92 LIGHT AND THE BEHAVIOR OF ORGANISMS 



of the spiral and that E. deses swims with one side facing 

 the axis. 



Euglenae in the crawHng state, just as in the free-swim- 

 ming state, may be either negative or positive in their 

 light reactions. The crawhng specimens worked on were 

 however negative to Hght of surprisingly low intensity 

 throughout the entire work. But very few were found 

 which were positive even in diffuse sunlight during the 

 middle of the day, unless the sky was covered with very 

 dense clouds. The cause of reversal in the sense of orien- 

 tation will be discussed elsewhere. 



/. Accuracy of orientation. — In the study of their 

 reactions to light, the Euglenae w^ere exposed either to 

 sunlight direct and diffused, or to light from a Nernst 

 glower, a Welsbach burner or a carbon filament. When 

 exposed to light from a single source, e.g., a Nernst glower 

 so arranged that there is as little reflection as possible, 

 Euglenae orient and move nearly straight toward or away 

 from the light with little deviation, if they are strongly 

 positive or negative; but if they are not, as is frequently the 

 case, they deviate much. Even under the most favorable 

 conditions there is however little similarity between Eugle- 

 nae moving toward a source of light and iron filings moving 

 toward a magnet, a comparison sometimes met with in the 

 literature on reactions to light. In studying Euglenae one 

 always finds specimens which do the unexpected thing. 

 Their reactions are very much less dependent upon external 

 conditions than are the reactions of iron filings. To come 

 to a full realization of this, one need only consider the fact 

 that these organisms may be negative or positive in almost 

 any light intensity or they may not react at all. To predict 

 with any degree of accuracy what these organisms are going 

 to do under given conditions, it is necessary to know much 

 about the history of their past reactions. 



g. Mechanics of orientation in Euglena x in the crawling 

 state. — Nernst glowers mounted in front of a non-reflect- 

 ing background (see Fig. 4) and properly screened in a large 



