SUMMARY 257 



moreover, opens out possibilities of inquiry in new directions, 

 such as the comparison of the excitatory effects of different 

 stimuU by the Method of Balance. 



The Torsional Balance 



A beam of light falling on the left flank of the pulvinus 

 of Mimosa initiates a torsion against the hands of the clock. 

 A second beam falling on the right flank initiates a contrary 

 movement ; the resultant effect is, therefore, determined 

 by the effective stimulation of the two flanks. The pulvinus 

 thus becomes a delicate indicator by which the effectiveness 

 of two stimuli may be compared with each other. The 

 following experiment is cited as an example of the application 

 of the method of phototropic balance. 



Experiment 139. — A parallel beam of light from a small 

 arc-lamp, passing through a blue glass, falls on the left 

 flank of the pulvinus ; a beam of blue light also strikes 

 the pulvinus on the right flank, the intensity of the latter 

 being so adjusted that the resultant torsion is zero. The 

 blue glass on the left side is now removed, the unobstructed 

 white light being allowed to fall on the left flank of the 

 pulvinus. This is found to upset the balance, the resultant 

 torsion being anti-clockwise, proving that white light 

 induces greater excitation than blue light. A red glass is 

 now interposed on the left side, with the result that the 

 balance is upset in the opposite direction, showing that the 

 phototropic effect of red light is comparatively feeble. It 

 is thus possible to compare the tropic effect of one form of 

 stimulation with that of another. It is enough here to draw 

 attention to the various investigations rendered possible by 

 the Method of Torsional Balance. Examples of some of 

 these will be given in a subsequent chapter. 



Summary 



Lateral stimulation induces a torsional response in a 

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



