PHOTOPERIODISM 497 



Several theories have been advanced to correlate flowering responses 

 as effected by photoperiodism. Some references to these theories are 

 Bakhuyzen (1947), Biinning (1946), Cajlachjan (1937), Chouard (1943), 

 Gregory (1948), Hamner (1942), Harder and Bode (1943), Lang and 

 Melchers (1943), Lona (1946), Parker et al. (1948), and Withrow and 

 Withrow (1944). In general, these theories separate the phenomena into 

 several parts and postulate one or more active compounds. They are, 

 however, little more than hypotheses to facilitate work. None is dis- 

 cussed here in detail, but some of the basic facts are presented. 



Most of the theories postulate that a photoperiod preceding a dark 

 period is required for formation of a specific active compound. That the 

 photoperiod becomes less effective in promoting flowering when greatly 

 shortened is shown by Xanthium saccharatum (Snyder, 1940) and soybean 

 (Parker and Borthwick, 1940) for photoperiods shorter than 3 hr. It is 

 also known in hundreds of other plants studied in less detail that a num- 

 ber of cycles of light and darkness will induce flowering. In fact, the 

 term "photoperiodism" is closely identified with this behavior and was 

 so conceived by Garner and Allard (1920). Several observations, how- 

 ever, indicate that the Hght period is required only for limiting the dark 

 reaction and replenishing metabolic reserves when these are limiting. 

 Thus controls of leaf and internode elongations in dark-grown seedlings 

 drawing upon the metabolic reserves of the seed are effected with very 

 low absorbed energy (Goodwin and Owens, 1948; Went, 1941). Flow^er- 

 ing of Kalanchoe blossfcldiana (Harder and Glimmer, 1947), a plant with 

 possibly high metabolic reserves, takes place when the photoperiod is 

 only 1 sec in 24 hr with an intensity of 6000-8000 ft-c. In both these 

 cases effect of light on a dark reaction alone is required. Albino plants 

 of Zea mays have been brought to flowering and seed formation, in light 

 of very low intensity, when supplied with sucrose (Spoehr, 1942). Infil- 

 tration of Chenopodium amaranticolor (Lona, 1950) roots and Xanthmni 

 pensylvanicum (personal communication from J. Bonner) leaves with 

 sugar solution in darkness also resulted in floral induction. 



Melchers and Lang (1942) found that Hyoscyamus niger would initiate 

 flower primordia when held on 14-hr dark periods if the leaves were 

 infiltrated with glucose, sucrose, fructose, or maltose. Control plants 

 whose leaves were infiltrated with water remained vegetative. Hyoscya- 

 mus was observed to form flower primordia after adequate cold treatment 

 even though the plants were completely defoliated and held in continuous 

 darkness (Lang and Melchers, 1941). These facts led Melchers and Lang 

 to postulate that certain processes in the leaves of long-day plants hinder 

 floral initiation. 



Other theories attempt to explain the apparentlj'- opposite flowering 

 responses of long- and short-dark-period plants to the same initial photo- 

 reaction. The identity of the initial reactions is best shown by the 



