122 'Journal of Comparative Neurology and Psychology. 



Moderate increase or decrease in light intensity does not appear 

 to affect the degree of deflection, e. g., the path of a given colony 

 in a light intensity of 400 candle meters was found to be so nearly 

 the same as that of the same colony in an intensity of 100 candle 

 meters that the difference could not be measured. From numer- 

 ous experiments, it appears that in order to influence deflection, 

 the increase or decrease in intensity must be great enough to affect 

 the positiveness of the organism; that is, the intensity must be 

 decreased to somewhere near the threshold or increased to near the 

 optimum. Now the threshold and optimum in different colonies, 

 and in the same colony under different conditions, vary extremely. 

 It is therefore to be expected that the effect of variation in intensity 

 on deflection varies much. This was found to be true experi- 

 mentally. 



The above discussion on the effect of change in light intensity 

 on deflection might lead one to assume that all Volvox colonies 

 could be made to move parallel with the rays, if the proper light 

 intensity were used. This, however, was not found to be true. 

 To bring about such a reaction, not only the proper light intensity 

 is necessary, but the organisms must also be in a certain physio- 

 logical state. Immediately after taking colonies from darkness 

 or very intense light in which they have been for some time, they 

 are in such a condition that no light intensity was found in which 

 they travel parallel with the rays. And many colonies under 

 various other conditions could not be made to swim parallel with 

 the rays. In the above discussion deflection up or down is not 

 considered; by parallel we mean merely without lateral deflection. 



c. Effect of Gravitation on Orientation. 



If Volvox is killed in formol and then transferred to water, it 

 gradually sinks to the bottom, showing that its specific gravity is 

 greater han one. When first dropped into the water there is, of 

 course, no indication of orientation; the longitudinal axis of the 

 different colonies point. in all directions, but as they sink, it is soon 

 found that their axis becomes approximately perpendicular, z. e., 

 the colonies become oriented with the anterior end up. Such 

 orientation is especially marked in organisms which contain num- 

 erous daughter-colonies, but it is apparently accidental or absent 

 in those without. Since the colonies are dead this orientation can 

 be brought about only by a difference in the specific gravity of the 



