THE GONYAULAX CLOCK 



569 



stimulated. Luminescence in this instance is assayed in terms of the 

 total amount of light emitted when the cells are stimulated to exhaus- 

 tion. Stimulation is accomplished by bubbling air through a cell sus- 

 pension in a test tube for a period of 1 min (Sweeney and Hastings, 

 1957a). When cultures are grown in conditions of alternating light and 

 dark periods of 12 hr each (=LD), the luminescence during the dark 

 period is greater by 40 to 60 times than the luminescence during the 



40 50 60 



TIME -HOURS 



Fig. 2. The curves illustrate the nature of the rhythm of luminescence 

 in Gonyaulax. (A) The cells were exposed to alternating light and dark 

 (black bars) periods of 12 hr each. (B) The cells were maintained at 

 constant temperature (21°C) and at a constant light intensity of 120 ft-c 

 subsequent to treatment with 12-hr light and dark periods as in (A). Note 

 that in (A) the period is exactly 24 hr; in (B) the natural period is 

 about 24.4 hr. 



light period, as shown in Fig. 2A. As shown in Fig. 2B, these fluctua- 

 tions will then continue as a persistent endogenous diurnal rhythm if 

 the cultures are transferred to conditions of constant dim lisht (100- 

 200 ft-c) at constant temperature (Hastings et al., 1956). If the LD 

 cultures are, instead, transferred to darkness, the rhythm will continue 

 for several days, but with a decreasing amplitude (see Fig. 7). This 

 amplitude decrease occurs because the nutrition of the organism re- 

 quires light. 



