408 LIFE MOVEMENTS IN PLANTS 



pulvinus from the right side, and the intensity of the latter 

 is so adjusted that the resultant torsion is zero. Blue glass 

 is now removed from the left side, the unobstructed white 

 light being allowed to fall on the left flank of the pul- 

 vinus. This was found to upset the balance, the resultant 

 torsion being anti-clockwise. This showed that white light 

 induced greater excitation than blue light. "We next inter- 

 pose a red glass on the left side, with the result that the 

 balance is upset in the opposite direction. This is because 

 the phototropic effect of red light is comparatively feeble. 

 We may thus compare the tropic effect of one form of 

 stimulus against a totally different form, phototropic against 

 geotropic action for example. It is enough here to draw 

 attention to the various investigations rendered possible by 

 the method of balance. Concrete examples of some of 

 these will be given in a subsequent chapter. 



DETERMINATION OF THE DIRECTION OF STIMULUS. 



I have shown that the torsion, clockwise or anti-clock- 

 wise, is determined by the direction of incident stimulus. 

 Hence it would be possible to determine the direction of 

 incident stimulus from the observ^ed torsional movement. 

 In the case of light, the direction of incident stimulus 

 is quite apparent. But it is difficult to determine the direc- 

 tion of stimulus which is itself invisible. In such cases, 

 the torsional movement gives us infallible indication of 

 the effective direction of stimulus. The application of this 

 principle will be found in a later chapter. 



SUMMARY. 



Lateral stimulus induces a torsional response in a dorsi- 

 ventral organ. This is true of all modes of stimulation. 



