512 



RHYTHMS IN PLANTS AND ANIMALS 



proved to be constant for many generations (Aschoff, 1955a,b; 

 Biinning, 1932; 1935a). The average values are between 22 and 28 

 hr. Within a population we shall find a certain variability due to 

 modifications and to genetic differences (Biinning, 1932; Fig. 8). 



a X b 



22 23 24 2S 26 27 23 21 22 23 2< 2S 26 27 28 



Length of Periods, hours 



Fig. 9. Phaseohis nudtijiorus. Variability of periods in strain a and b 

 (left), and in Fj and F., after crossing a x b (right). (After Biinning, 

 1935a,b.) 



After crossing individuals that differ in the length of their genetically 

 fixed periods, we shall observe in the next generation average values. 

 It is not yet clear whether or not the splitting in the F2-generation 

 occurs according to Mendelian laws (Biinning, 1935a; Fig. 9). 



INFLUENCE OF TEMPERATURE ON LENGTH OF PERIOD 



Immediately after a sudden increase or decrease of temperature 

 which had previously remained constant, we may observe the expected 

 increase or decrease of the length of the periods (Biinning, 1931, 

 Kalmus, 1935; Table I). The first experiments of Kalmus, published 

 in 1935, showed the astonishing result that in bees this temperature 

 influence apparently can be nullified by some process of compensa- 

 tion. 



The same conclusion can be drawn from experiments of longer 

 duration. Several days after the influence of a low-temperature period. 



