THE SYNTHESIS OF FLOWERING HORMONE 157 



Incidentally, a number of interesting experiments can be performed 

 to show that the flowering hormone must move with assimilates from 

 the leaf. Thus if one branch of a two-branched plant is induced, the 

 other will flower much sooner if all its leaves are removed so that it 

 is dependent upon the induced branch for its sugar supply. Removing 

 the leaves from the receptor plant of a graft pair also hastens its 

 flowering. 



2. Fractional Induction 



In many short-day plants, long days spaced between the inductive 

 short days cause a positive inhibiting eff'ect. W. W. Schwabe (69) in 

 England has shown in a series of elegant experiments with Kalanchoe 

 blossfeldiana that the inhibitory effect of a long day is on the short 

 day which /o//<9W'5 it. Thus it may require 1.5 to 2 short days to 

 overcome the harmful effects of one previous long day. It appears 

 that an inhibitor is produced on long days. The story is more com- 

 plicated than mere removal of inhibitor by short days, however, 

 since the effects of long days do not accumulate while the effects of 

 short days do. Furthermore, Hamner has shown that the inhibiting 

 effect of long days disappears when the days are shortened, even 

 though the days are still considerably longer than those required to 

 allow flowering. Thus long days must indeed produce an inhibitor, 

 and short days must indeed remove it (and more than one short day 

 may be required for removal), but in such species short days also 

 produce a positive promoting substance which is still being produced 

 long after the long-day inhibitor is gone. The inhibitor probably 

 interferes with production of the flower promoting hormone rather 

 than itself controlling flowering. Zeevaart wonders if the long-day 

 inhibitor might not be F-phytochrome. 



3. Leaf Removal 



If flowering were caused by cessation in production of a flower 

 inhibitor by the leaves, one should be able to cause flowering simply 

 by removing the leaves. With Hyoscyamus this critical experiment 

 is successful. Here, however, the story is also complicated by the 

 fact that grafting experiments with Hyoscyamus provide evidence for 

 a positive, flower promoting substance. Could this substance be 

 produced by the developing floral bud after flowering is initiated in 

 response to removal of the inhibitor ? 



