10 ■ S. O. MAST 



this increase in activity is due to a reduction of light energy on 

 the sensitive tissue in the zooids and that it is dependent upon the 

 time-rate of reduction, not upon the absolute amount of reduc- 

 tion. 



If the light intensity in a beam in which positive colonies are 

 oriented is suddenly decreased without in any way changing the 

 direction of the rays, the rate of movement for a short period of 

 time suddenly increases, but if the intensity is suddenly increased 

 there is no response. In negative colonies, however, just the 

 opposite is true. They respond in precisely the same way to a 

 sudden increase but not to a sudden decrease of intensity. This 

 response of the colonies is very striking. It gives one the im- 

 pression of a very marked forward spring, and seems to be in 

 all essentials like the shock-reactions in Euglena. And just as 

 in Euglena it does not occur if the light-energy is gradually 

 changed. Obviously then, this response is dependent upon the 

 time-rate of change of energy and not upon the absolute change. 



The increase in activity in the zooids farthest from the source 

 of Hght, during the process of orientation, appears to be of pre- 

 cisely the same nature as the increase in activity of all the zo- 

 oids, due to a sudden decrease of the light-intensity in the entire 

 field; consequently it would seem reasonable to conclude that it 

 also is due to a sudden decrease of intensity on the sensitive tis- 

 sue in the zooids involved. How can this occur? 



In unoriented colonies the light strikes the anterior face 

 obliquely (fig. 6), and as these colonies rotate it is evident, just 

 as in Volvox, that in each zooid the surface exposed to the light 

 continuously changes. This necessarily causes changes of in- 

 tensity owing ,to the movement of the shadows cast by the trans- 

 lucent bodies in the zooids, particularly the opaque portion of 

 the eye-spots. By referring to figure 6 it will be seen that in the 

 zooids on the side of the colony nearest the sources of light the 

 hyaline portion of the eye-spot is fully exposed, while in those on 

 the opposite side this structure is shaded by the opaque portion. 

 There is consequently a great reduction in the intensity of the 

 light on it as the zooids, owing to the rotation of the colony, are 

 transferred from the former to the latter position and an equally 



