ENDOGENOUS DIURNAL PERIODICITY 527 



controlled by the internal clock. It seems, however, that this clock 

 may have an influence on diurnal changes in the well-known red far- 

 red pigment system. In photoperiodic experiments, we may establish 

 antagonistic diurnal changes in the sensitivity to red and far red 

 (Biinning and Konitz, 1957). 



Time memory is a further phenomenon in which the endogenous 

 periodicity definitely is involved. Bees use the clock as an alarm clock. 

 They fix the time of day on this clock because the search for honey at 

 a certain distant place proved to be successful on preceding days. 

 They are even able to mark several points that mean different spots on 

 their alarm clock. 



Several animals use the clock in a more striking way. It is known 

 that certain birds, bees, and crabs use a solar compass for their homing 

 movements. This solar compass could possibly mislead them in case 

 they neglect the movement of the sun during their absence from their 

 homes. The animals, however, do not neglect this; instead they correct 

 the angle needed between the direction of the sun and the direction 

 of their way home. They use their internal clocks to decipher this 

 (Hoffmann, 1954; Kramer, 1953; Papi, 1955). 



In summary, we may state that apparently for very different 

 phenomena, such as photoperiodism in plants and animals, sense of 

 time in lower and higher animals, and regulation of the diurnal sleep- 

 ing and activity rhythm, the same cellular clock system is used. 



A ddendum 



Further experiments in plants and animals (Biinning, 1958a,b) 

 suggest that the cycles of the endogenous periodicity are periods of a 

 relaxation oscillator. This is confirmed by offering low temperature or 

 poisons in different phases of the cycle. There is, in both plants and 

 animals, one phase of several hours which cannot be delayed very 

 much by chilling; but low-temperature treatment of the other phase 

 makes the oscillator drop to its zero value, thus causing the delay 

 mentioned in the paper. The former is the relaxation, the latter the 

 tension phase. If chilling is less extreme, the system still may oscil- 

 late, but the tension is interrupted by relaxation earlier than normally. 

 This explains the extreme short cycles with these temperatures (for 

 instance, 10°C) mentioned in the paper (Fig. 12). 



