THE GONYAULAX CLOCK 575 



cisely balanced. Each of the three overt rhythms shows this same rela- 

 tionship to temperature. Bruce and Pittendrigh (1956) had also con- 

 cluded that compensation was the most likely mechanism for tempera- 

 ture-independent periods in the Eiiglena clock. 



The natural period is also a function of the light intensity. We have 

 studied this only with respect to the rhythm of luminescence, where at 

 higher light intensities the period is shorter than at lower intensities 

 (see Table I). 



Since the rhythmic mechanism in Gonyaulax is capable of marking 

 off 24-hr periods with a fair degree of accuracy, irrespective of tem- 

 perature and light intensity (within limits), the use of the word 

 "clock" seems justified. Although the term "clock" carries with it a 

 functional connotation, the fact that the functional importance of the 

 clock in Gonyaulax is not apparent to us does not seem to be a basis 

 for excluding the use of the word. The overt rhythms which we have 

 observed may, in fact, have a clock significance which we do not under- 

 stand. Also, it is clearly possible that the overt rhythms may be inci- 

 dental results of another, but as yet unidentified, cellular rhythmicity, 

 which does have an important clock function. As Pittendrigh (1958) 

 and Pittendrigh and Bruce (1957) have pointed out, a "master" clock 

 mechanism might necessarily result in a rhythmic aspect to a whole 

 variety of physiological processes. 



Under "natural" conditions, the alternating light and dark periods 

 serve to hold the periods at precisely 24 hr and to determine the phase 

 so that maximum luminescence occurs during the middle of the dark 

 period. But it seems adequately clear from a variety of experiments 

 that the endogenous rhythmicity in Gonyaulax does not require, and is 

 not the result of, previous treatment with alternating light and dark 

 periods. 



After the cells have been kept in bright light (800 to 1600 ft-c). the 

 rhythm of luminescence, and also of cell division, is lost. In bright 

 light, the amount of luminescence which can be evoked from the cells, 

 and also the number of pairs observed, does not vary with time of day. 

 As would be expected, the growth curve is then perfectly straight 

 (Fig. 4). If such cells are then placed in darkness, or if the light in- 

 tensity is reduced to 100 to 200 ft-c, a pronounced diurnal rhythm of 

 luminescence is initiated (Sweeney and Hastings, 1957a; Hastings and 



