Flower Promotion or Flower Inhibition? • 79 



substance or substances that prevent initiation until they are 

 removed by the proper conditions. 



It may be surprising that most of the very evidence presented 

 in the preceding section for movement of a florigen can be reinter- 

 preted as indicating simply deinhibition (see von Denffer, 1950). 

 Under this interpretation, noninduced leaves constantly produce a 

 flowering inhibitor that moves to the growing point along with the 

 products of photosynthesis; induced leaves no longer produce this 

 inhibitor. Hence the removal or darkening of noninduced leaves 

 often promotes flowering not, as under the florigen hypothesis, by 

 preventing interference with the carbohydrate stream in which the 

 florigen moves, but by reducing still further the sources of the 

 inhibitor; flowering thus occurs simply as a result of sufficient 

 quantities of inhibitor-free assimilates. It has been suggested, on 

 the basis of work to be discussed later, that the inhibitor in 

 question might be an auxin, and the general form of this hypothesis 

 fits some of the experimental data well enough. At least, it often 

 fits no worse than the other hypothesis, as a brief reconsideration 

 will show. 



Stout's (1945) work with "three-headed" beet plants indicated 

 that the presence of a shoot on short-day conditions did not inhibit 

 the response of a darkened receptor shoot to the long-day donor 

 shoot; thus if the noninduced (short-day) shoot produces an inhib- 

 itor, it is not detectable. This does not help the inhibitor hypoth- 

 esis. On the other hand, the further result that even 4 hours of 

 light per day (compared with 17 hours for the donor) prevents a 

 shoot from being an effective receptor also does not help the simple 

 fiorigen-movement-with-carbohydrates hypothesis, since it is un- 

 likely that the predominant direction of carbohydrate movement 

 would be reversed under these conditions. Another ambiguous 

 situation is of course that the inhibitory effect of noninduced 

 Xanthium leaves appears to be a photoperiodic phenomenon in its 

 own right, not simply a matter^ of affecting carbohydrate (and 

 florigen) flow. 



The florigen hypothesis can be saved from many difficulties, 

 including these, by the suggestion that noninduced leaves act not 

 by producing an inhibitor but by destroying florigen. On balance, 

 the simple inhibitor hypothesis is probably less satisfactory; the 

 strongest argument against it is the effectiveness of small amounts 



