HELIOTROPISM IN EUGLENA 423 



reaction does prove is that the continuous action of the hght may 

 affect the locomotor mechanism of Euglena in the same way as it 

 is affected during the gradual heliotropic turning. 



b. Orientation maintained by continuous light action. On page 

 411 it was shown that in bright light the Euglenae are kept 

 oriented in spite of stimuli tending to destroy that orientation. 

 Here it should be emphasized that since the organisms remain 

 oriented, and there is consequently no opportunity for any photo- 

 sensitive region to be subjected to changes in light intensity, that 

 the orientation must be maintained by the continuous action of 

 the light. In this case it seems probable that the mechanism 

 which maintains the orientation is identical with that which pro- 

 duces the orientation, for in strong light all the heliotropic indi- 

 viduals remain oriented, and as the light intensity is increased 

 the prompter turning which results is always accompanied by 

 the maintenance of the more accurate orientation. Accordingly, 

 since these two phenomena always vary together, and since the 

 maintaining of the orientation is undoubtedly a function of the 

 continuous light action, we have very strong evidence that in 

 Euglena the heliotropic turning is also a function of the continuous 

 action of the light as has been maintained by Loeb for many other 

 organisms, 



c. Heliotropism and variations in the rapidity of the change in 

 illumination. The more rapidly Euglena revolves upon its long 

 axis, the more rapid will be the changes in light intensity. If it 

 is these changes in intensity which are responsible for the orienta- 

 tion, then it might be possible by decreasing the rapidity of the 

 revolution on the long axis to abolish the heliotropism without 

 preventing locomotion. A positive result would tell strongly in 

 favor of Jennings's theory, but a negative result would not be 

 decisive either way. My results were negative but are given here 

 because the method seemed to be the most promising one for set- 

 tling the question. 



Hanging drop preparations were made up of mixtures of cul- 

 ture fluid and Irish moss jelly containing one-half, three-quarters 

 and seven-eighths of the jelly. These mixtures reduced the for- 

 ward locomotion more than they did the revolution on the long 



