ENDOGENOUS DIURNAL PERIODICITY 



521 



8 12 16 20 

 TIME OF DAY 



U 



Fig. 16. Phaseolus midtifiorus, epidermal cells of leaf blade. Diurnal 

 changes of volume of nuclei during the first day of continuous light after 

 normal liqht-dark cycles. (After Banning and Schone-Schneiderhohn, 

 1957.) 



hypothesis, the primary processes in the clock are structural changes 

 in the nuclei. Secondary processes are diurnal fluctuations, such 

 as in metabolism, in enzyme activities and growth. Also the well- 

 known diurnal periodicity of mitosis may be one of those secondary 

 processes. But the diurnal structural changes in the nuclei are still 

 going on while mitosis, owing to the ag-ino; of the cells, is no longer 

 possible. 



At any rate this would mean that the internal clock has a cellular 

 mechanism, which actually was almost self-evident for plants, but 

 not for animals. 



1 



4 5 6 



DAYS 



8 



Fig. 17. Daitcus caroui. Diurnal fluctuations in the area of a tissue 

 culture. (After Enderle, 1951.) 



The cellular nature of the internal clock is also contirmed by the 

 possibility of endogenous diurnal processes in tissue cultures of plants 

 (Enderle, 1951; plg. 17). 



