SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 359 



a greater chemical change or transformation resulting in a greater 

 expenditure of mechanical energy. 



Holmes ('03, p. 323) described the swimming of Volvox toward 

 a source of light as follows: 



It was found that, as the Volvox traveled toward the light, their 

 movement was at first slow, their orientation not precise, and their 

 course crooked. Gradually their path became straighter, the orienta- 

 tion to the light rays more exact and their speed more rapid. After 

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

 uniform until the end of the trough was reached. If the light is so 

 intense that one end of the trough is above the optimum intensity of 

 illumination, the speed of the Volvox is decreased as it approaches this 

 optimum where it finally stops. 



In our experiments the distance to be traversed was only 26 

 mm. We observed that the rate of locomotion became uniform, 

 as Holmes describes, but the organisms always went the whole 

 distance, so that the effect of intensity on the rate of movement 

 which he described is not in evidence. We have, however, often 

 observed that the rate of movement can be from the first rel- 

 atively very rapid, one might say, explosive, as if the colony had 

 been catapulted or had bounded away in the direction of the 

 source of lights. The impression was gained that in many cases 

 the velocity of movement slackened a little after the first out- 

 burst of speed, and that the colony settled down to a uniform 

 rate which was held until the end of the trough was reached. 

 This type of movement was only to be observed in the reactions 

 to the lights of greatest stimulating efficiency. Its rate was not 

 measured, and we are not sure that it can be measured, although 

 it is possible that by dividing the distance by transverse lines it 

 may be done. It was also observed with great clearness in the 

 experiments on the determination of the presentation time. In 

 these experiments, particularly in the responses to the most ef- 

 ficient lights, the Volvox would often continue to the end of the 

 trough, although, as above mentioned, the exposure lasted for a 

 very short time and was followed by the latent period. In many 

 cases, however, the colony would only proceed a short distance. 

 This sometimes happened even in those cases where the first 

 movement was of the explosive nature described, the colony stop- 



