PHOTOTAXIS IN VOLVOX. 323 



would lead us to expect ? Oltmanns' observation on the relation 

 of the rate of movement of I ^olvox to the intensity of illumination, 

 while they apparently do not conform to this theory, were not 

 made sufficiently in detail to form a crucial test of its validity. In 

 order to obtain evidence which would be somewhat more conclu- 

 sive I placed specimens of \~ofoox in a narrow glass trough 

 through one end of which light from an arc lamp was passed 

 after having filtered through an alum cell. The trough was 

 placed over a paper ruled off in spaces a centimeter in width. 

 The specimens were placed at such a distance as was found by 

 previous trials was about the point where they would begin to 

 orient themselves and travel towards the light. The number of 

 seconds required by a specimen to travel across successive inter- 

 vals in its passage towards the light was noted. The experiment 

 was repeated many times, both by using different specimens and 

 by using the same one over again. It was found that, as the 

 Volvox travelled towards the light, their movement was at first 

 slow, their orientation not precise, and their course crooked. 

 Gradually their path became straighter, the orientation to the 

 light rays more exact and their speed more rapid. After travel- 

 ling over a few spaces, however, their speed became remarkably 

 uniform until the end of the trough w r as reached where they 

 would remain. If the light is so intense that one end of the 

 trough is above the optimum intensity of illumination the speed 

 of the }^ohox is decreased as it approaches this optimum where 

 it finally stops. In going away from very intense light ]\th'o.\~ 

 moves at a nearly uniform rate until within a few centimeters of 

 the optimum when the speed begins to diminish. There is thus 

 a lessening of speed as the optimum is approached from either 

 direction. The distance over which there is either a marked 

 increase or decrease of speed is considerably less, however, than 

 the space over which the speed is nearly uniform. 



When we attempt to explain the foregoing facts on the theory 

 that orientation is effected through the differences of the intensity 

 of light on the two sides of the organism we inevitably get into 

 difficulties. If the Volvox acts as a lens concentrating the rays 

 on the side farthest from the light so that that side is more in- 

 tensely illuminated the behavior of the organism would meet the 



