The Life Cycle of the Single Cell 19 



For example, if a growing culture of Gonyanlax is exposed alter- 

 nately to 12-hour periods of light and dark (i.e., a regular day-night 

 cycle), cell division is confined to a very short time just before the end 

 of the dark period. Thus the population increases like a staircase rather 

 than steadily and continuously with time as do most microbial cultures. 

 If now the culture is placed in constant dim light or total darkness, the 

 population continues to mark off 24-hour cycles of rhythmic cell division. 

 Thus the rhythm is persistent even though light, temperature, and other 

 environmental conditions may be constant. ( See Fig. 8. ) 



The same was found to be true of its luminescence. A culture of 

 Gonyaulax was incubated during alternating 12-hour periods of light and 

 dark. At frequent intervals, measurements were made of the intensity at 

 which the cells luminesced, with an instrument somewhat like an ordinary 



Fig. 8. Rhythmic cell 

 division in Gonyau- 

 lax. The upper graph 

 shows the growth 

 curve of Gonyaulax 

 as a staircase rather 

 than as a continuous 

 line, because the 

 cells do not divide at 

 all times of the day 

 and night but only 

 during a short period 

 just before "dawn." 

 The lower graph 

 shows that even 

 when the organisms 

 are removed from 

 the ordinary day- 

 night cycle and 

 placed in constant 

 dim light, the 24- 

 hour rhythm of cell 

 division persists. 



2, 3000 



Period of cell growth 



Period of cell growth 



^ 



Light 



Constont dim light 

 ne in hours 



