326 S. J. HOLMES. 



falling upon them at such an angle as would result if the rays 

 diverged from a spot in front of the colony and in line with its 

 long axis the conditions for orientation would be fulfilled. Since 

 the eye spots in all the cells face the anterior end of the colony 

 this supposition appears very probable. The foregoing explana- 

 tion of the orientation of Vofaox may or may not be the true one, 

 but it enables us to see a significance in the peculiar arrangement 

 of the eye spots in this form and is consistent with the results of 

 the experiments we have described. 



How are we to explain the fact that Volvox becomes nega- 

 tively phototactic in light of strong intensity ? We certainly can- 

 not explain the reversal on the supposition that it is due to a dif- 

 ference of emphasis in the phases of the stroke of the flagella. 

 In positive phototaxis the backward phase of the stroke of the fla- 

 gella is the stronger, and if we suppose that with increase of stimu- 

 lation the reverse phase comes to predominate the organism would 

 go backwards instead of forwards. As it is the anterior end of the 

 organism that is directed away from the light in negative photo- 

 taxis it is obvious that the same phase of the stroke of the flag- 

 ella predominates is both cases. The theory of Holt and Lee 

 applies here very well ; if the effective beat of the fiagella is great- 

 est on the more illuminated side the organism would naturally 

 turn about so as to point away from the light. This may or 

 may not be the true explanation of the negative reaction ; I see 

 at present no way either of proving or of disproving it. The 

 probability of the supposition is somewhat weakened, however, 

 by the fact that it cannot also be applied to positive phototaxis. 

 It seems probable that light accelerates the action of the .flagella 

 not only by stimulating the eye spots but by acting on other 

 parts of the cells, and we might suppose that, as the light is in- 

 creased, a point would be reached where the stimulus to the 

 other parts of the cell would outweigh the effect upon the eye 

 spots, and the flagella on the side of the organism nearest the 

 light, beating more vigorously than the others, would thereby 

 bring about the negative orientation. We can thus give at least 

 a formal explanation of the phenomenon. There are many in- 

 stances in which an increase in the intensity of a stimulus causes 

 a reversal of the usual reaction, but the reason for this in any 

 case is still an unsolved problem. 



