Mast, Light Reactions in Lower Organisms. 131 



tion nearly parallel with the general direction of motion to a posi- 

 tion nearly perpendicular to it. This swinging of the axis, it is 

 thought, was due to contact with the bottom and counter-clock- 

 wise rotation, owing to which the posterior end seemed to roll to 

 the left more rapidly than the anterior. This appeared to turn 

 the anterior end of the axis sharply to the right, and since the colo- 

 nies tend to move parallel with their axis, it would cause deflection 

 to the right. Some such reaction must be at the basis of the deflec- 

 tion to the right when the organism is in contact with the vertical 

 wall nearest the light. It may also explain why the larger colonies 

 are found to deflect more to the right than the smaller, since the 

 specific gravity of the two is different. 



I have discussed the cause of the movement of Volvox to the 

 right along a vertical wall at some length because of its importance 

 in the study of the reactions of the colonies in aggregating in regions 

 of optimum intensity in graded light, which will be taken up later. 



7. ORIENTATION TO LIGHT FROM TWO SOURCES. 



In the preceding pages we have conclusively demonstrated that 

 while Volvox moves in general toward a given source of light, it 

 seldom travels parallel with the rays, excepting when they are ver- 

 tical, and it swims upward. But while the colonies do not usually 

 swim parallel with the rays they still orient in a definite way. That 

 is, if a colony is swimming at a given angle with the rays and the 

 source of light is moved, it so changes the direction of motion that 

 its course again makes the same angle with the rays that it did 

 before the position of the source of light was changed. What is 

 the cause of orientation .^ 



Oltmanns, as has been stated (p. 100), came to the conclusion 

 that difference in light intensity is the principal cause of orientation 

 of Volvox, but he presented no direct evidence in favor of this 

 view, and his indirect evidence is based upon experiments which 

 have since been proved to be defective. Holmes was not able 

 to explain orientation by assuming difi^erence in light intensity on 

 opposite sides of the organism to be the cause, and he is inclined 

 to believe that it is due to the direction of the rays. He writes 

 ('°3' P- 324): "It seems not altogether improbable that light 

 in passing through a nearly transparent organism like Volvox 

 exercises a directive efi^ect upon its movements, in a similar way, 

 whatever it may be, to that produced by the current of electricity. 



