SUMMARY 311 



1 am going to describe shows (i) the enhancement of 

 phototropic excitability on the introduction of coal-gas, 

 and (2) the depressing effect of excess of coal-gas in depriving 

 the organ of its supply of oxygen. After taking the norma] 

 curve of geotropic torsion, 



light was applied below at 

 — L and an exact balance 

 obtained in the course of 



2 minutes, as seen in the top 

 of the curve becoming hori- 

 zontal. Coal-gas was now 

 introduced into the plant- 

 chamber at C. This induced 

 an enhancement of photo- 

 tropic reaction with resulting 

 overbalance shown by the 

 reversal of torsion. This 

 enhancement persisted for 

 more than 3 minutes. By 

 this time the plant-chamber 

 was completely filled with 

 coal-gas, and the resulting 



effect on the phototropic reaction is seen in the second upset 

 of the balance, this time to be depression (fig. 188). It 

 would seem that the cells which respond to light are situated 

 nearer the surface of the organ than those which react to 

 geotropic stimulus. Hence an agent which acts on the 

 organ from outside induces phototropic change earlier than 

 geotropic. 



Fig. 1 88. Effect of coal-gas on photo- 

 geotropic balance. Geotropic tor- 

 sion, G, is exactly balanced by 

 opposing action of light, — l. Appli- 

 cation of coal-gas at c at first 

 caused enhancement of phototropic 

 reaction with resulting reversal. 

 Prolonged application induced 

 depression of phototropic reaction, 

 the geotropic effect thus becoming 

 predominant. 



Summary 



Under lateral action of geotropic stimulus, a dorsi- 

 ventral organ exhibits a torsional response such that the 

 less excitable half of the organ is made to face the stimulus. 



The direction of incident geotropic stimulus is the same 

 as the direction of the vertical lines of gravity. 



