Mast, Light Reactions in Lower Organisms. 169 



about three hours for the organisms to collect in the region ol 

 optimum illumination. The light intensity of this region was 

 16 zt candle meters at the left side and 71 ± at the right. In repeat- 

 ingthis experiment, the apparatus was several times so modified that 

 the more highly illuminated end of the aquarium was to the left. 

 Under these conditions the colonies reacted precisely as they did 

 when this end was to the right. All of them aggregated in the 

 right hand corner of the aquarium, now the region of lowest light 

 intensity, and then gradually spread out until they reached the 

 optimum. 



The reactions of Volvox were also studied with the light grader 

 in such a position that the rays were perpendicular to the bottom of 

 the aquarium in place of parallel with it as they were in the pre- 

 ceding experiments, and with so little water in the aquarium that 

 the organisms were forced to swim at right angles with the rays. 

 In some instances under these conditions, there was no evidence 

 of any aggregation whatever, but in others the colonies collected 

 in regions of optimum light intensity. The limits of the regions 

 in which they collected were, however, not well defined. In a 

 few of the exposures some specimens of Euglena viridis were put 

 into the aquarium with the Volvox colonies. These aggregated in 

 a very definite narrow band, the center of which was in a light 

 intensity of approximately 35 candle meters in every exposure. 

 The Euglenae reached the region of optimum intensity in the course 

 of a few minutes, but it required one hour for any indication of 

 aggregation of Volvox in any of these experiments. 



There was absolutely no evidence of orientation and direct 

 movement toward the region of optimum intensity, neither when 

 the light rays were parallel with the bottom of the aquarium, nor 

 when they were perpendicular to it. If there had been, we should 

 certainly expect the colonies to have reached the optimum in 

 much less time than was required in any of the above experiments. 

 The fact that the colonies reach the optimum seems to be a matter 

 of mere chance, the result of swimming about aimlessly. They 

 are more active in sub- and supra-optimum light intensities than 

 in the optimum and, therefore, tend to come to rest in the latter. 

 It is evident that this would tend to cause them to aggregate in 

 the region of optimum intensity. 



Oltmanns ('92, p. 186) states that he found the optimum light 

 intensity for colonies bearing asexual cells to be higher than that 



