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TEXTBOOK OF BOTANY 



tensity of light, through its influence upon photosynthesis, is a very 

 important factor in the growth of flowers and fruits after they are 

 initiated. 



The daily duration of light to which plants mav be exposed is often the 

 most potent of all external conditions in the initiation of flower primordia. 



Fig. 141. The efiect of long and short days on the evening primrose. Both 

 plants were brought into the greenhouse in November. The one at the left was 

 exposed to natural winter daylight and also to illumination from an electric light 

 from sunset to midnight for about two months. The one at the right was kept 

 under the same conditions, except that it was exposed only to the natural winter 

 daylight. This is a typical long-day plant, in nature flowering when the days are 

 long. W. W. Garner and H. A. Allard, U.S.D.A. 



This fact was first clearly recognized in 1920 by two American botanists 

 (W. W. Gamer and H. A. Allard), of the U. S. Department of Agricul- 

 ture (Figs. 141 and 142). They suggested the term photoperiod for the 

 daily light period, or for any other period of continuous light to which 

 the plants were exposed; and the term photoperiodism to refer to the 

 recognized effects of the light period on the development of the plant. 

 The photoperiod. The daily light period to which plants may be ex- 



