TIMING AND THE FLOWERING PROCESS 139 



The hour glass theory implies that a red light interruption of the 

 dark period inhibits flowering because it resets the flowering clock. 

 Figure 7-6 in the last chapter, showing the results of an experiment 

 in which the dark period was interrupted at various times with red 

 light, was first interpreted on the basis that the light interruption 

 resets the clock. Results with cobaltous ion (Fig. 7-7) made this 

 interpretation seem too simple, however. The problem is discussed 

 again below. 



Is Timing in The Flowering Process Based 

 Upon an Oscillator Principle? 



In spite of the rather simple explanation of timing given above, 

 there is ample reason to believe that the problem is more complex, 

 and that an oscillator might be involved. We are faced with the 

 problem of trying to describe and define how an oscillator might 

 work. The basic idea is that the oscillator is in continuous operation, 

 and the phytochrome system acts in some way to "entrain" or couple 

 it with the flowering process and with the environment. We might 

 imagine, for example, that when the plants are first put in the dark, 

 F-phytochrome begins to change to R-phytochrome, and as the 

 F-phytochrome disappears this begins to remove some block 

 between the oscillating time measurer and the flowering process. 

 Time is then measured by the oscillator until the critical dark period 

 is reached, after which flowering hormone begins to be synthesized 

 (Chapter 9). We can state what must be happening in principle, and 

 we could easily construct a physical model, but it is quite difficult to 

 postulate a satisfactory biochemical explanation. Nevertheless there 

 are a number of observations and ideas which do not fit well into 

 the hour glass theory but can be made to fit the entrainment with an 

 oscillator idea. The difficulties encountered, however, make us 

 wonder if anyone has yet thought of the proper explanation. 



1. The Use of Various Light-dark Cycles or Interruption Sequences 

 Karl Hamner (15) at the University of California at Los Angeles 

 and his students have been studying the effects of diff'erent combina- 

 tions of day and night-length upon flowering. They have found that 

 while the soybean, as might be expected for a short-day plant, flowers 

 only when the dark period exceeds a certain length, optimum 



