Adaptation of Pea Roots to Auxins and Homologucs 117 



adapicd segments in a range o£ 2,4-D solutions (Figure 2B) . In these 

 experiments concentrations up to 10-s g/ml had only slight addi- 

 tional depressive effects and the growth of these segments was slight- 

 ly (although not signiEcantly) greater than normal segments in the 

 same concentration (10-^ g/^il)- 



Convincing evidence of a change in auxin sensitivity comes from 

 the response of 2,4-D-grown segments to lAA. Here, instead of re- 

 duction, there is a progressive increase in segment extension which 

 in 10 "^ g/ml reaches 19 per cent over those in sucrose, a difference 

 which is highly significant (Figure 2A) . The cell length distribution 

 results of Figure 2D show a surprising feature: the mature basal cells 

 of the adapted segment are completely insensitive to these high lAA 

 concentrations, but in the immature cells at the apical end extension 

 is stimulated, their growth equaling that of corresponding normal 

 cells and being at least twice that of corresponding adapted cells 



in sucrose. 



In the first place, it is very unlikely that changes in lAA-oxidase 

 activity of the 2,4-D-grown cells have any part to play in these 

 changed responses. Direct measurement of lAA inactivation by ex- 

 tracts of adapted roots gives mean rates only 12 per cent above those 

 of normal roots, the difference being completely nonsignificant 

 (Table 1). This is supported by previous observations by Audus and 

 Thresh (3), which showed that such 2,4-D treatments had no effect 

 on the internal levels of lAA in pea roots. It seems possible that the 

 lack of sensitivity of the more mature basal cells could be caused at 

 least in part by the retention of 2,4-D at the growth centers of the 

 treated cells. If all these centers were occupied by 2,4-D to the exclu- 

 sion of the lAA penetrating from the medium, then one would ex- 

 pect the results obtained. Even if lAA were to replace 2,4-D, no great 

 change in growth would be expected since, molecule for molecule, 

 they have the same order of inhibiting effect on root cells. 



But the almost normal growth of the younger apical cells is very 

 much more puzzling. The following speculations may be worth con- 

 sideration. First, there might be a mutual, and therefore chemical 

 neutralization of these two substances in apical meristematic cells. 

 This would mean that 2,4-D adaptation involved the generation of 

 an enzyme catalyzing this reaction and would necessitate that this 

 enzyme should disappear as soon as extension commenced; this would 

 explain the persistence of inhibition (either by 2,4-D or lAA or both) 

 in cells just beginning to extend at the time of excision and lAA 

 treatment. From what is known at present of the chemical proper- 

 ties and biochemical behavior of these two substances, this would 

 seem most unlikely. Alternatively, we could explain this virtually 



