Gibberellic Acid and Auxin in Extension Growth 655 



that of the same region on the phnit, since these increases that you 

 obtained, of 10 or perhaps 20 per cent at the most, seem rather small? 

 Dr. Brian: I can't answer the first question with any certainty. Cer- 

 tainly by just putting them on filter paper you do get the same kind 

 of effect, but it isn't so pronounced. I called it starvation. I'm not at 

 all sure that it isn't starvation plus leaching. The second point is 

 that certainly the response to gibberellin is never as great as the 

 growth the same piece of tissue would make in the intact plant. The 

 over-all extension is, if you are using a dwarf section, however, very 

 much greater than the extension of the same tissue in the intact plant. 

 Dr. Tolbert: Dr. Haber and I have, some years ago, extensively 

 studied the distribution of carbon-14 in the products from long and 

 short term photosynthetic C^^Oo fixation by gibberellin-treated plants. 

 We found no difference, that could be detected by paper chromatog- 

 raphy, in the rate or amount of C^^ incorporated into the sugars, 

 amino acids, or organic acids, of plants treated with gibberellin as 

 compared to controls. 



Dr. Lockhart: I'd like to make a comment on the importance of 

 the tip in the response to gibberellic acid. I also agree with the other 

 workers that in light-grown plants or plants that have been treated 

 with light, the response to gibberellin is very much reduced if you 

 remove the tip. However, if the plants are grown and maintained in 

 continuous darkness, then the tip appears to be completely unneces- 

 sary for a response to gibberellin — the tip then can be completely 

 replaced in the promotion of elongation by gibberellic acid. This is 

 true with the 'Alaska' - a tall variety, but not when a dwarf variety 

 is used. 



Dr. Evenari: First of all, do you know anything about the action 

 spectrum of this interesting high-intensity light effect? We know now, 

 as far as germination is concerned, there is definitely a red-far-red 

 low-energy, and a red-far-red high-energy process involved. And it 

 would be very interesting to know what the action spectrum of your 

 own high-energy effect is; that part which is not photosynthetic. Also 

 is there any effect of a dark period? I mean, is there any difference 

 between continuous light or interruption by dark periods. 



Dr. Brian: I'll take your second point first. So far as we have in- 

 vestigated it, the dark period has no effect except that it isn't light. 

 We've only just started work on the spectral effects and I really can't 

 make any useful comment on this at all. 



Dr. Gordon: Considering the irradiances employed, may I sug- 

 gest an alternative explanation for the light effect you observed? 

 Your own experiments have demonstrated the interaction between 



