480 RADIATION BIOLOGY 



ter is turned to that part of the work deaUng with the initial reaction 

 rather than to the details of the final biological expressions, although 

 many of these are given as examples. 



Perhaps one of the most striking peculiarities of photoperiodic responses 

 is that they are often clearly shown by a particular variety of a species 

 but are not apparent in closely related varieties or forms. This negative 

 condition should be held in mind, since it too is a subtle part of the phe- 

 nomenon indicative of a delicately poised system. It dictates careful 

 consideration of the experimental conditions and reference to the forms 

 studied. 



DISCOVERY 



The clear-cut recognition of photoperiodism was due to Garner and 

 Allard in 1920. They found that Nicotiana tabacum var. Maryland 

 Mammoth was entirely vegetative during the normal summer days and 

 those of early autumn in latitude 39° N. In the greenhouse, as the day 

 further shortened and the night lengthened, the variety came into flower 

 even when the plants were very young. Garner and Allard called plants 

 of this type "short-day." They found that floral induction could be sup- 

 pressed by shortening the dark period with light of low intensity. 



Flowering of Raphanus sativus var. Scarlet Globe was promoted by 

 long natural days, Avhereas its root enlargement took place on short days. 

 Garner and Allard emphasized the flowering of this variety and called it 

 "long-day." Climbing hempweed, Mikania scandens, which Allard 

 (1938) studied later in greater detail, was found to be of an intermediate 

 type, flowering most abundantly when the photoperiod was between 13.5 

 and 14.5 hr. Flowering of A^. tabacum var. Connecticut Broadleaf, on 

 the other hand, apparently did not depend upon day length. This 

 variety was recognized as "indeterminate," or "day-neutral." 



Garner and Allard in their first paper raised the question of photo- 

 periodic control in animals, taking migration of birds as an illustration. 

 Rowan (1926) was at that time observing the seasonal response of the 

 finch, Junco hyemalis, and became convinced that photoperiodism was 

 an important factor in the control of sexual reproduction. Marcovitch 

 (1924) earlier had made extensive observations on control of reproduc- 

 tion in Aphididae and had come to similar conclusions. 



During the same period Schaffner (1931) was concerned with the sexual 

 irregularities that had been frequently observed in the flowering of hemp, 

 Cannabis sativa. Hemp grown when the days are long and the nights 

 short is normally fully dioecious, but on shorter days and longer nights 

 male flowers frequently occur on female plants and a few female flowers 

 sometimes appear on male plants. 



Seasonal changes in reproduction are lore for animals. The common 

 opinion seems to have been, perhaps quite naturally, that they were 



