HELIOTROPISM IN EUGLENA 399 



the increased illumination produced by swerving towards the light 

 during the previous half of the spiral revolution. Consequently, 

 according to this view, positive heliotropism is conditioned by and 

 should be accompanied by shock-movements produced by shading 

 (= shading reaction), and negative heliotropism should always 

 be accompanied by shock-movements produced by sudden illumi- 

 nation ( = illumination reaction) . 



It has been found, however, that this usual association of 

 shock-movements with tropism is not a necessary one, but that it 

 can be destroyed if the proper means be taken. Consequently 

 the view that the heliotropic swerving is a shock-movement must 

 fall. 



"When Euglenae from Culture B were placed in the rays of the 

 arc light, at a distance of four or five feet from the light, they were 

 strongly positively heliotropic and gave the shading reaction 

 ( f , B., fig. 4). When, however, they were gradually brought 

 nearer to the light a point was reached at which the heliotropism 

 disappeared but the shading reaction persisted ( g, B., fig. 4). 

 When moved still closer to the light they became negatively helio- 

 tropic but still without any change of the shading reaction (fig. 

 4, B,, i^). When moved still closer to the light, there was a short 

 time when no shock-movements could be obtained (Fig. 4, B., 

 ^), but soon the illumination reaction appeared (7)- At the 

 same time the negative heliotropism became more prompt and 

 precise. Finally, when the light was still further increased and 

 allowed to act for a considerable time, even the illumination reac- 

 tions frequently disappeared completely (p), and a most pro- 

 nounced and compelling negative heliotropism held full sway. 



The difference between the intensity of the light required to 

 reverse the heliotropism and that required to change the shading 

 into the illumination reaction varied with different samples, and 

 with the same sample at different times. With hanging drops of 

 culture fluid and Euglenae which had just been taken from the 

 jar this difference was sometimes quite small so that care was 

 needed to reverse the sign of the tropism without changing the 

 shading reaction, and even then the negative heliotropism was 

 sometimes only poorly developed. But when the same prepara- 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 4 



