REVERSION IN ORIENTATION TO LIGHT * 375 



adapted specimens, they are likely to be more strongly positive 

 than the light-adapted .specimens; whereas if they are in the 

 negative stage they will be found to be less strongly positive. 

 Other contradictory features in these results can be similarly 

 explained. 



The cause of the passage through the various stages mentioned 

 above, especially the return to positive orientation after having 

 been negative, is not known. It may be associated with what 

 is ordinarily called acclimatization or adjustment or with some 

 sort of a periodicity. However this may be, reversion is un- 

 doubtedly associated with physiological processes or states and 

 these processes are dependent both upon time and intensity 

 of illumination. This the evidence presented clearly indicates. 

 How are these processes related to the amount of light energy 

 received? 



RELATION BETWEEN REVERSION IN ORIENTATION AND THE 

 QUANTITY OF LIGHT ENERGY 



The relation between reversion and the amount of light energy 

 received was ascertained as follows: Colonies of Pandorina 

 adapted to darkness or to weak light were put into each of 

 seven observation aquaria. These were then exposed in various 

 intensities of light from the 1,000-watt lamp as indicated in 

 table 1. Observations were then made, first at intervals of 

 thirty minutes and later at intervals of one hour. After each 

 observation the position of the colonies in each aquarium was 

 recorded. From these records the approximate time at which 

 reversion occurred in each intensity was ascertained. The re- 

 sults thus obtained appear in table 1. 



When first exposed the colonies in all of the aquaria were 

 neutral and relatively inactive. Gradually they became more 

 and more active, and as they became more active they began to 

 swim toward the side of the aquarium nearest the source of light, 

 where in the course of several minutes practically all had col- 

 lected in a dense mass. This occurred first in the aquarium in 

 the highest and last in that in the lowest illumination. Later 



