Mast, Light Reactions in Lower Organisms. 119 



many cases a single colony was selected and its course studied, in 

 place ot that of a number of colonies in a group. To my surprise, 



1 found that whereas during the preceding season, 1904, Volvox 

 colonies deflected, with scarcely an exception, to the left, they 

 now deflected to the right more often than to the left. We shall 

 consider the cause of this somewhat in detail later. In all I have 

 records of 174 paths, only a few of which were observed in light 

 of uniform intensity. Seventy-eight of them deflect to the left 

 from 2 to 45 degrees; seventy-five deflect to the right from 2 to 

 45 degrees; and only twenty-one are found in the area between 



2 degrees to the right and 2 degrees to the left, and very few of these 

 are parallel with the rays. 



In these experiments, however, only deflections to the sides 

 were recorded; it is important to note that marked deflection up or 

 down was also to be observed. It becomes clear then, that the 

 colonies which appeared to be moving nearly parallel with the 

 rays when seen from above, w^ere in all probability slowly ascend- 

 ing or descending as they proceeded toward the source of light. 



The cause of deflection — the inability to orient accurately — is 

 complicated. The direction of movement in Volvox is affected 

 by internal as well as by external factors. The effect of some of 

 these factors on orientation or deflection will be discussed under 

 the following headings: {a) Effect of internal factors on orienta- 

 tion; (b) Effect of light intensity on orientation; {c) Effect of 

 gravitation on orientation; (d) Effect of contact stimulation and 

 rotation on orientation. 



a. Effect of Internal Factors on Orientation. 



If a number of Volvox colonies, varying in size, are put into the 

 aquarium at the same time and allowed to swim horizontally 

 toward any concentrated source of light, it will be seen that the 

 larger colonies, especially such as contain numerous daughter- 

 colonies, soon collect along the right side of the group, and the 

 smaller ones, and such as contain only very small daughter-colonies 

 along the left side. In some experiments there was such striking 

 difference between the deflection of different colonies in a group 

 that two distinct columns were formed, which moved across the 

 aquarium at quite a definite angle with each other. The right 

 column in such cases invariably contained most of the larger 

 colonies, and the left most of the smaller ones. 



