HELIOTROPISM IN EUGLENA 385 



fied independently. Thus, according to Jennings negatively helio- 

 tropic Euglenae always react by the motor reaction to sudden illu- 

 mination, and not to sudden shading. Jennings's whole theory is 

 based upon the association of negative heliotropism with motor 

 reactions by illumination and of positive heliotropism with the 

 motor reactions by shading. But it was found possible to produce 

 at will negatively heliotropic Euglenae that react to sudden shad- 

 ing, and not to sudden illumination, by the motor reaction. Posi- 

 tively heliotropic Euglenae which gave the motor reaction only 

 upon sudden illumination were also observed. These, as well as 

 many other facts, are not at all in accord with Jennings's theories 

 on the subject, but show that Torrey's ('07, p. 517) contention that 

 Euglena is unterschiedsempfindlich, and nevertheless turns di- 

 rectly towards or away from light is correct. Thus it turns out 

 that Euglena furnishes a beautiful example of both types of 

 light reactions described by Loeb : and it will also be shown that 

 the mechanism of these reactions is best explained by the theories 

 on the subject advanced by Loeb. 



2. HISTORICAL REVIEW 



In this controversy concerning heliotropism in Euglena there 

 are two questions which have sometimes been considered together, 

 but which for clearness' sake it is essential to separate. They 

 are: 



1. Does Euglena become oriented to light as directly as its 

 method of locomotion admits; or does it orient indirectly, by the 

 method of trial and error? In either case the reaction will be 

 heliotropic, but the method of orientation will be different? 



2. Is heliotropism in Euglena brought about by response to 

 temporal changes in light intensity, or is it caused by the continu- 

 ous action of the light independent of changes in intensity? 



It is only the first question that will be considered in this his- 

 torical review as it is the answer to this that determines whether 

 the heliotropism in Euglena is considered to take place after the 

 manner advocated by Loeb and most of those investigators who 

 consider the tropisms of great importance in the behavior of 



