20 The Life Cycle of the Single Cell 



photographic Hght meter. It was found that the organisms luminesced 

 only at night, reaching a peak of brightness at midnight and a low at 

 noon (the upper part of Fig. 9). Then the animals were placed in con- 

 stant dim light, and they still continued to mark off 24-hour cycles of 

 luminescence over the course of many days. Finally, photosynthesis was 



Light 



24 36 48 60 



Dark Light Dark Light h 



72 84 



Constant dim light 



6 12 18 24 30 36 42 

 Light Dark Light Dark Light Dark Light h 



54 66 



Constant dii 



light 



Fig. 9. Rhythnnlc luminescence in Gonyaulax. The upper curve shows that 

 the rhythmic luminescence displayed during the usual day-night cycle per- 

 sists even when the organisms are placed in constant dim light. The lower 

 curve shows that although the luminescent rhythm can be altered by ex- 

 posing the organisms to a different day-night cycle (in this case 6 hours of 

 light, 6 of dark), it returns to the usual 24-hour rhythm as soon as the 

 organisms are placed in constant dim light, and no memory of the unusual 

 rhythm is retained. 



also shown to be rhythmic even in constant dim hght. The cells photo- 

 synthesized only during the daylight hours, reaching a peak at noon. Con- 

 sider how remarkable must be the control mechanism that could force 

 cells incubated in constant light to cease photosynthesis during the night 

 hours even though night was as bright as day! 



