HELIOTROPISM IN' EUGLENA 409 



to much weaker light than the weaker arc hght used in these 

 experiments, it is evident that Jennings's theory is still less able 

 to account for the heliotropism in these weaker intensities. 



F. No blending of shock-movements and tropism as the light 



is diminished 



If, as Jennings believes, the gradual swerving is a series of shock- 

 movements, then each swerve must be considered a much weaker 

 shock-movement than the usual kind in which the change of direc- 

 tion is much greater. Presumably, then, each swerve towards 

 or away from the light during the gradual orientation is produced 

 by a smaller photic stimulus than the usual shock-movements. 

 T therefore studied the shock-movements in very bright light and 

 in light which was just able to cause the shock-movements, in 

 order to see if, with the weaker light, the shock-movements tended 

 to take on the character of the slight swerving which produces the 

 gradual orientation. The results, however, were the reverse of 

 what was to be expected according to Jennings's view. 



Euglenae from Culture A were observed in the hanging drop in 

 the light of the arc lamp. An incandescent electric light at some 

 distance gave just enough illumination to see the Euglenae when 

 the arc light was cut off by interposing a screen. The arc light 

 was allowed to produce orientation, then the screen was interposed. 

 An individual which had not changed its orientation was selected, 

 and observed after the screen had been removed again. 



With the weaker arc light, the sudden action of which sufficed 

 to produce shock-movements in about half the Euglenae, the 

 orientation was good in the majority of the individuals. When 

 the oriented individuals were suddenly illuminated, in the way 

 indicated above, the shock-movements usually threw them out of 

 orientation, arid the orientation was only gradually regained by 

 the slow swerving process. Thus in 20 shock-movements that 

 were closely observed only 7 left the organism oriented. In four 

 cases two or three shock-movements instead of a single one fol- 

 lowed the sudden illumination ; and in two of these four cases the 

 first shock-movement left the organism oriented, but was followed 

 by another movement which destroyed the original orientation. 



