156 THE FLOWERING PROCESS 



Flower iNmsiTiNG Substances (3, 14, 22, 37, 38) 



We can interpret the observation that the leaf responds to the 

 inductive light-dark cycle not only by assuming that a flower pro- 

 moting substance is made in the leaf and sent to the bud, but by 

 assuming that a flower inhibiting (or vegetative promoting) substance 

 is continually sent to the bud under non-inductive conditions, and 

 that the proper environmental stimulus causes a cessation in produc- 

 tion of this substance. It is even possible that the bud produces such 

 a flower inhibitor which is removed by the leaves under inductive 

 conditions. These interpretations do not fit well with the second 

 two evidences listed above (why should grafting stop production of 

 an inhibitor, or removal of leaves following induction produce 

 results such as those in Fig. 9-1 ?). Nevertheless, there are a number 

 of observations which do support the inhibitor idea for some species. 

 We might summarize in advance by saying that the evidence usually 

 indicates promoting substances, rather often there is also evidence 

 that inhibitors are involved, and in a few cases it appears possible 

 that flower initiation is completely controlled by removal of an 

 inhibitor. The literature on this topic is extensive and somewhat 

 confusing, but fortunately Jan A. D. Zeevaart (37, 38) has recently 

 classified the evidence into three categories as follows : 



1. Interference with Translocation of Flowering Hormone 



In a typical experiment, a single leaf is given the inductive long 

 dark period(s) (e.g. using cocklebur or Biloxi soybean), but a leaf 

 between the induced one and the bud is left under non-inductive 

 conditions. The non-inductive leaf actively inhibits the flowering 

 process. Is this because non-inductive conditions cause the leaf to 

 produce an inhibitor, or do these conditions simply upset the trans- 

 location patterns of assimilates within the plant, so that the flowering 

 hormone is unable to get from the induced leaf to the bud ? Careful 

 experiments using radioactive tracers have shown that the second 

 explanation is clearly the most likely one. The non-induced leaf does 

 indeed upset the pattern of translocation, and tracers applied to the 

 induced leaf do not reach the bud. This then, is not an example of a 

 true inhibiting substance, but only an inhibiting effect. Yet many 

 reports in the literature base their claim for inhibiting substances 

 upon such experiments. 



