422 FRANK W. BANCROFT 



Here, then, we have a widening of the spiral which is undoubt- 

 edly due to the continuous action of the light; for during the 

 whole reaction the Euglenae remained accurately oriented^^ to the 

 arc light and consequently there was no opportunity for any part 

 of the organism to be subjected to changes in light intensity. Fur- 

 thermore this continuous light affects the locomotor mechanism 

 in the same way that it is affected during the gradual heliotropic 

 turning. There is a slight widening of the spiral which stops 

 instantly when the light is reduced and does not continue for an 

 appreciable time after the stimulus as is usually the case with the 

 motor reactions. The only difference is that during the helio- 

 tropic turning the plants are not oriented and so they swerve more 

 dur'ng one portion of the spiral than during the other. 



But while this reaction does prove that the continuous light 

 action may cause a widening of the spiral, and makes it very prob- 

 able that it also causes the gradual heliotropic turning, it does 

 not prove that the heliotropism is caused by the continuous action 

 of the light. For, during the heliotropic orientation, the lateral 

 illumination gives an opportunity for the sensitive region to be 

 subjected to changes of intensity resulting from the revolution 

 on the long axis ; and it might be urged that these changes furnish 

 the orienting stimulus. 



It is not contended that the mechanism which underlies this 

 reaction is identical with the heliotropic mechanism. For, helio- 

 tropism ordinarily occurs without this widening of the spiral 

 during illumination, and conversely some of the exceptional 

 individuals which are not heliotropic even in a bright light occa- 

 sionally show the widened spiral during ilhimination. 



However, it may be that this mechanism is identical with that 

 which controls the gradual heliotropic orientation. We know that 

 the stronger the light and the longer it lasts the more pronounced 

 the negative heliotropism becomes. It may be that this contin- 

 uous widening of the spiral is merely the last sl^ep in the increase 

 of the strength of the negative heliotropism. But what this 



'* These experiments were performed with horizontal parallel rays and the Eu- 

 glenae were placed in a trough with parallel glass sides so as to reduce the lateral 

 light to a minimum. 



