300 MOTOR RESPONSES 



the stroke. Changes in the direction of the stroke of the flagella are not 

 involved. They are relatively slow responses which occur, even if the 

 luminous intensity is gradually changed. The responses are primarily 

 dependent upon change in luminous intensity, not upon the rate of 

 change. Consequently, there are, in Volvox, two different types of re- 

 sponse: (1) typical shock reactions, and (2) responses which consist 

 merely in changes in activity. 



Holmes (1903) maintains there is no consistent correlation between 

 luminous intensity and rate of locomotion in Volvox. But his methods 

 did not exclude the effect of adaptation. Further work concerning this 

 correlation is therefore highly desirable. 



Orientation. — If a colony of Volvox in a beam of light is laterally 

 illuminated, it turns gradually until it is oriented, and then proceeds 

 either toward or away from the light source. When it is laterally illumi- 

 nated, the zooids, owing to rotation of the colony on the longitudinal 

 axis, are continuously transferred from the light side to the dark side, and 

 vice versa. As the zooids pass from the light side to the dark side, the 

 photosensitive substance in the eyespots becomes shaded by the pigment 

 cup. As they pass from the dark side to the light side, this substance be- 

 comes fully exposed. A rapid decrease in the illumination of the sensitive 

 substance on the dark side of photopositive colonies induces shock reac- 

 tions on this side, and the flagellar stroke increases in its backward phase. 

 A rapid increase in illumination of the sensitive substance on the light 

 side of a colony induces shock reactions. The latter consist of increase in 

 the lateral phase of the stroke of the flagella (Fig. 110). This difference 

 in the direction of the stroke of the flagella causes the colonies to orient 

 gradually, until they are directed toward the light, after which all sides 

 are equally illuminated. Rotation on the longitudinal axis then no longer 

 produces changes in the illumination of the photosensitive substance, and 

 the shock reactions cease. The Volvox colonies continue directly toward 

 the light because, in the absence of external stimulation, they tend to 

 take a straight course. Furthermore, if they are forced out of their course, 

 opposite sides immediately become unequally illuminated, the intensity 

 of the illumination of the photosensitive substance in the eyespots 

 changes, and consequently reorientation occurs. 



If photopositive colonies are exposed in a field of light consisting of 

 two horizontal beams which cross at right angles, they orient and swim 

 toward a point between the two beams. The location of this point de- 



