Adaptation of Pea Roots to Auxins and Homologues 113 



less mature apical cells is much more restricted than that of the 

 corresponding normal cells. It seems likely that cells in all segments 

 may be limited in their extension by some growth factor or factors 

 (not auxins) coming either from the root tip or from the mature 

 regions of the root, since they never reach the size attained by cells 

 in intact roots. In this respect the immature cells would tend to 

 suffer most, since the more mature cells would make earlier inroads 

 into this limited growth-factor supply. On the other hand, one might 

 equally well postulate that growth limitation may be set by the 

 accumulation of a staling factor, which in intact roots might be 

 removed upwards. Again immature cells would be the most affected 

 since, being the last to commence extension, they would be most 

 affected by the accumulating products from the maturer cells. It is 

 difficult to see how lAA adaptation could aggravate this restriction 

 of the extension of younger cells except perhaps by augmenting the 

 accumulation of staling products, of which it might even be the 

 precursor. 



The response of normal segments to the direct action of lAA at 

 the concentration used for adaption is a reduction of over-all exten- 

 sion of 58 per cent (Figure lA) which is seen to affect all the cells 

 of the segment to virtually the same extent (Figure IC). lAA-grown 

 segments are, however, very much less sensitive. Figure lA shows 

 that in normal segments 10 » g/ml lAA can produce a growth re- 

 duction which, in adapted segments, requires a concentration of 

 10-^ g/ml, i. e., a drop in sensitivity of at least ten times. The cell 

 distribution analysis of Figure ID shows that this loss of sensitivity 

 takes place over the whole segment but is most marked in the imma- 

 ture apical cells, which extend to virtually the same extent whether 

 lAA is present in the medium or not. The small inhibition which 

 does occur is confined to the mature, more completely extended 

 cells. 



Such behavior might be explained in terms of enhanced lAA- 

 oxidase of the adapted cells. Direct estimations for these roots (see 

 Table 1) show that extracts of treated roots inactivated lAA almost 

 50 per cent faster than those of normal roots, an increase that is 

 highly significant. Although no enzyme distribution studies were 

 made, the results in Figure ID suggest that possibly inactivation was 

 more complete in the immature apical cells which were virtually 

 unresponsive to exogenous lAA. 



This, however, is a very much oversimplified picture of the sit- 

 uation, since lAA adaptation also involves changes in sensitivity to 

 2,4-D, which is certainly not destroyed by lAA-oxidase. 



