The Reversal of Tropisms in 



a sub-optimum intensity favors the formation of 

 substances represented by X and a supra-optimum 

 those represented by Y; and (3) that the colonies 

 are neutral in reaction when there are Y substances 

 in one member of the equation and X in the other; 

 positive when one member contains (X +) sub- 

 stances and the other (Y ), and negative when 

 one contains (X) and the other (Y+)." Vol- 

 vox is positive in weak light and negative in strong, 

 but when placed in light of supra-optimal intensity it 

 does not immediately change its response. This, as 

 Mast suggests, may be due to the fact that some 

 time is required to transform the substance X upon 

 which the positive reaction is supposed to depend. 

 After more of Y was produced and the amount of 

 X diminished through the action of more intense 

 light the negative reactions would be initiated. 



This interpretation, which is confessedly very spec- 

 ulative, not improbably contains elements of truth. 

 It is probable that reversible chemical reactions ef- 

 fect the general restoration of organic equilibrium 

 which had been disturbed by the influence of the 

 stimulus. Whether positive and negative photo- 

 taxis depend on the relative proportions of the sub- 

 stances belonging to a single equation is of course 

 very problematical. It might be that the formation 

 of a substance A which is favored by intensity of 

 light brings about a negative reaction, while the posi- 

 tive reaction might depend upon the formation of 

 another substance B which was not connected with 



