572 



PHOTOPERIODISM IN INVERTEBRATES 



10 20 30 40 



TIME- HOURS 



Fig. 4. Growth curves for Gonyaulax. Under conditions of constant 

 bright light (X) the logarithm of the cell number increased linearly with 

 time and the average generation time was 1 day. Under conditions of alter- 

 nating light and dark periods (O) the growth curve had a staircase shape, 

 and the average generation time was 1.5 days. Dark periods indicated by 

 black bars. Light intensity in both cases was 900 ft-c; temperature, 21.5°C. 



The rhythm of cell division has all the properties of an endogenous 

 rhythm. It will persist for long periods under conditions of constant 

 dim light and constant temperature, as shown in Fig. 5. 



The average growth rates measured in phased or rhythmic cultures 

 have always been found to be less than one division per day. In experi- 

 ments such as those shown in Figs. 4 and 5, therefore, the generation 

 time for some cells must have been approximately 24 hr, while for 

 others it was 48 hr, or a higher integral multiple of 24 hr. This indi- 

 cates that the population is heterogeneous with respect to life cycle and 

 mitotic events. 



