HELIOTROPISM IN EUGLENA 401 



motor reaction when shaded, and then to become oriented again 

 and to swim away from the Ught when the shading screen was 

 removed. 



It might be contended that when the individuals of Culture 

 B are shaded, the weaker light resulting from the shading, makes 

 them positively heliotropic, and so of course they give the shad- 

 ing reaction. While this objection can be met, it is hardly 

 worth while to consider it at length here, for two other obser- 

 vations have been made to which this objection cannot possibly 

 apply. 



Ordinarily the only heliotropism that could be observed in 

 individuals of Culture A was the negative, and the only differ- 

 ential reaction was the illuminating reaction (fig. 4 upper A). 

 But in a few exceptional samples most of the Euglenae, while 

 preserving their negative heliotropism, gave the illuminating 

 reaction in a strong light, and a pronounced shading reaction in 

 a weaker light (fig. 4 lower A). Since these individuals never 

 become positively heliotropic the above objection will not hold. 



In one exceptional sample from Culture B the individuals 

 did not exhibit the usual negative heliotropism when subjected 

 to the strong arc light; but became still more strongly positive. 

 When they were shaded by interposing a screen, no shock- 

 movements resulted. But when the shading screen was re- 

 moved the Euglenae gave pronounced shock-movements, and 

 then rapidly became oriented, and swam towards the light. 

 Here also the above objection will not hold, because the light 

 which produces the illumination reaction is the same in which 

 they become oriented towards the light. 



It is very evident, then, that the invariable correlation of 

 positive hehotropism with the shading reaction, which is re- 

 quired by Jennings' theory, does not exist. But both kinds of 

 heliotropism may be associated with either the shading or the 

 illumination reaction. Accordingly, it must be concluded that 

 the heliotropic mechanism does not depend upon the mechanism 

 for the shock-movements, but that the two mechanisms are 

 independent. 



