120 Journal of Comparative Neurology and Psychology. 



On July 26, 1905, the paths of two such diverging columns, 

 observed in light of uniform intensity, were recorded. The one 

 containing the larger colonies deflected to the right, making an 

 angle of nine degrees with the light rays, while the one containing 

 the smaller colonies deflected to the left, making an angle of fifteen 

 degrees. Both columns, however, sometimes deflect to the right 

 or to the left of the light rays. 



Deflection then varies with different periods in the life of the 

 colonies; but it also depends upon the physiological state of the 

 organisms, as is shown by the following observations. 



In the morning, after being in the aquarium all night, Volvox 

 colonies were repeatedly found lying on the bottom, apparently 

 perfectly quiet. They were in a state which may be termed dark 

 rigor. When light is thrown on them while they are in this condi- 

 tion, they do not respond at once. After a time, however, they 

 begin to swim about, slow^ly at first, without orienting; but soon, 

 more rapidly, until they become normally active, and move toward 

 the light. Apparently there is a certain chemical change neces- 

 sary to bring the organisms out of the state of dark rigor into such 

 a condition that they can respond readily to light; and this change 

 appears to be induced by light. The production of carbon dioxid 

 in darkness suggests itself as the probable cause of dark rigor. 



When a colony, after having been in darkness all night, first 

 begins to respond to light, it moves toward the surface of the water 

 and deflects strongly either to the right or left as it proceeds toward 

 the source of light. But if it be made to cross the aquarium several 

 times in succession, it is found that the deflection gradually decreases 

 until it has traveled 25 to 30 cm. Then it reaches an apparently 

 stable condition; and on the following trips it takes a fixed course 

 which may be at almost any angle with the light rays, but is usually 

 at an angle of from 5 to 10 degrees. Such reactions were ob- 

 served many times, mostly in experiments performed for other 

 purposes. The following detailed experiment is typical. 



On August 7, 1905, Volvox was collected at 6.15 a. m. and left 

 in total darkness until 8.30 a. m., at which time the colonies were 

 still moving about, but very slowly. One of them was put into 

 the aquarium in the light grader in a light intensity of nearly 

 400 candle meters. This colony moved about irregularly at first 

 and deflected strongly to the right, but it soon became more active 

 and moved quite rapidly toward the light. On its first trip across 



