PHOTOPERIODISM 509 



ate photosynthesis is not a factor in floral induction (Snyder, 1940). 

 Photoperiodic control of flowering is effected by radiant energies far 

 lower than these, whether they are used to extend a natural photoperiod 

 or to interrupt a dark period. 



REVIEWS ON PHOTOPERIODISM 



Interest in the subject of photoperiodism is world-wide, and articles 

 are disseminated in the journals of many countries. Since the phenome- 

 non is exhibited by both plants and animals, articles are likely to appear 

 in almost any journal devoted to natural science. For this reason reviews 

 by authors in various countries and fields of activity are of special value. 

 Articles on photoperiodism in plants have been reviewed at frequent 

 intervals, beginning as early as 1925. Garner discussed the subject in 

 1936 and listed several of these earlier reviews. The literature was well 

 summarized again in the period 1936-1939 by Burkholder (1936), Cho- 

 lodny (1939), and Murneek (1937). More recent reviews include those 

 by Borthwick, Parker, and Hendricks (1948, 1950), Chouard (1949), 

 Hamner (1942, 1944), Lang (1949), Melchers and Lang (1948), Murneek 

 (1948), and Parker and Borthwick (1950). Samygin (1946) gave exten- 

 sive lists of plants grouped by families, showing the nature of the photo- 

 periodic response and a reference to the original work. Review papers 

 dealing with photoperiodic responses of animals include those by Bisson- 

 nette (1936a), Burger (1949), Jenner (1951), Marshall (1936), Rowan 

 (1938), and Yeates (1949). 



COGNIZANCE 



The significance of the many observations mentioned in the several 

 sections is only suggested, for each work is circumscribed. Does a pat- 

 tern exist and is knowledge adequate to permit even the barest outline 

 to be measured? The outline is taking shape, and photoperiodism, 

 instead of being only a minor phenomenon, is one expression of a pri- 

 mary process controlling development. The nature of the primary proc- 

 ess is essentially established, but the following steps by which control is 

 effected are still unknown, though subject to experiment. 



How universal is the phenomenon? In bold summary, a similitude 

 exists in control by darkness of reproduction in plants and animals, even 

 though the course for each is unique. The control affects other func- 

 tions that can best be classed together as development. The initial 

 reaction takes place in cells of one part of the plant or animal and affects 

 development of remote parts. Control is shown both in highly developed 

 forms, such as the chordates, and in more primitive ones. 



The control mechanism involves a dark reaction that is recognized in 



