406 FRANK W. BANCROFT 



the negative tropism in strong light can be easily accomplished, 

 as described on pages 400 and 402 above, by subjecting the plants 

 to an intense light for half or three-quarters of an hour. 



This fatiguing of the illuminating reaction was sometimes 

 obtained with the hanging drop cultures, but was obtained better 

 when the Euglenae were exposed to the light in an open trough of 

 tap water. It is thus possible in this case that there may be a 

 chemical or osmotic as well as a light effect. 



We see, then, that in weak light the mechanism for the helio- 

 tropic and the shading reactions become sensitized at different 

 rates; and that the negative heliotropism cannot be fatigued under 

 conditions which easily fatigue the accompanying illuminating 

 reaction. It is evident that here again Jennings's theory will not 

 account for the facts. 



D. Orientation with and without shock-movements 



We have seen on page 391 that Jennings found that when begin- 

 ning to orient to light from a new direction Euglena at first swims 

 in a wider spiral which is evidently to be interpreted as one or 

 more shock-movements.^^ With the negative material from Cul- 

 ture A I found it easy to obtain this reaction when an arc light was 

 allowed to shine suddenly on weakly illuminated individuals. 

 The first result of the bright light was a widening of the spiral 

 which soon merged into the narrow spiral swimming which com- 

 pleted the orientation. We have seen (p. 391) that Mast found 

 that orientation may take place without any widening of the spiral. 

 This observation I confirmed for the negative individuals from 

 Culture A in a weak light. In this light the illuminating reac- 

 tion could not be demonstrated. 



instance, it seems very probable that the photochemical substances responsible for 

 the temporary positive tropism in individuals of Culture A described above are 

 quite different from those responsible for the usual positive heliotropism which 

 can be obtained at will with weak light. 



i^For mechanical, chemical and most other stimuli Jennings interprets such a 

 widened spiral as a single shock-movement. But in positive heliotropism he 

 interprets each part of the spiral in which the swerving is towards the light as a 

 single shock-movement. It seems to me that in this case also the whole of the 

 widened spiral represents a single shock movement. 



