310 



CHAP. XXVIII. GEOTROPIC TORSION 



continuously diminished by regulation of the diaphragm till 

 exact balance was obtained, as shown by the horizontal part 

 of the record. It is with great surprise that one comes to 

 realise the fact that the effect of one form of stimulus can be 

 so exactly balanced by that of another entirely different, and 

 that the stimulus of gravity can be measured, as it were, in 

 candle-powers of light ! After securing the balance, light 

 was cut off, and geotropic torsion was renewed on the 

 cessation of the counteracting phototropic reaction. 



Experiment 172. Comparative balancing effects of white 

 and red light. — ^White light was at first applied at— L, in 



opposition to the geo- 

 tropic movement. The 

 intensity of light was 

 stronger than was necess- 

 ary for exact balance, and 

 its effect was at first to 

 retard and then to reverse 

 the torsional geotropic 

 response. When thus 

 overbalanced, a red glass 

 was interposed in the path 

 of the light at R. As the 

 phototropic effect of this 

 light is comparatively 

 feeble, the geotropic torsion became predominant, as seen in 

 the subsequent up-curve. The red glass was then removed, 

 white light being substituted at — L to stimulate once 

 more in opposition ; the result is seen in the final over- 

 balance, and reversal of torsion (fig. 187). 



Experiment 173. Effect of coal-gas on the balance. — The 

 method of balance described above opens out new possi- 

 bilities in regard to investigations on the relative modifica- 

 tions of geotropic and phototropic excitability by any given 

 external change. Traces of coal-gas are known to enhance 

 the phototropic excitability of an organ, while continued 

 absence of oxygen is found to depress it. The experiment 



Fig. 187. Application of white light at 

 — L in opposition causes reversal of 

 torsion. Red light, r, is ineffective, 

 and geotropic torsion is restored. 

 Reapplication of white light causes 

 once more the reversal of torsion. 



