358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



In addition to the action of the different wave lengths of light, 

 the factors of intensity and duration exert definite effects on the 

 growing plant. Without going too much into detail we shall first 

 consider some of the interesting growth responses induced by different 

 lengths of daylight. 



DURATION EFFECTS 



Everyone has observed the remarkable regularity with which our 

 common plants come into flowering with the advent of the different 

 seasons. Among the early blooming plants in spring are the arbutus 

 and f orsythia, then the dogwood, and later the iris, and so on into the 

 summer and fall when the asters and chrysanthemums hold the cen- 

 ter of the stage. Although temperature plays an important role, yet 

 the main contributing climatic factor controlling flower production 

 is the length of daylight. Plants like the cosmos which normally 

 flower in autumn when the days are short can be made to flower at 

 other seasons of the year by artificially limiting them to definite 

 hours of light. 



Numerous experiments have been carried out by Drs. Garner and 

 Allard (1920), of the United States Department of Agriculture, 

 which conclusively demonstrate that plants may be made to produce 

 flowers or to continue their vegetative growth by merely regulating 

 the number of hours of exposure to daylight. The lengths of the 

 daily light and dark periods were controlled by moving the plants 

 in and out of darkened sheds. 



These authors conclude from their many studies that plants which 

 are sensitive to length of day fall naturally into two groups which 

 are divided by a fairly definite critical light period. "In the short- 

 day group flowering is initiated by day lengths shorter than the criti- 

 cal, and in the long-day group flowering is initiated by day lengths 

 in excess of this critical period. * * * The essential characteris- 

 tic of the less sensitive or indeterminate group of plants is that they 

 possess no clearly defined critical light period." 



An interesting economic application made of the influence of the 

 length of daylight on plants is that relating to Maryland Mammoth 

 tobacco. This unusually large plant was discovered in southern 

 Maryland, but under the long periods of summer daylight it would 

 not flower or set seed. By growing the plant in the greenhouse dur- 

 ing the winter, seed could be produced. Likewise seed could be se- 

 cured easily by growing the plant in southern Florida during the 

 winter. On the other hand, this short-day plant could be kept grow- 

 ing vegetatively in the winter by supplementing the daylight with 

 electric light. Two such winter-group plants are shown in plate 1, 

 figure 1. The one on the left was exposed to the short-day length. 



