HORMONES IN RELATION TO 

 VERNALIZATION AND PHOTOPERIODISM 



by 

 Karl C. Hamner 



United States Plant, Sail and Nutrition Laboratory, Ithaca, N. Y. 



In recent years a great many investigations have been concerned with 

 the internal changes in plants which lead to the differentiation of flower 

 primordia and the development of flowers. Much interest has centered in 

 the earliest changes which take place in the plant which mark the initial 

 transition from the vegetative to the flowering condition. This present 

 discussion will deal primarily with these early changes and with several 

 theories which postulate the sequence of events leading to the differentia- 

 tion of flower primordia. No attempt will be made to give an adequate 

 review of the literature since it has been carefully covered in other chapters. 

 Selected references will be made to illustrate particular points. 



Many years ago, Sachs postulated the existence of "organ-forming 

 substances" in plants. He produced no definite evidence for the existence 

 of flower-forming substances. Garner and Allard (12) working with 

 Cosmos sulphureous, a typical short-day plant, demonstrated the localiza- 

 tion of the flowering responses to photoperiod when one part of the plant 

 was exposed to short day and the other part to long day. They demon- 

 strated that a portion of the plant exposed to continuous darkness ex- 

 hibited some response when an adjacent portion of the plant was exposed 

 to short day, indicating some transmission of the short-day stimulus to 

 the darkened portion of the plant. Knott (19) working with spinach, a 

 long-day plant, found that exposure of the bud to long-day conditions did 

 not induce flowering if the leaves were exposed to short day. On the other 

 hand, exposure of the buds to short day did not inhibit flowering if the 

 leaves were exposed to long day. Knott concludes, "the part played by 

 the foliage of spinach in hastening the response to a photoperiod favorable 

 to reproductive growth may be in the production of some substance, or 

 stimulus, which is transported to the growing point." Most of the credit 

 for the recognition of the green leaves as the organs for the perception 

 of the photoperiodic stimulus, however, must go to two Soviet botanists, 

 Cailahjan and Moskov, both of whom, apparently working independ- 

 ently, performed conclusive experiments which indicated that the green 

 leaves first perceive the photoperiodical stimulus which is then transmitted 

 to the growing point. Since then, many investigators have obtained simi- 

 lar results in photoperiodic studies. 



Perception of the Photoperiodic Stimulus : — There can be little 

 question that the green leaves are the organs which perceive the photo- 

 periodic stimulus. This has been shown for both long-day and short-day 

 plants (5, 10, 17, 19, 25). The young expanding leaves seem to be in- 



