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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



perature insensitive, as it is in the circadian 

 rhythms. Phytochrome shift would be expected 

 to be more responsive to temperature. It was 

 shown (43) that the critical night, as well as fhe 

 time of maximum sensitivity to a light break 

 (39. 48). was not affected by threshold light of 

 intensities capable of inhibiting flowering (fig. 

 T). If this light inhibits flowering by converting 

 the phytochrome system, then time measurement 

 can clearly not be a matter of phytochrome 

 changes. 



Hamner and his students (24) have found that 

 various short-day and long-day plants exhibit 



alternating periods of sensitivity to light (fig. 8) 

 in a manner reminiscent of circadian rhythms, 

 lie and others suggest that time measurement in 

 photoperiodism is more closely related to oscillat- 

 ing time measurement in the circadian rhythms 

 than it is to an hourglass mechanism of timing, 

 such as pigment shift. 



We have investigated this problem for several 

 years (4^- 43)- When experiments were set up 

 as in fig. 9. critical day could be studied. It was 

 found that flowering would not occur unless an 

 intervening light period exceeded about n hours. 

 The experiment of fig. i) indicates an increase in 



100-1 



4 8 12 



HOURS OF DARKNESS 



16 



24 36 48 60 



TOTAL CYCLE LENGTH 



INCLUDING EIGHT HOURS OF LIGHT 



72 



Figube 7. — Flowering response of cocklebur to length of 

 night (top, J,S) and time of light break (bottom, 48), 

 as affected by threshold light (10 (i\\ em."-) during the 

 dark period. Floral stages represent increasing levels 

 of flowering nine days after treatment (for example, 

 0=vegetative, 3 = first visible flower primordia, 6=flow- 

 er primordia covering all but tip of inflorescence pri- 

 niordium). 



Figure 8. — Rhythmical flowering responses of Biloxi soy- 

 bean after 7 light-dark cycles (H). Top: Effects of 

 time of 4-hour interruptions of a 64-hour dark period. 

 Upper bar on abciscca shows plan of experiment : 8 

 hours light, 64 hours dark: bottom bar shows postu- 

 lated photophile (white) and skotophile (black) phases. 

 Bottom : Effects of length of dark period, following 

 8-hour light period. 



