372 S. O. MAST 



This was repeated many times and the same results were per- 

 sistently obtained. Thus it is clear that, under certain conditions, 

 Pandorina is definitely positive in strong and definitely negative 

 in weak light. What are the conditions under which this occurs? 



It was repeatedly observed that colonies exposed in strong 

 diffuse light, in which they were strongly positive, usually become 

 negative in the evening at the approach of twilight. This sug- 

 gests that the phenomena may be associated with the night and 

 day variations in illumination and that it may be analogous to 

 the so-called sleep movements in higher plants. The fact, how- 

 ever, that it does not occur in dark-adapted colonies and in colo- 

 nies which have been exposed to relatively weak light does not 

 support this contention. 



The physiological condition in which the colonies are positive 

 in strong and negative in weak light is, in all probability, de- 

 pendent upon the amount of light energy received in the immedi- 

 ate past as the following results indicate. 



At 8 A.M., August 23, dark-adapted specimens in observation 

 aquaria were exposed in illuminations of 16,000 m.c, of 4,000 

 m.c. and in lower intensities. Twenty minutes later, 8.20 a.m., 

 a few of the small colonies in 16,000 m.c. were negative, the 

 rest were all strongly positive. The aquarium was left in this 

 illumination in the dark-room and observations were made every 

 hour. The temperature remained practically constant through- 

 out the day. At 9 and 10 a.m., the reactions were practically 

 the same as they had been at 8.20 a.m. At 11 a.m. practically 

 all of the small colonies and a few of the large ones were negative. 

 More and more continued to become negative, until at 3 p.m., 

 practically all were negative. At 4 p.m. a few of the colonies 

 were scattered, all the rest were negative. From this time on 

 gradually more and more became scattered. At 7 p.m. many of 

 the colonies were scattered, some were negative and some were 

 distinctly positive. At 8 p.m. there were more positive colonies, 

 and at 9 p.m., when the experiment was closed, there was a 

 large positive collection. 



In the aquarium in 4,000 m.c. the colonies were at first all 

 positive. An hour later, 9 a.m., a few were negative. From 



