[Chap. XXXI 



INITIATION OF FLOWERS 



341 



^'■^ 



y 



Fig. 142. Effects of length of day on tobacco plant. Both plants grew in a green- 

 house during the winter. The plant at the right was exposed to daylight and also 

 to electric light from sunset to midnight, while the plant at the left was exposed to 

 natural daylight only. This is a typical short-day plant. When exposed to long 

 days, this variety will grow 15 feet or more in height and produce upward of 100 

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



posed influences not only the initiation of flowers, but also physiological 

 processes in plants that affect the growth of all plant organs. Here only a 

 few of its relations to the initiation of flower primordia will be discussed. 



Since the species and varieties of plants are inherently different, they 

 are not all affected in the same way when exposed to the same photo- 

 period. Garner and Allard recognized three different groups of plants 

 with reference to length of day and flower formation. 



First, there are the long-day plants, which bloom only when the days 

 are relatively long, usually more than 12-14 hours. The shortest daily 

 photoperiod under which one of these species will bloom is called its 

 critical photoperiod. When exposed to continuous light many of these 

 species grow and reproduce; others fail to survive. These are the plants 

 that regularly bloom in the long days of summer. During the short days 

 of the growing season they remain vegetative, often as rosettes. They 

 will also bloom in greenhouses in winter if the daily photoperiod is 

 lengthened by means of electric lights. This supplemental electric light 



