402 FRANK W. BANCROFT 



B. Correlation of gradual heliotropic orientation and differential 



reactions with respect to the presence and absence 



of these two factors 



We have seen that there is no necessary correlation between the 

 sign of the heliotropic and differential reactions which co-exist in 

 the same individuals. We have now to consider whether helio- 

 tropic Euglenae always show some kind of a differential reaction 

 (either a positive or a negative one) or whether we may have the 

 one without the other. According to Jennings since the helio- 

 tropic orientation is a shock-movement of a somewhat modified 

 kind it ought to be possible to find some kind of a constant relation 

 between shock-movements and tropism. 



Although usually the cultures studied were heliotropic in light 

 of ordinary intensity it occasionally happened that some cultures 

 when treated in certain ways exhibited no heliotropic reactions 

 that I could be sure of. In one of these cases the medium was pea 

 decoction in y^o citric acid. The Euglenae showed no tropism 

 or only doubtful traces of it in diffuse day light, and required 

 a strong arc light to bring about certain heliotropism. In the 

 weaker light, however, the shading reaction was at all times con- 

 spicuous. 



With Euglenae from Culture B it was always possible to bring 

 about the absence of heliotropism by subjecting them to light 

 which was just too bright to cause the positive and too weak to 

 cause the negative tropism. In this case, as we have already seen 

 in the previous section, the shading reaction persisted with undi- 

 minished vigor. Consequently the absence of tropism in these 

 two cases cannot be attributed to the failure of the shading reac- 

 tion ; and we see that it is possible for the differential reaction to 

 exist without the tropic one. Similarly even in strong light in 

 which over 99 per cent of the individuals are strongly negative a 

 few individuals are often seen in which no heliotropism can be 

 observed but which give pronounced illuminating reactions. 



On account of lack of time only one experiment was made to 

 test the correlation of tropism and differential reaction on the 

 positively heliotropic Euglenae from Culture B. Two hanging 



