240 G. E. Blackman 



Brassica and Pisiun the rates of absorption change with time Trom a 

 positive to a negative value. With the Brassica segments there is an 

 interaction between the negative rate and the external concentration. 

 At 45 and 3 mg/1 the loss between 6 and 12 hrs. is significant, but 

 there is no loss at 0.5 mg/1. In contrast, the losses from segments of 

 pea stem are all significant. 



SPECIFIC DIFFERENCES IN THE MECHANISM 



CONTROLLING THE UPTAKE OF 



2,4-DICHLOROPHENOXYACETIC ACID 



In considering the possible physiological factors which might ac- 

 count for specific differences in the course of uptake, there were 

 clearly many experimental approaches. In the light of previous 

 studies on L. minor, it was evident that the influences of tempera- 

 ture and external concentration on the rates of entry and egress 

 would be of interest, while on the basis of the investigations of John- 

 son and Bonner (5) the effects of enzyme inhibitors ought also to be 

 assessed. In weighing up the most likely point of attack, it was 

 recalled (4) that the pattern of uptake of phenoxyacetic acid (POA) by 

 L. yninor was quite unlike that of 2,4-D since there was a continuous 

 accumulation over the 24 hrs.; indeed, the general trend was akin 

 to that of the uptake of 2,4-D by a resistant species, such as Avena. 

 If for L. minor the variations in the pattern of uptake brought about 

 by alterations in chemical structure are linked with shifts in the path- 

 ways of absorption and changes in the natme of the reactive sites, 

 it could be further postulated that the specific differences in the 

 course of uptake of 2,4-D are due to the same basic causes. Thus, 

 where there is a continuous accumulation, e.g., Avena, the sites are 

 unaffected or do not become affected as the auxin accumulates; that 

 is, they are like those in L. minor which permit the accumulation of 

 POA. In contrast, in a susceptible species, such as Gossypiiim, at least 

 a proportion of the sites are different in the sense that they do not 

 react similarly with POA and 2,4-D. On these postulates it could be 

 further advanced that when POA is added to the external solution, 

 its competitive influence on the uptake of 2,4-D would be greater for 

 a resistant than a susceptible species. 



To test this hypothesis the first experiment aimed at determining 

 the influence of pretreatment with POA on the subsequent uptake of 

 2,4-D by segments of Avetia coleoptiles and Gossypiuni stem, employ- 

 ing the procedures described in the previous section. Preliminary ex- 

 ])erimcnts demonstrated that in terms of gain in fresh weight the 

 optimal concentrations of POA lay between 200 and 100 mg/1. Ac- 

 cordingly, some segments were immersed in buffer solution cfintain- 



