PHOTOTAXIS AND PHOTOPATHY 



205 



organisms is determined by "the direction of the light rays. 

 There is, then, such a thing as phototaxis. 



But is the direction of locomotion ever determined by a dif- 

 ference of intensity of light in adjacent regions, without refer- 

 ence to the direction of the light rays ? Whole series of obser- 

 vations make this probable ; for a migration to a definite part 

 of the trough has followed unequal illumination by rays 

 perpendicular to the trough. Thus LUBBOCK found that 



J F 



FIG. 60. Two test tubes a and b, containing Porthesia larvae c, which move towards 

 the window FF, although in doing so they pass from a brighter to a darker region. 



(LOEB, '90.) 



FIG. 61. Diagram to show how Porthesia larvae move in the test tube ab towards 

 the window FF, although in doing so they leave the part of the tube more brightly 

 illumined by the sun's rays SS. (LOEB, '90.) 



Daphnias, placed in a trough nearly perpendicular to the rays 

 dispersed by a prism, moved towards the brighter part of the 

 spectrum. GKABEB employed screens of diverse translucency 

 and color, which were placed adjacent to one another, and 

 found that the organisms tended to aggregate opposite the 

 one or the other. OLTMANNS ('92, p. 195) has offered certain 

 new experiments pointing in the same direction. These experi- 

 ments were made upon Volvox minor and Volvox globator, 

 which were placed in a trough between which and the source 



