REVERSION IN ORIENTATION TO LIGHT 373 



this time on gradually more became negative, until at 6 p.m. 

 nearly all were negative. At 9 p.m., when the experiment was 

 closed, more of the colonies were scattered than earlier, but 

 there were no positive colonies. In 62.5 and 250 m.c. many of 

 the colonies became negative, but some of them remained positive 

 throughout the experiment. In all intensities lower than 62.5 

 m.c, none of the colonies became negative at all. 



This experiment demonstrates that a certain intensity of 

 hght is necessary to induce reversion in the sense of orientation 

 from positive to negative, that the time required depends upon 

 the intensit}^, and that in strong illumination the colonies, after 

 having become negative, become positive again if they are ex- 

 posed long enough. Whether or not they continue positive in- 

 definitely after the last reversion was not ascertained, nor was 

 it ascertained in this experiment whether or not they would have 

 been negative in low illumination after they had become positive 

 in high. But in all probability these colonies would have re- 

 mained positive in strong and would have been negative in weak 

 illumination indefinitely, for negative orientation in weak light 

 was never observed in dark-adapted colonies; it was observed 

 only after long exposure to intense light, and continued exposure 

 to such light failed to make the colonies negative after they had 

 once become positive in this light. 



The changes that occur in the reactions of Pandorina as indi- 

 cated by the results described above may be visualized by means 

 of a graph presented in figure 2. This graph indicates that 

 dark-adapted colonies of Pandorina when first exposed to light 

 are neutral for a short time, i.e., they do not orient (fig. 2, a-6), 

 that they then become positive, increasing rapidly to a maximum 

 (6-c) then decreasing slowly to a minimum (c-d), after which 

 they become negative, passing through a maximum at e, and that 

 they finally become positive again (f-g). It is probably during 

 this last period that the colonies are positive in strong and nega- 

 tive in weak light. 



As previously stated, this reaction is found only in light- 

 adapted colonies, never in dark-adapted ones. What, now, is 

 the difference in different illuminations in the reaction of colonies 

 in these two states? 



