[Chap. XXXI 



INITIATION OF FLOWERS 



34-; 



Fig. 147. Preconditioning effects of temperature on the development of flower 

 primordia in celery plants. The plants pictured in the center row grew in a cold 

 frame (40-50° F.) for 35 days during their seedling stage before they were 

 transferred to the field. The plants in the next rows in the foreground were kept 

 in a warm greenhouse during their seedling stage. They have continued to grow 

 vegetatively, while those in the center row have borne flowers and seeds. Photo 

 from H. C. Thompson. 



plant physiology is not sufficient to enable us to explain these precondi- 

 tioning effects either of temperature or of light. 



Since the initiation of flowers may be either promoted or inhibited 

 by temperature or by the photoperiod, one of these external conditions 

 may either annul or accentuate the effects of the other. What happens 

 when the effects of the one are opposed to the effects of the other in a 

 particular plant? At the present time one may obtain an answer to 

 this question by only one of two ways. Someone may have already 

 discovered the answer and reported it. If the answer cannot be found 

 in botanical literature, the only way to obtain it is by a series of experi- 

 ments. 



When a plant is exposed to a length of day near its critical photo- 

 period, it is more readily influenced one way or the other by changes in 

 temperature or some of the factors that affect photosynthesis and protein 

 synthesis. For example, barley is a long-day plant; when exposed to a 

 photoperiod near its critical day-length it will bloom or remain vegeta- 

 tive, depending upon the temperature and upon the factors that affect the 

 relative rates of photosynthesis and protein synthesis. What has just 

 been said may be made clearer by the following summary: 



