12 • Photoperiodism: An Outline 



and in which the term "photoperiodism" first appeared, although 

 the definition favored above is not their original one. These papers 

 are among the classics of plant physiology; not only do they outline 

 many of the major problems still facing students of photoperiodism, 

 but they are also models of the critical, at first almost reluctant, 

 demonstration of what then seemed a revolutionary concept. 

 Although there is no intention here to maintain a historical 

 approach, a brief outline of two practical problems faced and 

 explained by Garner and Allard will serve as a concrete introduc- 

 tion to photoperiodism. 



GIANT TOBACCO AND SEPTEMBER SOYBEANS 



The preceding heading might well have been used by Garner 

 and Allard to summarize the problems that led to their dis- 

 covery. The tobacco, Nicotiana tabacum, was a mutant named 

 Maryland Mammoth since it grew over 10 feet high in an experi- 

 mental plot at Beltsville, Maryland. It nevertheless remained 

 vegetative, thus frustrating its growers who wanted to use it in 

 breeding experiments. Propagated by cuttings and grown in the 

 greenhouse in the winter, however, the mammoth flowered and set 

 seed when less than five feet high. Equally puzzling was the 

 behavior of the Biloxi variety of soybean, Glycine (or Soja) max. 

 When successive sowings were made at two-week intervals from 

 early May through July, all of them showed their first flowers in 

 September, so that the earliest planted had taken some 120 days 

 to flower and the latest about 60. It was as if all were waiting for 

 some signal at which to start flowering, irrespective of their age 

 from germination— an improbable notion that turned out to be 

 correct. 



After eliminating other factors such as temperature variations, 

 nutrition, and light intensity, Garner and Allard concluded that 

 the length of day was controlling flowering in both situations. 

 Both Biloxi soybean and Maryland Mammoth tobacco are short- 

 day plants, a term introduced by Garner and Allard. Neither will 

 iflower unless the daylength is shorter than a certain critical number 

 of hours (which happens to be different for the two plants). On 

 sufficiently short days, flowering takes place. Thus Maryland 

 Mammoth flowered in the greenhouse in winter under the naturally 



