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DISCUSSION 



Dr. Henderson: Have you measured the lAA content of these 

 plants after treatment? 



Professor Blackman: I haven't measured the lAA content. In fact, 

 I don't know how I would measure the content of lAA which is 

 physiologically active. 



Dr. Henderson: In a variety of plants, we found that 2,4-D reduced 

 the rate of disappearance of I A A, while in others auxin disappear- 

 ance was increased by 2,4-D. 



Professor Blackman: With TIBA, if you add lAA to the external 

 solution at concentrations of 30 to 50 mg/1, you can prevent the loss 

 of TIBA but not from roots. That's the only case I know where 

 there is a clear-cut effect of averting the rate of loss by adding lAA. 



Dr. Burstrom: With the high addition of 2,4-D to your susceptible 

 species, as well as in one of the cereals, you had the peak of accumula- 

 tion after 8 to 12 hrs. In these species have you ever observed any cy- 

 tological disturbances which may account for the inability of the 

 tissues to retain the amounts of 2,4-D initially taken up in these 

 concentrations? 



Professor Blackman: In the root uptake investigations we only used 

 one mg/1, a very low concentration unlikely to cause cytological dis- 

 turbances in a few hours. It is true that we have observed in Brassica 

 alba that you can have injection of water into the intercellular spaces 

 at high concentrations and still have 2,4-D being taken up by the stem 

 segments. In this instance, this type of disturbance does not prevent 

 accumulation of 2,4-D. For Triticiim I do not know whether there was 



