OBSERVATIONS ON UNICELLULAR FORMS 1 23 



the organism becomes directed from the source of Hght. 

 Thus it is that the rotation itself constitutes a trial move- 

 ment. 



(7) There is no evidence that light acting continuously 

 has any influence on orientation. These organisms are not 

 heliotropic in accord with Loeb's definition of this term. 



(8) Stentors probably are more active in higher than in 

 lower light intensity. But even here it is impossible to say 

 whether the greater activity is due to stimulations pro- 

 duced by constant intensity or by changes of intensity, 

 since even in a field of absolutely constant light intensity 

 the movements of the organism cause the more sensitive 

 parts to become alternately shaded and illuminated. 



2. CEdog07iium Swarm-spores 



The reactions of swarm-spores to light have been studied 

 but little. Most observers have merely recorded the fact 

 that they do respond to stimulation by light and that they 

 may be negative or positive. Strasburger (1878, p. 591) 

 found that if exposed in glass jars they collect near the 

 surface of the water at the side facing the window, but he 

 says that they orient very indefinitely and that he therefore 

 did not attempt to analyze the reactions. 



These organisms are very nearly radially symmetrical. 

 It is in such forms, rather than in asymmetrical forms like 

 Euglena and Stentor, that one might expect to find a defi- 

 nite relation between the direction of turning and the side 

 illuminated. In such forms one might also expect orienta- 

 tion to be the result of light acting constantly as a directive 

 stimulation. I was therefore much interested in working 

 out the details in the reactions of these creatures. 



a. Description. — (Edogonium swarm-spores are in gen- 

 eral very much Uke an egg in form (Fig. 15). At the smaller 

 end, the anterior, there is a colorless mound-shaped eleva- 

 tion. The rest of the body is green. At the base of this 

 mound-shaped elevation there is a band of cilia. I was 



