252 CONTROL OF REPRODUCTION 



well be the temperature-independent critical night-length-determining 

 process which we have mentioned earlier. Pigment decay and time 

 measuring having been consummated, the processes which we know as 

 hormone synthesis begin. These are the processes which we know by 

 virtue of the proportionality between number of flowers or rate of 

 flowering and amount by which the dark period exceeds the critical 

 value. Hormone synthesis is inhibited by applied auxin in short-day 

 plants (Bonner and Thurlow, 1949), by dinitrophenol, by cyanide, and 

 by other respiratory inhibitors (Sahsbury, 1957; Nakayama, 1955). 

 We do not know the nature of the active material which is produced. 

 We do know, however, that during the succeeding light period this 

 material must be stabilized, must be transformed, in the presence of 

 high-intensity light, into a new form with thermal sensitivity different 

 from that of the material produced during the dark (Lockhart and 

 Hamner, 1954). Hormone stabilization in the light having been 

 achieved, the floral stimulus now begins its exit from the leaf: the 

 process of translocation gets underway. It is possible for us to time 

 and determine the characteristics of the translocation of the flowering 

 substance by experiments on defoliation such as those of Lockhart 

 and Hamner (1954), of Sahsbury and Bonner (1956), and of Lincoln 

 (1954). Suffice it to say that the translocation of the flowering 

 hormone is a slow process and is not finished until perhaps 20 to 45 

 or more hr after the end of the dark period. When the flowering 

 stimulus arrives at an actively growing bud, the processes of induction 

 proper get underway (Carr, 1953). These are the processes which 

 lead to differentiation, to the initiation of reproductive growth. 



Such are some of the characteristics of the photoperiodic process 

 and of photoperiodic induction. These facts constitute a part of the 

 framework upon which a conference on photoperiodism may build. 



REFERENCES 



Bonner, J., and J. Thurlow. 1949. Inhibition of photoperiodic induction in 



Xanthium by applied auxin. Botan. Gaz., 110, 613-24. 

 Borthwick, H. A., S. B. Hendricks, and M. W. Parker. 1952. The reaction 



controlling floral initiation. Proc. Natl. Acad. Sci. U. S., 38, 929-34. 

 Cajlachjan. M. Ch. 1936. New facts in support of the hormonal theory of 



plant development. Compt. rend. acad. sci. UR.S.S., 4, 79-83. 



