516 RHYTHMS IN PLANTS AND ANIMALS 



for the functioning of the endogenous periodicity. The more the 

 constant temperature deviates from the optimum, the more often we 

 shall find that the clock system fails to work; this means that no diurnal 

 cycles can bs observed (Table VI). 



— 21*'-ki-28%fk 



B 



Fig. 1L As in Fig. 10, but increased temperature for 8 hr, from 0-8 

 (A), 8-16 hr (B). (Unpublished data.) 



With extreme values of temperature the clock fails completely. In 

 that case, for instance in Phaseolus exposed to a temperature of about 

 10°C, the diurnal cycles stop suddenly, and a very irregular periodicity 

 occurs, in many cases showing cycles of 8, 10, or 14 hr (Fig. 12; 

 BUnning and Tazawa, 1957). There is no gradual shifting from the 

 diurnal cycles to these very short ones. Either the clock operates on 

 the 24-hr cycle or it fails completely (Fig. 12A). This seems to be 

 true also with respect to animals. Brown and Webb (1948), in their 

 experiments on the diurnal rhythmic color change in the fiddler crab, 



