418 FRANK W. BANCROFT 



these two types of stimulation and between the reactions which 

 depend upon them has long ago been pointed out by Loeb ('93, pp. 

 100-103) . The power to react to differences he calls Unterschied- 

 sempfindlichkeit (differential sensibility) . In the tropisms, on the 

 other hand, he thinks the stimulus is some constant or continuous 

 action of the lighti^ ('97, p. 400). 



It is not always easy to determine whether a given reaction is 

 produced by changes in the light or by its continuous action. 

 It may even be possible that future work will show that we have 

 here only one, and not two fundamental reactions. But, in the 

 present state of our knowledge, these two types of reactions appear 

 to be fairly distinct. 



A . Criteria for distinguishing reactions due to differential and 

 constant light reaction 



Loeb ('10, p. 465) has pointed out that if it can be shown that 

 the Bunsen and Roscoe law holds for any reaction then we have 

 the best evidence that we are at present capable of furnishing 

 that the reaction depends upon the continuous action of the light. 

 According to this law the photochemical effect depends upon the 

 quantity of light. If the light intensity is constant its effect 

 depends upon the duration. If the duration is constant the effect 

 is proportional to the intensity. If both intensity and duration 

 of the light vary the photochemical effect is a function of the 

 product of the two. Unfortunately it is not easy to apply this 

 law to heliotropism and so far it has only been found to hold for 

 the heliotropism of several plants (Blaauw '08; Froschel '08).^^ 



^^ But Loeb does not think, as has been assumed by some writers, that if a trop- 

 ism is shown to be due to differential sensibiltiy that by definition it ceases to be 

 a t'ropism. 



i^But so little is the importance of this law realized by Mast that he says ('11, 

 p. 258): "It should however be emphasized again that in no case has it been demon- 

 strated that orientation is 'a function of the constant intensity' as maintained by 

 Loeb." The truth is that in all those cases where it has been found possible to 

 demonstrate which of these two light effects has been responsible for the orienta- 

 tion it has been found that the orientation is "a function of the constant intensity." 

 And of these cases the one which has been most conclusively demonstrated and 

 which (granting the correctness of the facts) cannot possibly be disproved in our 



