336 GROWTH FACTORS AND FLOWERING 



half its final size and is at its most rapid phase of expansion (Khudairi 

 and Hamner, 1954b; Salisbury, 1955), that is, a leaf which is pre- 

 sumably at its top auxin level. In Biloxi soybean, expanding leaves 

 seem somewhat less sensitive to photoinduction than leaves which 

 have just reached full size (Borthwick and Parker, 1940), but this 

 sensitivity change is much less than the change in auxin content of 

 bean leaves during a comparable age interval (see Shoji et ciL, 1951). 



One may of course argue that this evidence is not conclusive; photo- 

 periodic sensitivity of a leaf may be hmited by some other factor that 

 changes with age. One can also raise other arguments, for example, 

 that only one of the several auxins which seem to occur in plant 

 tissues is important in induction, or that it is not the total auxin of a 

 leaf but only a small, "physiologically effective" fraction thereof. I 

 cannot deny that the evidence 1 can present leaves many loopholes 

 through which a staunch defender of a more direct part of auxin in 

 photoinduction can wiggle out; in fact, he does not have to wiggle at 

 all, he can walk out, his head high in the air with disdain. But what 

 evidence there is is not in favor of auxin being more than a modifying 

 influence in photoinduction of short-day plants. 



One last argument that I would like to make also supports this con- 

 clusion. Photoinduction in obligate short- (and long-) day plants 

 contains a qualitative or all-or-none element; above (or below) a cer- 

 tain photoperiod, no flower formation whatever will occur, regardless 

 of the duration of the treatment. The auxin effect on photoinduction in 

 short-day plants, however, seems to be a purely quantitative matter. If 

 a plant is given minimum induction, for example, a cocklebur one dark 

 period of, say, 10 or 12 hr, it is fairly easy to suppress its effect 

 completely by applying a moderate concentration of auxin. But if the 

 inductive treatment is repeated, some flowering response will as a rule 

 be obtained even in plants treated with high auxin concentrations. In 

 one experiment in which I used a concentration of 1000 mg /liter of 

 indoleacetic acid, 1 was not able to suppress induction completely when 

 the plants (Xanthium) received continuous short-day treatment. This 

 pronouncedly quantitative character of the auxin effect, in contrast to 

 the partly qualitative of photoinduction, likewise suggests that auxin 

 has only a modifying influence on photoinduction in short-day plants. 



