HELIOTROPISM IN EUGLENA 421 



function of the constant intensity of the Ught. The first criterion 

 may be of value in deciding this question, but it is not necessarily 

 conclusive. A discussion of the possibilities involved would not 

 be advisable here. But it may be well to point out that Mast's 

 conception of how it is that the stimulation is unquestionably 

 due to changes in light intensity is somewhat difficult to under- 

 stand in the light of his additional statement that the factor which 

 is primarily involved in allowing the male to recognize the flash 

 of the female to which he becomes oriented is probably the dura- 

 tion of the flash. 



B. Experimental data 



a. Continuous widening of the spiral. In the experiments 

 mentioned on page 30 in which the mechanism for the shock- 

 movements was fatigued without fatiguing the heliotropic mech- 

 anism, another phenomenon was observed which is of interest 

 in this connection. After the Euglenae had been subjected to 

 the light for half an hour or more some were noticed to be mov- 

 ing away from the light in a wider spiral than usual. If these 

 individuals were watched as the arc light was shaded the wide 

 spiral was at once seen to change to the usual narrow type. But 

 it reappeared with the bright light and was seen to persist as long 

 as the light did. No attempt was made to observe this wide 

 spiral for longer than one or two minutes. During that period 

 the width of the spiral was frequently seen to diminish gradually; 

 but its width was still sufficient so that there was no doubt that 

 the spiral was suddenly narrowed when the strong light was 

 shaded. When these selected Euglenae were first subjected to 

 the arc light the usual response was a weak shock-movement, 

 manifesting itself as a very wide spiral, which rapidly merged into 

 the moderately wide spiral which lasted as long as the bright light 

 did. In a few individuals, however, in which the mechanism for 

 the shock-movenaents was still more fatigued, it was found possi- 

 ble by slowly removing the screen from in front of the arc light 

 to bring about a gradual increase in the width of the spiral. In 

 these cases there was no sign of shock-movements, and the wide 

 spiral persisted during the intense illumination. 



