THE GONYAULAX CLOCK 577 



Sweeney, 1958). The amplitude of the rhythm progressively diminishes 

 when the cells are in the dark, but it remains undamped when they are 

 in dim light. The phase of the initiated rhythm is dependent only upon 

 the time at which the light intensity was changed. Experiments of this 

 nature have been carried out with cells from cultures grown in bright 

 light for a year or more (200 to 400 generations). There is, therefore, 

 no recent history of exposure to day-night conditions, or of informa- 

 tion from Hght or temperature concerning a 24-hr period. 



The converse type of experiment, namely, attempting to "train" cells 

 to periods which differ greatly from their natural period, is illustrated in 

 Fig. 6. The cells were grown under conditions of alternating light and 

 dark periods, as illustrated in the figure, for about 90 hr. In most in- 

 stances, the luminescence changes showed a period of the same length 

 as the imposed light-dark cycle. Subsequent to this "nondiurnal" al- 

 ternating light-dark treatment the cells were left in constant dim light. 

 In each case it is evident that a rhythm with a period of approximately 

 24 hr is resumed. The same result was obtained when the cells were 

 left in darkness after the "nondiurnal" alternating light-dark treatment. 

 Preliminary studies indicate that the rhythm of cell division behaves 

 identically. It may therefore be concluded that a diurnal fluctuation in 

 environmental factors is not obligatory for the functioning of the 

 Gonyaulax clock. It will be of interest to see the results of an experi- 

 ment, now in progress, in which the cells are maintained on a 16-hr 

 cycle for several months or years. 



An important point, demonstrated and discussed in detail in the 

 work of Pittendrigh and Bruce (Pittendrigh, 1954; Bruce and Pitten- 

 drigh, 1957; Pittendrigh and Bruce, 1957; Pittendrigh, 1958), is that 

 the phase of an endogenous rhythm may be shifted (= resetting the 

 clock) by a nonrepeated, relatively brief exposure to changed environ- 

 mental conditions, such as light. Such a treatment was appropriately 

 called a perturbation. As the above authors have noted, this feature 



Fig. 6 {Continued). 



noted from the results with 12-hr cycles at 200 ft-c and 800 ft-c. With 

 the dim light, a type of frequency demultiplication is observed (maxima 

 every 24 hr); with the bright light, entrainment or repetitive resetting 

 occurs, with maxima every 12 hr. The distinction between these phe- 

 nomena is discussed by Bruce and Pittendrigh (1957) and by Pittendrigh 

 (1958). It appears that little consideration has been given to the effect of 

 varying the energy input to the system in experiments such as these. 



