Concluding Remarks • 95 



response versus daylength, but only on the assumption that day- 

 lengths above the critical had no other effect than to be noninduc- 

 tive. Work mentioned in Chapter Three, however, indicates now 

 that such daylengths are often positively antiinductive, not merely 

 ineffective, and that this antagonistic effect is quantitatively related 

 to the amount by which the noninductive daylength exceeds the 

 critical. While no generalization is likely to hold for all plants, 

 it is possible that the processes involved in induction proceed con- 

 tinuously, and that only the ratio of the rates of, say, two or more 

 of them differs under different daylengths. The critical daylength 

 would then be that value at which the ratio neither promotes nor 

 inhibits the train of events finally leading to flowering. 



Many of the subjects touched on in the preceding chapters, 

 including the question of the degree of difference between the 

 structure of vegetative and floral meristems, bear on this sort of 

 problem, but cannot be enlarged upon now. The relevance of such 

 theoretical considerations to more concrete questions is largely 

 in the suggestion that flowering does not represent a sudden change, 

 some sort of developmental "quantum-jump," but is probably under 

 controls similar to those affecting vegetative growth, to the small 

 degree that these are understood. 



Consider, for instance, the nature of floral stimuli. That some- 

 thing moves between induced and noninduced parts of a plant, or 

 between grafted plants, cannot be doubted. Movement of active 

 substances from vegetative to reproductive tissue is also highly 

 probable. In physiological terms, then, both florigen and anti- 

 florigen appear to be valid concepts, but in the absence of extracted 

 samples one can only speculate as to their nature and whether they 

 are the same in all plants. In the light of the considerations above, 

 it appears extremely unlikely to the writer that florigens, whether 

 simple substances or as complex as a virus, are likely to be specific 

 floral hormones in the sense that they are involved only in the 

 processes of floral initiation and development but no others. Julius 

 Sachs's concept of specific organ-forming substances has not stood 

 the test of experimentation, since most vegetative systems studied 

 indicate that particular aspects of development can be controlled 

 by the concentrations and interactions of substances that affect many 

 other processes as well. A few examples will be helpful here. 



The use of the auxin indoleacetic acid in rooting cuttings is 



