524 RHYTHMS IN PLANTS AND ANIMALS 



shifting is also possible after the synchronization. Even short light 

 periods of a few minutes, offered during the normal dark period of 

 diurnal light-dark cycles, are effective. By exposing the plant or animal 

 to a reversed 12: 12-hr light-dark cycle, the phases are shifted by 12 

 hr. In both plants and animals, about 2 to 8 days are necessary for 

 the complete inversion (Aschoff, 1955b; Brown and Webb, 1949; 

 Hemingsen and Krarup, 1937; Kleinhoonte, 1929). 



The regulating influence of light-dark cycles is also made evident 

 by the fact that under normal conditions the length of the periods is 

 regulated to exactly 24 hr, whereas under constant conditions the 

 periods may be between some 22 and 28 hr. But this modification 

 works only within certain limits. In higher and lower animals, as well 

 as in many plants, the internal clock will not follow external cycles of 

 less than about 16 to 20 hr. If for instance, we offer cycles of 6 hr of 

 light and 6 hr of darkness, the organism will show only its specific 

 cycles of approximately 24 hr (Aschoff, 1954; Hemmingsen and 

 Krarup, 1937; Kleinhoonte, 1929; Tribukait, 1956). 



In certain green plants this regulation works only with orange and 

 far-red light. In a fungus we could establish only a blue light efficiency. 

 In certain animals all the qualities of visible light are effective (Rem- 

 mert, 1955). 



We may also mention that other factors, such as cycles of high and 

 low temperature, have a regulating effect. 



PROCESSES CONTROLLED BY ENDOGENOUS PERIODICITY 



The internal clocks may, as we mentioned before, control processes 

 such as mitosis, growth, turgor pressure, spore discharge, motility, and 

 luminescence. Of special importance is the influence of the internal 

 clock on quantity and quality of the sensitivity to light and tempera- 

 ture. Because of this influence, the internal rhythm becomes important 

 for photoperiodism and thermoperiodicity. 



Photoperiodic reactions enable plants and animals to get the essen- 

 tial information on the course of the seasons. This means that the 

 organisms have inherited a measure which they compare with the 

 actual length of day or night. 



Our first conception of this measure was rather simple. With the 



