6o2 



FACTORS AFFECTING GROWTH 



during the winter months. This commercially valuable variety of tobacco 

 does not ordinarily blossom in the summer v^^hile growing out-of-doors in the 

 latitude of Washington, D. C. When sown in a greenhouse during the 

 winter months, although the plants which develop were much smaller (field 

 grown plants in the summer attained heights of ten to fifteen feet; green- 

 house plants grown in the winter did not exceed five feet in height) they 

 blossomed profusely and produced excellent crops of seed. These observations 

 led to the hypothesis that the dissimilar development of the tobacco plants 

 during the two seasons was due to the difference in the length of day, relatively 



Fig. 133. Effect of the length of the photoperiod on flowering of lettuce, a long day 

 species. Photograph from Arthur et al. (1930). 



short days apparently favoring reproductiveness in this species. Subsequent 

 more critically performed experiments confirmed this hypothesis. 



Later these experiments were extended to include a large number of addi- 

 tional species. "Short day" conditions were provided, during the summer 

 months, by transferring the plants to a dark house after exposure to the desired 

 number of hours of daylight, while exposure to the normal summer day-length 

 provided "long day" conditions. Long day-lengths were obtained during the 

 winter months by supplementing the hours of daylight with the necessary 

 number of hours of artificial illumination. The exact intensity of the artificial 

 light used does not appear to be critical ; intensities no greater than one-thou- 



