134 THE FLOWERING PROCESS 



of light was required following darkness. Thus the plant receives 

 light while it is in the photophile phase and receives darkness only 

 when the cycle had finally shifted to the skotophile phase. Short-day 

 plants, on the other hand, complete their photophile phase in a 

 relatively short time after the lights come on and are beginning the 

 skotophile phase when the night overtakes them. This theory fails 

 to take into account some of the facts of photoperiodism, such as 

 the greater importance of the dark period in relation to the light 

 period, but with a little improvising, most of the facts could be quite 

 well accommodated by Biinning's basic concept. 



To me the important aspect of Biinning's theory is that it postulates 

 an endogenous timer which is basically of an oscillating type. The 

 flowering process is supposed to be tied to this endogenous oscillator, 

 which is capable of measuring the length of the light and the dark 

 periods. Of course Bunning's theory is not this simple. The various 

 phases, along with a number of details which we shall not take time 

 to discuss here, compUcate it. Nevertheless, the fundamental idea 

 is that the requirement for a specific photoperiod for flowering is 

 controlled by an internal oscillator. 



Considerable controversy developed about the theory, which was 

 tested in a number of ways including the following: certain flowering 

 plants were given extended dark periods (2 or 3 days) which were 

 interrupted by flashes of fight ("flashes" lasting sometimes from 2 to 

 4 hr) at various times. According to Biinning's theory, the light 

 should promote flowering if it fell during the photophile phase and 

 inhibit if it fell during the skotophile, and in some experiments this 

 proved to be the case. Furthermore, peaks in promotion (or 

 inhibition) were often separated by approximately 24 hr. Figure 8-1 

 is an example of such an experiment, using soybean on a 72-hr cycle. 

 On the other hand, other workers performed the same experiment, 

 often with diff'erent plants, and were unable to obtain any such 

 clear-cut results. D. J. Carr succeeded beautifully with Kalanchoe 

 while G. Hussey failed completely with Anagallis and Arabidopsis. 

 Biinning said the ones which failed would exhibit persistent rhythms 

 only for 1 or 2 cycles anyway. 



As noted above, Biinning's theory stated that the leaf movement 

 phases were set by the onset of the light period. There were some 

 impressive evidences which seemed to relate this to flowering. A 

 series of soybean varieties, for example, exhibited different degrees 



