PHOTOPERIODISM 



507 



initiation of Biloxi soybean, growing on favorable photoperiodic cycles, 

 is less at 13° and 30°(^ than at intermediate temperatures for the dark 

 period irrespective of light-period temperatures between 13° and 30°C 

 (Parker and Borthwick, 1939). This temperature effect is on the leaf 

 blade, which is the organ of perception (Parker and Borthwick, 1943). 

 That the effect was not on translocation was demonstrated by cooling 



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I 31 I 31 30 30 29 29 28 27 27 26 26 

 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



Fig. 10-5. Light and dark periods at 10°-50° north latitudes. Light periods include 

 morning and evening twilight, beginning and ending, respectively, with the sun 6° 

 below horizon. {Data from American Nautical Almanac, 1943, U.S. Naval Observatory.) 



the petioles of leaves receiving photoperiodic cycles favorable to flowering 

 (Borthwick et al., 1941). It was necessary to cool the petiole to 4°C to 

 block the response, and at this temperature the transport of other meta- 

 bolic products was blocked. Flowering of Allium cepa is inhibited by 

 high temperatures under all photoperiodic conditions (Heath, 1943; 

 Holdsworth and Heath, 1950). Bulb formation in A. cepa, which 

 involves cessation of mitoses in the primordial leaf tips and consequent 

 production of bladeless scale leaves (Heath and Holdsworth, 1948), is 

 also markedly temperature-dependent, being favored by short dark 



