302 MOTOR RESPONSES 



pends upon the relative intensity of the Hght in the beams. The higher 

 the intensity in one of the beams in relation to that in the other, the 

 nearer to the former the point is. If the intensity in the two beams is 

 equal, the point is halfway between them. The colonies are oriented 

 under these conditions, opposite sides are equally illuminated, both in 

 reference to intensity and direction of the rays, i.e., the angle of inci- 

 dence at the surface of the colony. If the intensity in the two beams is 

 not equal, the illumination of the oriented colonies is higher and the 

 angle of incidence greater on one side than on the other. However, when 

 a colony is oriented in a field of light, no matter how unequal the in- 

 tensity from different directions may be, transfer of the zooids from side 

 to side in consequence of rotation on the longitudinal axis causes no 

 responses. In other words, the effect of unequal illumination on opposite 

 sides is equal. This obviously must be correlated with the difference in 

 the angle of incidence. 



Mast (1927a) and Mast and Johnson (1932) demonstrated that the 

 location of the point of focus in the eyespot varies with the angle of 

 incidence. By ascertaining the location of these points in the eyespots on 

 opposite sides of the colonies, in relation to the relative intensity of the 

 two beams, they calculated the distribution of sensitivity and found that 

 the photosensitive substance is much more sensitive in the central regions 

 of the eyespot than at the periphery (Fig. 109). The stimulating effi- 

 ciency of light, therefore, depends upon the location of the point of 

 focus; this, in turn, depends upon the angle of incidence. The equal effect 

 of light on the sides of colonies which are unequally illuminated on op- 

 posite sides when they are oriented, is therefore due to the fact that the 

 point of focus in the eyespots is more nearly centrally located on the side 

 which receives the least light than on that which receives most. 



In photonegative colonies the process of orientation is precisely the 

 same as it is in photopositive colonies, except that decrease in intensity 

 causes increase in the lateral phase, and increase in the light intensity 

 increases the backward phase of the stroke of the flagella. In consequence, 

 the illuminated side moves more rapidly than the shaded side. The 

 colonies therefore turn away from the light source. 



Orientation of Volvox in light is the result of qualitative differences 

 in the action of the locomotor appendages on opposite sides. These dif- 

 ferences are due to shock reactions induced by rapid change in the in- 



