408 FRANK W. BANCROFT 



lumination lasting not longer than the time required for Euglena 

 to make half a revolution on its long axis. 



The negatively heliotropic Euglenae from Culture A (which 

 were the only ones available when these experiments were per- 

 formed) made on an average ten revolutions about their long 

 axis in 5.8 seconds. Half a revolution then would require 0.29 

 second. It should be possible, then, to produce shock-movements 

 by sudden illumination last'ng not longer than 0.3 second. 



The attempt was made to time the stimulus with a photographic 

 focal plane shutter. But the longest illumination which the 

 shutter would give automatically was 0.1 second; and it was 

 never found possible to obtain shock-movements with such a 

 short illumination. Consequently the method of procedure con- 

 sisted in opening the shutter, leaving it open, and then timing 

 the interval between the beginning of illumination and the first 

 shock-movement with a stop watch. The method is unsatisfac- 

 tory, but the results were so clear that further refinement was 

 dispensed with. 



With a comparatively weak arc light which was just strong 

 enough to produce the illuminating reaction in most of the individ- 

 uals, but which caused a pronounced negative heliotropism, it 

 always required an illumination lasting at least 1.0 or 1.4 seconds 

 before the first illuminating reactions appeared. With a much 

 stronger light and sensitive unfatigued individuals the reaction 

 was obtained after 0.5 and 0.4 second. It was probably obtained 

 after still shorter illuminations, though I could not succeed in 

 making the stop watch register less. However, the time could 

 not have been less than 0.1 second for the automatic shutter failed 

 to give results. It is evident that the time required for stimulation 

 with the weaker light is much longer than the 0.3 second which 

 Jennings's theory demands. Consequently, since effective stim- 

 ulation must take place within the time interval of 0.3 second, it 

 cannot be the shock-movement mechanism that is stimulated, 

 but it must be some more sensitive mechanism that is responsible 

 for the gradual heliotropic orientation. Furthermore, since the 

 time required for effective stimulation evidently varies inversely 

 with the light intensity and since Euglena is markedly heliotropic 



