244 NATURE OF PHOTOSENSORY PROCESS 



experiments on Ciona (Hecht, 1918-19, a) and Mya have demons 

 strated that the distinction on which this controversy rests is without 

 any but the most superficial basis. 



The facts are simple enough. Blaauw (1909) and Froschel (1909) 

 demonstrated that the orientation of certain plants obeys the Reci- 

 procity Law of Bunsen and Roscoe; therefore a definite amount of 

 Hght is required for a stimulus. Loeb (1918) and his associates 

 proved the same to be true for the animals with which they experi- 

 mented. All these organisms are, of course, to be classed in the group 

 which responds to an obviously continuous source of illumination. 

 On the other hand, Mya and Ciona belong decidedly in the group of 

 organisms which is sensitive to hght only when the illumination in- 

 tensity has been increased rapidly. And yet both Mya and Ciona 

 must receive a definite amount of light before they respond. In 

 Ciona this is approximately 5,000 meter candle seconds; and in Mya, 

 as we found in this paper, the required energy is about 5 meter candle 

 seconds. The stimulus for both groups of organisms is therefore the 

 same, a definite quantity of hght energy producing a specific photo- 

 chemical effect. Certainly no distinction can be made between them 

 on this score. 



One point remains to be elucidated. Since both groups of organ- 

 isms require fundamentally the same stimulus, how is it that animals 

 Hke Mya and Ciona respond apparently to sudden illumination only? 

 This is indeed a paradoxical situation, yet the answer to the question 

 is simple. The explanation depends on the reversible character of 

 the sensory process. 



The presence of a regenerative mechanism in the sense organ is 

 shown by the course of dark adaptation. The photosensitive material 

 decomposed by the Hght is automatically regenerated as soon as some 

 products of decomposition accumulate. The velocity of this regen- 

 erating reaction depends on the concentration of the precursor de- 

 composition products. Therefore a httle time must elapse before the 

 effect of the regenerating reaction will become apparent. It is pre- 

 cisely during this short interval of time that the required amount of 

 energy must be received by the sense organs in order to produce a 

 response. 



