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RHYTHMS IN PLANTS AND ANIMALS 



EVOCATION OF RHYTHMS BY SINGLE STIMULI 



Plants or animals grown under constant temperature and light 

 conditions sometimes show no diurnal processes, but a single stimulus 

 can evoke the periodicity. This single stimulus may be a light break 

 within continuous darkness. Also the shifting from continuous dark 

 to continuous light, or from continuous light to continuous dark, is 

 sometimes sufficient (Fig. 18; Ball and Dyke, 1954; Biinning, 1931, 

 1935b; Hastings et al, 1956). 



How can we explain this evocation of a periodic process by a single 

 stimulus? It seems that even prior to this evocation, diurnal processes 

 are going on within the cells, although they are not yet synchronous 



Fig. 18. Drosophiki emergence. Evocation of the periodicity by shift- 

 ing from continuous darkness to continuous light. (After Biinning, 

 1935a,b.) 



within the several cells. The external stimulus would have the function 

 of synchronizing the processes within the several cells. There is 

 evidence in favor of this explanation. We can bring about a desyn- 

 chronization by certain light stimuli. The periodicity is now sub- 

 divided into several processes, each of which has periods of about 24 

 hr. A single strong light stimulus offered later on may cause a new 

 synchronization (Biinning, 1931). 



Pittendrigh's experiments on Drosophila have demonstrated this syn- 

 chronization within a population; there is still another evidence for 

 the single individual plant. Without this synchronization we shall 

 observe at any time of the day a great variability of the nucleic 

 volumes. After the synchronization, however, the variability at a 

 certain moment is much lower. In this case, the diurnal fluctuations 



