Mast, Light Reactions in Lower Organisms. 167 



of the light was 5000 candle meters, the general estimate of the 

 intensity of the strongest direct sunlight, the decrease per milli- 

 meter in the aquarium was 12.5 candle meters. The difference in 

 light intensity on opposite sides of the largest colonies due to direct 

 light could, therefore, not have been greater than 12.5 candle meters. 

 It probably was much less. As previously recorded (pp. 139-141), 

 I found that if the decrease in light intensity is 6.4 candle meters per 

 millimeter in a field of graded light, the deflection is only 1.5 -\-°. 

 It is consequently evident that if the colonies in Oltmanns' appa- 

 ratus moved directly toward the region of optimum light intensity, 

 the direction of such movement was not caused by the difference 

 in light intensity due to rays perpendicular to the sides of the 

 aquarium. It is clear, then, that there must have been sufficient 

 diffusion in Oltmanns' apparatus to affect tne direction of motion 

 of the organisms. 



If diffusion is practically eliminated, will Volvox still be able to 

 reach the region of optimum intensity in graded light, and if so by 

 means of what reactions } These questions are answered in the 

 recorded observation and results of the following experiments. 

 These experiments were performed in the light grader so arranged 

 that the rays of light were horizontal and nearly perpendicular to the 

 sides of the aquarium which contained water 1.5 cm. deep. The 

 field of light gradually decreased in intensity from 238 ± candle 

 meters at one end to total darkness at the other. It was not quite 

 as long as the aquarium, and was a little narrower than the depth 

 of the water, so that the surface of the water and the sides of the 

 aquarium were not illuminated and thus reflection was prevented. 



At 10 a. m., August 26, 1904, a large number of Volvox colonies 

 were evenly scattered in the aquarium along the entire side farthest 

 from the source of light. They started toward the opposite side 

 almost as soon as they reached the water and all deflected to the 

 left, moving across the aquarium in nearly parallel lines, remind- 

 ing one of columns of soldiers. Those in the region of higher 

 light intensity, however, swam noticeably faster than those in 

 regions of lower. The deflection was toward the darker end of 

 the aquarium, but it must be remembered from what has been 

 stated in preceding pages, that this deflection was not in the main 

 due to the difference in light intensity. It would have been in the 

 same direction and only a little greater if the more highly illumi- 

 nated end of the field had been to the left instead of to the rieht. 



