Literature • 29 



is more often true in very recent research, since air conditioning 

 now makes it possible to grow plants entirely under high intensi- 

 ties of artificial light. This frequently results in comparisons between 

 plants grown, for example, in 8 and 16 hours of light per day, 

 comparisons with the implicit or explicit assumption that the 

 operative difference between treatments is in light duration, even 

 though the total light energies also differ proportionately (see, 

 for example, Galston and Kaur, 1961; also portions of Went, 1957). 

 It would help clarify the literature if the term photoperiodic were 

 properly restricted to effects that have been concurrently or pre- 

 viously shown to be controlled by light and dark duration and 

 timing, as indicated by light-breaks or low-intensity supplementary 

 illumination. Any other use of the term only results in confounding 

 photoperiodism with the effects of greatly increased or decreased 

 photosynthesis, or other light actions. 



LITERATURE 



The literature on photoperiodism is vast. Some of the most use- 

 ful reviews are by Lang (1952), Naylor (1953), Bonner and Liver- 

 man (1953), Borthwick, Hendricks, and Parker (1956), and Door- 

 enbos and Wellensiek (1959). A volume edited by the late R. B. 

 VVithrow (1959) contains many valuable reviews and original re- 

 ports on photoperiodism and related phenomena in both plants 

 and animals. 



