374 S. O. MAST 



Numerous observations were made to ascertain this. In all 

 of these observations dark-adapted colonies were put into one 

 rectangular aquarium and light-adapted ones into another. 

 These two aquaria were then placed side by side at the desired 

 distance from the source of light and the reactions of the colo- 

 nies compared. Frequently three sets of such aquaria were 

 exposed in different illuminations at the same time. 



The results obtained appear to be extremely contradictory. 

 Usually the light-adapted colonies were found to be more strongly 



Fig. 2 Graph representing changes in the sense of orientation in dark-adapted 

 Pandorina exposed to constant illumination of high intensity. The horizontal 

 axis represents time, the vertical axis degree of orientation, above the base line 

 positive, below the base line negative. The graph indicates that Pandorina was 

 first neutral (a-b) then became positive, increasing to a maximum (b-c) then de- 

 creasing to a minimum (c-d), after which it became negative, increasing to a maxi- 

 mum (d-e), then decreasing to a minimum (<?-/), after which it again became 

 positive (f-g). After the colonies reach the last stage they are probably in a 

 state such that they are positive in strong and negative in weak illumination. 



positive in strong and less strongly positive in weak light than 

 the dark-adapted ones, but in many instances precisely the oppo- 

 site was found to be true and in some there was no appreciable 

 difference in the reaction. Moreover, in a given intensity, e.g., 

 4,000 m.c, the light-adapted were in some instances more strongly 

 positive and in others less strongly positive than the dark- 

 adapted colonies. How can these puzzling results be explained? 

 Dark-adapted specimens, as we have seen, are frequently neu- 

 tral when first exposed to light, then they become strongly posi- 

 tive and later negative. If they are in the strongly positive 

 stage when their reactions are compared with those of light- 



