FLOWERING HORMONE MOVEMENT AND ACTION 



191 



P.. JO 



"^ 8 



z 



H 

 O 



g 0-6 



o 



a: 



Q. 



^ 0-4 



^ 0-2 



I 2 3 4 5 6 7 



RELATIVE TIME IN Kt 



Figure 10-3 



Two curves showing the amount of product as a function of time for 



an aulocatalytic reaction in which rate of the reaction is a function of the 



amount of product and the amount of precursor present at any time. 



The two curves are for different amounts of initial product. 



This is not analogous to induction of cocklebur. The initial 

 amount of flowering hormone seems to be determined by the length 

 of the inductive dark period (and, of course, other factors which may 

 influence its destruction, etc.), and under good growing conditions 

 this amount is maintained throughout the life of the plant. If a plant 

 is given a maximum amount of induction resulting in a maximum 

 amount of flowering hormone, development of flowers and seeds 

 will be very rapid (in the case of a cocklebur the development may be 

 complete in about 30 days under continuous short-day cycles). But 

 if a plant is given only a minimum induction (a single 9- or 10-hr 

 night in the case of a cocklebur), development of the flowers will be 

 extremely slow and may require many months for completion 

 (Fig. 10-4). Furthermore, the number of seeds on the plant will 

 be far fewer in the second case than in the first. Thus we have some- 

 thing different from the autocatalytic mechanisms of the chemist. 

 Flowering in cocklebur is an example of homeostatis, in which an 

 initial concentration of hormone, following its establishment by 

 induction, causes a given rate of development which is maintained 

 throughout the subsequent life of the plant — even though the plant 

 continues to increase its volume by growth. 



