PHOTOPERIODIC CONTROL OF FLOWERING 285 



answered. We know reasonably well, for example, that the plant 

 measures the duration of darkness, but we still do not know how it 

 does this even though time-requiring events that occur during the 

 dark period are known. For example, the plant is unresponsive to red 

 light at the beginning of darkness, but it becomes responsive after a 

 measurable period of a few hours. Reappearance of responsiveness to 

 red probably must occur before actual flower-promoting reactions 

 start, and these presumably must run for a time to reach the threshold 

 of flowering. The time required for change of the pigment system from 

 the far-red- to the red-absorbing form is probably an important feature 

 of the time-measuring mechanism, but it still is not clear why these 

 two events have a combined time requirement equal to a critical dark 

 period. 



The reappearance of reversibility itself also is not completely under- 

 stood. In germination of light-sensitive seeds, which is controlled by the 

 same reversible reaction as flowerins, there is good evidence that the 

 pigment undergoes thermal change in darkness. One surmises that the 

 reappearance of reversibility of flowering results from a similar type of 

 pigment change, but the time periods involved in flowering are only a 

 few hours whereas in seed germination they are about a day, the length 

 of the period depending on temperature. Lack of agreement of these 

 time periods in seed germination and flowering, however, is not a seri- 

 ous argument against dependence of both types of response on dark 

 conversion of the pigment from the far-red- to the red-absorbing form. 



Effect of Far Red at Beginning of Dark Period 



The promotion of flowering in millet {Set aria italica (L.) Beauv.) 

 (Downs, 1958) by a short far-red treatment at the beginning of each 

 8-hr dark period is in agreement with what one might expect of a 

 short-day plant. It was mentioned earlier that during a critical dark 

 period two things happen: first is the dark conversion of the pigment 

 from the far-red- to the red-absorbing form, which is followed by the 

 onset of flower-forming reactions. In the Downs experiment with millet 

 the dark period was less than the critical one for millet, but he con- 

 verted the pigment in 5 min in far-red-radiant energy instead of in a 

 longer time in darkness and thereby left almost the entire 8 hr for the 

 second event of the dark period, the flower-promoting reactions. 



