246 G. E. Blackman 



visible injury, but 1 would again stress that although the segments 

 at high concentrations eventually lost water, they still took up 2,4-D. 



Dr. Bennet-Clark: In our laboratory we have extremely similar 

 results which are to be published in the Journal of Experimental 

 Botany. lAA accumulates in disks of tissue in essentially the same 

 way that Dr. Blackman has described for 2,4-D. It rises to a maximum 

 concentration and decreases again. The disappearance of I.\.\ from 

 the tissue, which at one time we thought was due to the metabolism of 

 lAA, the formation of conjugates, oxidation to COo, etc., is partly 

 due to metabolism. Part is due to the output into the external 

 solution. 



The point of interest in this connection is that the uptake process 

 is oxygen-dependent, aerobic respiration-dependent, as Dr. Blackman 

 shows for 2,4-D. The output is not dependent on aerobic respiration. 

 The second feature of interest, as Dr. Blackman showed, is that the 

 gross concentration of 2,4-D in the tissue was smaller than the gross 

 concentration outside, and yet the material came out. This is defi- 

 nitely also the case in our lAA experiments. If )ou consider that the 

 lAA concentration inside is the weight of lAA per unit weight of 

 tissue water, this gross concentration of lAA inside is smaller than 

 the concentration of lAA outside during the phase at which the lAA 

 is coming out. 



And yet this output of lAA is not an active secretion process; it is 

 completely unaffected by cyanide, azide, anaerobic conditions, etc. 

 And, therefore, I would judge that the gross lAA concentration inside 

 and, by implication, the gross concentration of Dr. Blackman's 2,4-D 

 inside, is not equal to the concentration of these substances in the 

 effective site at which it is concentrated, and I would guess that it is 

 concentrated in the cytoplasm rather than the vacuole. I would quite 

 like to hear if that agrees with Professor Blackman's data. 



Professor Blackman: Yes, our ideas are very much like those of 

 Bennet-Clark. ^\'e do not think that 2,4-D has got past the cytoplasm. 

 We tlo, however, think that we have certainly obtained some very 

 puzzling results with Lemna. Thus, if Lemna is first allowed to take 

 up labeled 2,4-D and then transferred to culture solution, more, not 

 less, 2,4-D comes out if the external solution contains cold 2,4-D 

 (Blackman et al., Jour. Exper. Bot., 10: 'i'^, 1959). Egress is clearly not 

 a simple exchange reaction. My feeling is that the 2,4-D accumulat- 

 ing in the cytoplasm exerts a different effect from that located in the 

 outer cytoplasm cell wall interface, and it is the concentration here 

 which controls extension growth rather than what piles up behind 

 the cytoplasm. Johnson and Bonner (Physiol. Plant. 9: 102, 1956) did 



