ENDOGENOUS DIURNAL PERIODICITY 



511 



a js s 



TIME OF LIGHT BREAK 



Fig. 7. Kalanchoe hlossfeldiana. The plants were offered cycles of 10-hr 

 light and 62-hr darkness. Light period from 20*^ to 1^. The dark period 

 was interrupted by light breaks of 1 hr. Time of light break on abscissas. 

 Controls with continuous light (Ci) or without light breaks in the dark 

 period (Co). (After Melche^s, 1956.) 



cycle and yet continue to show the endogenous periodicity (Aschoff, 

 1955a,b). The same holds true for Drosophila after 15 generations 

 without external cycles (Biinning, 1935b). 



There are genetic differences in the length of the periods which 



Length of Periods 



Fig. 8. Phaseolus multiflorus, leaf movements in continuous darkness. 

 Frequency distribution of length of periods. (After Biinning, 1952.) 



