EFFECT OF RADIANT ENERGY ON GROWTH 



60 1 



and yellow light (550-620 m/x) more than twofold. In cultures exposed only 

 to infrared (850-1200 ;/z/i) or kept in complete darkness little change occurred 

 in the number of cells present, but in green light (500-560 mfi) a diminution 

 occurred in the number of cells in the culture. The green region appeared 

 therefore to have an actual destructive effect upon algal cells, and perhaps 

 may exert such an effect upon plant cells generally. 



3. Duration of the Light Period. — In all parts of the world except the 

 tropics and sub-tropics marked seasonal variations occur in the length of the 

 daylight period. At 39° N. latitude (approximately that of Washington, 

 D. C.) for example, on the shortest day (December 21) the sun shines for 

 only about 9^ hours; on the longest (June 21) for about 15 hours. The 

 actual daylight period is always somewhat longer than the number of hours 

 of possible sunshine. At higher latitudes the annual variation in day-length 

 is greater, at lower latitudes less (Table 57). Only in recent years, however, 

 has any systematic attempt been made to evaluate the influence of this factor 

 upon plant development. Investigations during the last two decades have 

 shown that the phenomenon of photoperiodis?n, as the development of plants 

 in relation to the length of the daily light period is now called, is one of the 

 most notable of all reactions of plants to their environment. 



TABLE 57 APPROXIMATE NUMBER OF HOURS OF SUNSHINE POSSIBLE AT VARIOUS 



DEGREES OF NORTH LATITUDE 



Degrees N 

 latitude 



Approximate latitude of 



Hours of sunshine possible 



Dec. 11 



Mar. 21 



Sept. 21 



25 

 27 



29 



31 

 33 

 35 

 37 

 39 

 41 

 43 

 45 



47 

 49 



Key West, Fla 



Palm Beach, Fla. . 

 San Antonio, Tex. . 



Mobile, Ala 



Charleston, S. C. . . 

 Memphis, Tenn. . . 

 San Francisco, Cal 

 Washington, D. C. 



Omaha, Neb 



Milwaukee, Wis. . . 



Portland, Ore 



Duluth, Minn 



Vancouver, B. C . 



0.6 



0.4 



i-5 



!.2 



12 2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 



12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.2 

 12.3 

 12.3 



The foundations of our present knowledge of photoperiodism were laid 

 when Garner and Allard (1920) observed the behavior of plants of the 

 "Maryland Mammoth" variety of tobacco while growing in a greenhouse 



