510 



RHYTHMS IN PLANTS AND ANIMALS 



TIME OF DAY 

 ^V 12 Z» 12 Z» n 



21 



n 



Zt 



12 



> 

 > 



in 



z 

 u 

 in 



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o 



X 



a. 



Fig. 6. Euglena gracilis. Diurnal changes of phototactic activity in 

 12: 12-hr hght-dark cycles and in continuous darkness. Dark period cross- 

 hatched. (After Pohl, 1948.) 



(Clauss and Rau, 1956; Melchers, 1956). Reviews on these several 

 phenomena have been published by Aschoff (1955b), Bruce and 

 Pittendrigh (1957), Marker (1958), and Bunning (1956b, 1957b, 

 1958b,c). 



HEREDITY AND LENGTH OF PERIODS 



The endogenous rhythm is estabhshed in the individual plant or 

 animal through heredity and is not induced in early stages of the 

 individual's development. If the young plants or animals are exposed 

 to constant external conditions, they will nevertheless show the en- 

 dogenous rhythm later on. If they are exposed to external conditions 

 with rhythms deviating from the normal 24-hr cycles, their internal 

 clocks will, in spite of this, operate on cycles of 24 hr thereafter. 



This has been established for plants as well as for animals. Klein- 

 hoonte (1932) tried in vain to influence the cycle by such a pretreat- 

 ment beginning with seed germination. Influences on the mother 

 plant during the embryonal development are also ineffective (Biinning, 

 1932). Bees show the normal cycle after having developed under 

 constant conditions (Wahl, 1932). The eggs of chickens and of 

 hzards may develop under constant conditions, but nevertheless the 

 animals later on show the diurnal cycle (Aschoff and Meyer-Lohmann, 

 1954; Hoffmann, 1957b). An abnormal cycle does not influence the 

 rhythm of the next generation of rats (Hemmingsen and Krarup, 

 1937). Mice may hve for several generations without an external 



