98 TROPISMS 



of the photosensitive end of the animal produced by swerv- 

 ing toward the light during the previous half of the spiral 

 revolution. Bancroft 21 showed that Jennings's theory- 

 was based upon incomplete facts. 



According to Jennings's view positive keliotropisrn is conditioned by 

 and shonld be accompanied by sbock movements produced by sudden 

 shading (= shading reaction), and negative heliotropism should always 

 be accompanied by shock movements produced by sudden illumination. 



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 Euglence 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. When, however, 

 they were gradually brought nearer to the light a point was reached 

 at which the heliotropism disappeared but the shading reaction per- 

 sisted. When moved still closer to the light they became negatively 

 heliotropic but still without any change of the shading reaction. When 

 moved still closer to the light, there was a short time when no shock 

 movements could be obtained, but soon the illumination reaction ap- 

 peared. 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 reactions 

 frequently disappeared completely, and a most pronounced and compell- 

 ing negative heliotropism held full sway. . . . 



It is very evident, then, that the invariable correlation of positive 

 heliotropism with the shading reaction, which is required by Jennings's 

 theory, does not exist. Both kinds of heliotropism may be associated 

 with either the shading or the illumination reaction. Accordingly, it 

 must be concluded that the heliotropic mechanism does not depend upon 

 the mechanism for the shock movements, but that the two mechanisms 

 are independent. 21 



The simplest method of determining whether or not 

 the orientation of flagellates depends upon rapid changes 

 in intensity of light or upon constant illumination can be 

 furnished with the aid of intermittent light. We know 

 that a striped muscle contracts only when a current is 



