PREPARATION FOR RESPONSE TO PHOTOPERIOD 97 



observation seems inconsistent with the reserve foodstuff viewpoint 

 of ripeness for vernalization. One might imagine that the ability of 

 a moist seed to be vernalized would be related to the amount of 

 reserve materials it contains, that is to its size. Such a relationship 

 does not exist. Some very small seeds can be vernalized and some 

 very large ones cannot. If it is a matter of reserve materials, quality 

 would seem to be more important than quantity. 



We have no specific information relating to the nature of ripeness 

 to flower in leaves of photoperiodically sensitive plants. We can, 

 however, probably state in principle that ripeness to flower must be 

 a matter of getting the machinery for response to photoperiod and/or 

 synthesis of flowering hormone constructed and in operating order. 

 The right precursors must be available, and the right enzymes must 

 be on hand; the pigment system (which is probably present all the 

 time) must be coupled to the clock and to the florigen producing 

 system in the proper way. Getting any or all of these processes into 

 functioning condition could be ripeness to flower. The fact that it 

 occurs in a specific organ of the plant, the leaf in photoperiodism or 

 the meristem in vernalization, is another example of the basic 

 biological question of how certain genes (the ones for producing 

 flowering machinery) can operate in certain places and not everywhere 

 in the plant. 



The High Intensity Light Process 



It was known that response to an inductive dark period in the case 

 of short-day plants was best if this inductive dark period was both 

 preceded and followed by light of high intensity. Dim light either 

 before or after (see Chapter 10) the dark period caused a reduction 

 in the degree of flowering. In 1940, Karl Hamner, then at Chicago, 

 clearly demonstrated the necessity for high intensity light before the 

 inductive dark period. He treated plants with short dark periods 

 separated from each other by brief intervals of light. The short dark 

 periods were not long enough to allow synthesis of flowering hormone, 

 nor were the interruptions with light bright enough to allow a 

 significant amount of photosynthesis. If a long dark period followed 

 this treatment immediately it was ineff'ective, and plants remained 

 vegetative, but if plants were exposed to 1 to 3 hr of sunlight after 



