THE PHOTOPERIODIC PROCESS 249 



produce rapid flowering. This is in fact the classical method for deter- 

 mination of the critical night length. If, on the other hand, few dark 

 periods are given to our short-day plant, these must be much longer 

 than the critical value in order to elicit the same flowering response. 

 The amount of flowering stimulus necessary to elicit a rapid rate of 

 flowering may thus be produced all at once during one or a few nights 

 much longer than the critical value, or it may be made little by little 

 during a long succession of dark periods which need then be but little 

 longer than the critical value. It helps us, too, to understand the effects 

 of temperature upon photoperiodic treatment. The critical dark length 

 is changed but Httle by temperatures as low as 5°C, but the number 

 of short-day, long-night cycles needed to achieve a given level or 

 intensity of flowering is greater, the lower the temperature (Hamner 

 and Bonner, 1938). Evidently the events which determine the critical 

 dark length are not greatly temperature-dependent (Long, 1939). 

 The events which follow the critical dark period and which result in 

 the quantitative aspect of flowering are, on the other hand, apparently 

 highly temperature-dependent. And this fact in turn tefls us that we 

 must differentiate sharply between at least two kinds of processes which 

 occur during the long-night treatment of long-night plants. Clearly the 

 temperature-independent processes which are rate-limiting for the 

 first 8.5 hr are different from the temperature-dependent processes 

 that follow. 



Now let us consider photoperiodic induction. We select a plant 

 and give it an appropriate photoperiodic treatment. For soybeans of 

 the Biloxi variety, at least two short-day, long-night cycles are re- 

 quired in order to assure subsequent flowering. In the case of 

 Xanthium, Chenopodium amaranticolor, and Pharbitis nil, but one 

 short-day, long-night cycle is required. In the case of Hyoscyamus, a 

 long-day plant, four long-day cycles must be given to the plant in 

 order to assure subsequent flowering. During the photoperiodic treat- 

 ment, nothing happens in the bud which can be detected even by the 

 most skilled anatomist. But suddenly and some time after the expira- 

 tion of the photoperiodic treatment, flower bud development begins. 

 In the case of Xanthium, the visible changes in the bud which initiate 

 the reproductive state are apparent to the anatomist about 2.5 days 

 after the beginning of the long-night treatment which incited this 



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