176 



THE FLOWERING PROCESS 



10 II 12 13 

 NIGHT LENGTH 



14 



15 16 



Figure 9-12 

 Summary curves showing the effects on flowering of various substances 

 appHed just before dark periods of various lengths. Most compounds 

 fail to change the critical night, even though flowering is inhibited. 

 Ethionine and picolinic acid inhibit flowering at all times by a fairly 

 constant floral stage, thus extending the critical night, and cobaltous ion 

 extends the critical night but does not greatly inhibit flowering at the 

 long night lengths. 



period would be extended. How can we distinguish between this 

 effect and an influence upon timing ? 



The critical experiment which seems to cleariy implicate cobaltous 

 ion as an inhibitor of timing is the one in which treated plants are 

 interrupted at various times during the inductive dark period, as 

 shown in Fig. 7-7. The same experiment was repeated using plants 

 treated with picolonic acid, ethionine, or cobaltous ion. Results are 

 shown in Fig. 9-13. Only the cobaltous ion curve is shifted in the 

 manner best explained by an effect upon timing. The other curves 

 are merely lowered, in a manner best explained by a quantitative 

 inhibition of flowering hormone synthesis. Thus eff'ects upon timing 

 and eff'ects upon hormone synthesis can be clearly separated. 



4. Compounds which are Effective during the Translocation Period 

 In Fig. 9-11 it can be seen that the auxins cause an inhibition of 

 flowering, even when they are applied after the end of the inductive 



