426 FRANK W. BANCROFT 



7. SUMMARY 



A. Direct orientation vs. 'Trial and error' 



According to Jennings's theory positive heliotropism is condi- 

 tioned by motor reactions produced by sudden shading; and nega- 

 tive hehotropism by motor reactions to sudden illumination. He 

 considers that the gradual heliotropic orientation is a series of 

 motor reactions. It has been found, however, that: 



1. There is no necessary relation between the sign of the helio- 

 tropism and the character of the motor reactions. Positive helio- 

 tropism may be accompanied by motor reactions to either sudden 

 shading or sudden illumination; and the same is true of negative 

 heliotropism. 



2. The ability to react to suddeti changes in illumination by 

 means of motor reactions is possessed by Euglenae which are not 

 heliotropic. Conversely there are heliotropic Euglenae that can- 

 not be made to respond to changes in illumination. 



3. The mechanisms for heliotropism and for the motor reac- 

 tions in response to light behave quite differently as regards sensi- 

 tization, fatigue and recovery. 



4. If sudden changes of intensity and direction of the light are 

 avoided, orientation may be brought about without any evidence 

 of motor reactions, and this under circumstances which are per- 

 fectly capable of producing these motor reactions if the changes 

 are sudden. 



5. In a weak arc light which produced good heliotropism and 

 motor reactions the time required for sudden illumination to 

 bring about motor reactions was three times the time required for 

 half a revolution of the organism on its long axis. Hence it is 

 impossible for the gradual heliotropic orientation to be produced 

 by a series of motor reactions. 



6. According to Jennings's theory the heliotropic swerving must 

 be considered weak motor reactions. But as the intensity of the 

 light is diminished there is no tendency for the character of the 

 gradual orientation and motor reactions to resemble each other,- 

 they remain perfectly distinct. 



