Kinds of Photoperiodic Flowering Responses • 13 



short days of that season, but merely vegetated and grew large in 

 the field in summer and fall. Biloxi soybeans, no matter when 

 they were planted, would not flower until the sufficiently short days 

 of late summer. Garner and Allard were able to show all this 

 experimentally both by artificially shortening the summer days 

 (placing the plants in light-tight sheds or cabinets at various times) 

 or artificially lengthening winter or fall days even with dim 

 incandescent lights. They also examined the effects of various 

 daylengths on other plants and discovered various kinds of flower- 

 ing responses, as well as many other effects. Work on photoperiodism 

 soon became world-wide and has remained so, with major contribu- 

 tions coming from Britain, France, Germany, Italy, Japan, the 

 Netherlands, Russia, the United States, and elsewhere. 



KINDS OF PHOTOPERIODIC FLOWERING RESPONSES 



The flowering responses of various plants to different day- 

 lengths in a normal 24-hour cycle can be roughly grouped into 

 the following classes, of which the first two are those commonly 

 studied. 



1. Short-Day Plants: The abbreviation SDP will be adopted 

 for these hereafter. Flower initiation in SDP is promoted by day- 

 lengths shorter than a particular value, the so-called critical day- 

 length, which differs widely from species to species. It is probably 

 actually the nightlength that is the most critical factor in such 

 plants; hence, they have been described as "long-night plants." 

 Much more work has been done with SDP than with the other 

 classes. Examples are Maryland Mammoth tobacco and Biloxi 

 soybeans, discussed above, also the common cocklebur, Xanthium, 

 and the succulent Kalanchoe blossfeldiana. See the illustration 

 facing page 1 and Fig. 2-1 for two examples of SDP. 



2. Long-Day Plants: The abbreviation LDP will be used for 

 these. Flower initiation is promoted by daylengths longer than a 

 particular value, the critical daylength, which differs from species 

 to species. Again, such plants have also been described as "short- 

 night plants." Examples are the Black Henbane, Hyoscyamus niger, 

 and some varieties of barley, Hordeum vulgare. 



3 and 4. Short-Long- and Long-Short-Day Plants: Flower 



