404 FRANK W. BANCROFT 



tions as is required by the theory of Jennings ; but that we have to 

 deal with two separate mechanisms that can be influenced sepa- 

 rately to a certain extent. The correlation which does seem to 

 exist between these two reactions in positively heliotropic Euglenae 

 in weak light merely serves to emphasize the lack of any regular 

 correlation for all conditions such as Jennings's theory demands. 



C. Sensitization, fatigue and recovery 



Some photochemical systems require the action of the light 

 for a certain time before they acquire the power to react to the 

 light in their usual way. They are said to require sensitizing. 

 Some substances are quickly destroyed by light, others more 

 slowly. Some reactions are irreversible, and when they have 

 been completed in an organism cannot take place again until the 

 metabolism has furnished a new supply of the reacting substances. 

 Other reactions are reversible and may take place in one direction 

 in the light and in the other in the darkness, etc. 



In the organism the phenomena of sensitization for the response 

 to light, of the fatigue in reactions to light and of the recovery from 

 fatigue are undoubtedly functions of some such reactions as those 

 enumerated above. If, as Jennings supposes, the differential 

 light reactions and heliotropic orientation are essentially the same 

 reaction and differ only in degree, then we should expect that in 

 both of them the same photochemical substances are involved, 

 and both should behave in the same way in regard to sensitiza- 

 tion, fatigue and recovery. 



When preparations from Culture B were brought into the light 

 after having been kept in the dark .over night or longer the shading 

 reaction often developed in less than a minute while no tropism 

 could be made out for ten or fifteen minutes. But this is not'con- 

 clusive because additional experiments are required to prove that 

 the light intensity employed was not just between the intensity 

 needed for positive and negative heliotropism. On one occasion, 

 however, perfectly distinct heliotropism developed before any 

 shading reaction was developed, and here we can be sure that sen- 

 sitization has taken place more rapidly for the heliotropic mechan- 

 ism than for the mechanism which produces the shading reaction. 



