HELIOTROPISM IN EUGLENA 



393 



.t,^—t 



b,' 



'^- 



X^ 



-)a 



Fig. 1 After Jennings ('04, p. 57). "Path followed by Euglenawhen the direc- 

 tion of the light is changed. From 1 to 2 the organism swims forward in the usual 

 spiral path. At 2 the position of the source of light is changed, so that it now 

 comes from behind. The organism then begins to swerve farther than usual toward 

 the dorsal side, owing to the decrease in the illumination of the anterior end. Thus 

 the spiral becomes wider, a and 6 showing the limits of the swerving. At 3 the 



normal amount of swerving is restored Now the organism swerves 



at each turn of the spiral a short distance away from the source of light, as at c, e, g, 



and a longer distance towards the source of light, as at d, f, h At h 



it has in this manner become directed toward the source of light, and there is no 

 further cause for swerving more to one side than the other; it therefore swims in a 

 spiral with a straight axis toward the source of light." 



