G = 



2. ,, ,, with light from above, T + G + L = 

 Light withdrawn ; complete recovery under T + G = 



3. Rate of torsion with light from below, T + G — L = 



Experiment 229. — Porana placed in A position. 



1. Rate of torsion without light, T + G = 



2. ,, ,, with light from above, T + G + L = 



3. ,, „ with light from below, T + G — L = 



16 



40 



16 



-8 



28 



58 

 -3 



