PHOTOTAX1S IN VOLVOX. 325 



parallel with the rays, and the other cells at various angles de- 

 pending upon their position in the colony. To account for the 

 orientation of the colony we are thrown back upon the problem 

 of the mechanism of the process whereby each individual cell 

 places itself so that its eye spot faces the source of light. The 

 behavior of the cells of the colony, according to this interpreta- 

 tion would fall into Prof. Mark Baldwin's somewhat extensive 

 category of imitative activity, in that each cell reacts so as to 

 secure more of the stimulus affecting a specialized portion of its 

 structure. How and why the cells so react we still have to ex- 

 plain, and various theories of orientation may be applied to the 

 individual cells. The orientation of the colony may be accounted 



for rather more simply, however, if we suppose that the eye spots 

 are most sensitive to licrht striking them at a certain angle such 



C> * i-J 



as is indicated in the diagram by the lines ab and cf. If rays of 

 light enter the colony in the direction of the lines ab and (/"some- 

 what obliquely to the long axis, AP, the flagella of the cell repre- 

 sented on the upper side of the diagram would beat more vigor- 

 ously and accelerate the motion of that side of the organism. The 

 opposite cell being struck by rays in the direction cd would be 

 less stimulated, and, as the flagella would beat less strongly than 

 those on the other side of the colony, the organism would swing 

 about until its long axis is brought parallel with the rays when, 

 being equally stimulated on both sides, it would move in a 

 straight course towards the light. We do not have to suppose 

 that each cell makes a special effort to orient itself at a particular 

 angle to the rays, but that it is so organized that the effective 

 beat of its flagella is most accelerated by light striking the cell 

 at a certain angle. If the cells were most stimulated by light 



