332 GROWTH FACTORS AND FLOWERING 



Table II. Reversal of Auxin Inhibition of Photoinduction in 



Xanthium pensylvanicum by Simultaneous Application of an 



Antiauxin (Bonner and Thurlow, 1949) 



a-Naphthaleneacetic 2,4-Dichloroanisol, 

 Acid, mg/ liter mg/liter 



10 — 



30 — 



— 10 



10 100 



30 10 



that the inhibition of photoinduction in short-day plants by applied 

 auxin is a typical auxin response. 



A definitive answer to question 3, is, to my mind, not yet possible, 

 but there exists some pertinent information on this point which should 

 ultimately help us to obtain such an answer. Photoinduction in short- 

 day plants can be subdivided into a series of consecutive partial reac- 

 tions or "steps." If any of these steps is blocked, flower formation will 

 not take place. The first step is a "high-intensity-light reaction" 

 (Hamner, 1940). It is followed by a series of reactions which can take 

 place only in the dark. Hamner ( 1940) showed that the events in the 

 early part of an inductive dark period, the period before the "critical 

 length" of the dark period has been reached, are different from those 

 in its later part, which probably consist in the actual synthesis of the 

 floral stimulus. The earlier events are completely negated by a light 

 interruption; they thus constitute the "timing mechanism" of photo- 

 induction. Floral stimulus synthesis is stopped by light, quite likely 

 because some condition which has been created by the "timing 

 mechanism" is now abolished, but the stimulus which had time to 

 accumulate is preserved. Salisbury and Bonner (1956) presented 

 evidence suggesting that the "timing mechanism" involves two distinct 

 steps, the first being the conversion of the "photoperiodic pigment" 

 from the far-red- into the red-absorbing form, the second a "prepara- 

 tory reaction" of unknown nature, but quite distinct from pigment 

 conversion. Lockhart and Hamner (1954), on the other hand, by ter- 

 minating an inductive dark period by a brief period of light and then 



