164 



THE FLOWERING PROCESS 



7-0 

 6-0 



< 



4-0 

 3-0 

 2-0 

 1-0 







13 14 15 16 



NIGHT LENGTH IN HOURS 



Figure 9-7 



Data such as those shown in Fig. 9-5 (flowering as a function of night 

 length applied at various temperatures) using plants grown in the green- 

 house in the summer. A number of the points fail to fit the curves as 

 shown, and at short night lengths the curves are drawn primarily with 

 reference to those of Fig. 9-5. The trends are very clear, however, at 

 longer night lengths and for lower and higher temperatures. The 

 symbols are the same as in Fig. 9-5. Data previously unpublished. 



The curves are different, however, after the end of this initial 

 active period (part B of Fig. 9-2). The low temperature curves 

 (15 and 20°C) are still increasing until the end of a 16-hr dark period, 

 indicating that synthesis of flowering hormone is still continuing 

 during this period. At 25°, however, the curve is level, and at 30°C 

 there is a very sharp decrease in amount of flowering during the 

 B part of the curves. 



This decrease can only be understood easily if we assume that 

 flowering hormone, present at the end of the active period, is some 

 way metabolically removed or destroyed or at least made less 

 effective by longer dark periods at the high temperatures. At 10°C 

 this process seems to be essentially ineffective, but the initial synthesis 

 is also reduced. At 15°C the destructive process does not seem to be 

 very important, but the temperature seems to be too low for optimal 



