EFFECT OF RADIANT ENERGY ON GROWTH 603 



sandth of that of sunlight were found to be adequate for the production of 

 photoperiodic effects on growth. More recently intensities as low as O.i foot 

 candle have been found to induce photoperiodic effects, at least in some species 

 (Withrow and Benedict, 1936). 



The fundamental principles of photoperiodism as discovered b}^ Garner 

 and Allard have been confirmed by a number of later investigators. The most 

 important effect of day-length is upon the reproductiveness of plants. Al- 

 though most plants exhibit better vegetative development when exposed to 

 long photoperiods than short ones this statement does not hold for reproductive 

 development. In general plants fall into three groups: 



(i) "Long-day" species. Species in this group flower more or less readily 

 in a range of day-lengths longer than a certain critical photoperiod. Many 



Fig. 134. Effect of length of the photoperiod on flowering of salvia, a short day species. 



Photograph from Arthur et al. (1930). 



such species flower and fruit even in continuous illumination. At shorter 

 day-lengths than the critical these plants produce solely vegetative organs, in 

 many species developing only rosettes (Fig. 133). 



(2) "Short-day" species. Species in this group flower more or less readily 

 in a range of day-lengths shorter than a certain critical photoperiod. Under 

 longer photoperiods, however, they develop only vegetatively (Fig. 134)- 



(3) "Indeterminate" species. The species falling into this category ex- 

 hibit no critical photoperiod, most of them developing both vegetatively and 

 reproductively over a wide range of day-lengths (Fig. I35)- 



The classification of plants into long-day and short-day tj'pes depends, 

 in brief, upon whether the critical period represents the lower or upper limit 

 of day-length conducive to reproductive growth. The critical photoperiod 



