ORIENTATION IN COLONIAL FORMS 137 



colonies are the more nearly parallel with the rays they 

 swim. If the position of the source of light is changed 

 without a change of intensity, they change their direction 

 of motion until the course bears to the light rays the same 

 relation that it had before. In thus changing their direction 

 of motion they always turn directly toward the source 

 of light without any preliminary movement. No matter 

 which surface is illuminated there is an apparent differential 

 response to localized stimulation. There is no evidence of 

 trial movements in the colonies taken as a whole. They 

 never turn in the wrong direction as Euglena and Stentor 

 frequently do, even if the intensity and the ray direction 

 are changed simultaneously. There is no evidence that the 

 sensitiveness of the colonies depends upon the surface 

 exposed as was found to be true in many unicellular forms, 

 nor is there any indication of an avoiding reaction when 

 the intensity is changed. If the intensity is much decreased 

 they merely stop forward progress and, because of the effect 

 of gravity, the anterior end turns up. They do not aggre- 

 gate extensively in highly illuminated areas in a dark field 

 as Euglena and various other forms do. They pass from 

 darkness into light and vice versa without any apparent 

 reaction. 



There is no evidence that the direction of rays through 

 the organism, in accordance with Sachs' theory, or that the 

 angle between the rays and the sensitive surface as suggested 

 by Loeb controls orientation. This process is regulated by 

 the relative intensity of light on opposite sides of the colony. 

 The following facts prove this to be true: i. If exposed to 

 light from two sources they swim toward any point between 

 them. The location of this point depends upon the relative 

 amount of light from the two sources as indicated in Fig. 18. 

 2. In the light grader (see Fig. 4) so arranged as to produce 

 a field of light which consists of parallel rays, but in which 

 the intensity gradually diminishes from side to side so that 

 one side of a colony, swimming toward the source of light, 

 is more strongly illuminated than the other, it deflects 



