Adaptation of Pea Roots to Auxins and Homologues 115 



Responses to inhibitory concentrations of 2,-f-D (10^^ g/»^^ ^""^ 

 70 « g/'ml) . Here only total segment extension has been measured, 

 and results are recorded in Figure IB. Growth of normal segments 

 is not significantly reduced by the lower but is reduced by about 

 48 per cent by the higher concentration. lAA-grown segments, on 

 the other hand, are not significantly affected by these 2,4-D concen- 

 trations, the total extension being the same as in sucrose alone. Un- 

 fortunately, time has not permitted analysis of growth on a cell 

 basis for these 2,4-D-inhibited segments, but it is clear from these 

 results alone that lAA adaptation has markedly reduced the sensi- 

 tivity of cells to 2,4-D inhibition. It might be suggested that the 

 lack of response to 2,4-D is due to the domination of the growth 

 centers by lAA carried over from the adapting solutions, but this 

 would certainly not be so if lAA-oxidase were at a high level in 

 the cells. It would be much more logical to suppose that here we 

 are dealing Avith a change in the growth centers themselves involv- 

 ing a great loss in sensitivity not only to 2,4-D but also most prob- 

 ably to lAA itself. It is very unlikely that lAA-oxidase induction 

 plays any direct part in these 2,4-D sensitivity changes. 



Segments From Roots Grown in 2,4-D (3 X 10 § g/ml) 



In this series of nine identical factorial experiments we tested the 

 responses of normal and 2,4-D-grown segments to three lAA concen- 

 trations (10-11, 10-9, and 10-' g/ml) . The pooled results of these ex- 

 periments are drawn in Figure 2A. Normal control segments show, 

 contrary to earlier experiments by Audus and Das (1), no stimulation 

 in 10-11 g/ml lAA but a progressive inhibition in higher concentra- 

 tions, reaching about 27 per cent in 10^ g/ml- The pattern of cell 

 extension in these normal segments (Figure 2C) closely resembles 

 that already described, with restricted growth of the cells at the 

 apical end. lAA (10 " g/ml) reduces the extension of all cells in the 

 normal segment, with a proportionately greater effect on the more 

 rapidly extending maturer basal cells. 



Root segments grown in 2,4-D extend much less (about 27 per 

 cent) in sucrose than normal segments (Figure 2A) . The analysis of 

 cell length distribution (Figure 2D) shows that this restriction of 

 extension applies to all cells, with perhaps a slightly greater effect 

 (as for L\A adaptation) on the immature apical cells. Such a reduced 

 extension could undoubtedly be explained in terms of residual 2,4-D 

 left associated with the growth centers after excision and exerting, at 

 least in part during subsequent extension, a continuing growth in- 

 hibition. But this reduction in extension is much less than that pro- 

 duced by the direct action of the adapting concentration (3 X 10-s 



