118 L. J. Audus and J. K. Bakhsh 



normal growth by invoking changes at the growth centers in both 

 the sensitivity to and the affinity for growth substances in these imma- 

 ture adapted cells. It would be necessary to postulate that residual 

 2,4-D is easily pushed off these cells by the lAA entering from the 

 medium but, at the same time, the sensitivity of the growth centers 

 to lAA is so reduced that it has virtually no effect and normal exten- 

 sion ensues. Furthermore, there would have to be a rapid increase in 

 2,4-D affinity at the commencement of extension in order to explain 

 the lack of lAA effect in the basal cells starting to extend at the time 

 of excision. 



Segments From Roots Grown in TIBA (5 X 10^ g/ml) 



A series of five identical factorial experiments has been carried out 

 to test the responses of these roots to IQii, 10 «, and 10 ' g/ml lAA 

 solutions. The pooled treatment means are drawn in Figure 3A. The 

 behavior of control segments is as in previous experiments. lAA at 

 10'^ g/ml giving an inhibition of extension of about 48 per cent. 

 TIBA-grown segments extend about 19 per cent less than normal 

 segments in sucrose. The cell length distribution curves of Figure 

 :^B and C show that this is due, as in the case of lAA-adapted seg- 

 ments, to a marked restriction of the extension of the youngest apical 

 cells. Although, for some unknown reason, the mature basal cells of 

 this particular sample of control segments grow less than usual, thus 

 emphasizing these differences, corresponding cells of the TIBA-grown 

 segments appear to extend more than normal, reaching a significantly 

 higher final length. 



The responses of these treated segments to lAA are very slight 

 (Figure 3A), amounting to a reduction of only 9 per cent in 10 "^ 

 g/ml, as compared with those of corresponding segments in sucrose 

 only. Whereas lAA inhibition is exerted along the whole length of 

 the normal segment, in TIBA-treated segments, the much reduced 

 inhibition is exerted mainly on the maturer basal cells. In all re- 

 spects the growth behavior of TIBA-adapted segments closely re- 

 sembles that of lAA-adapted segments and the similarity also extends 

 to augmented lAA-oxidase activity. Enzyme extracts of TIBA-treated 

 roots gave mean lAA destruction rates well over twice those of 

 normal roots, the difference being very highly significant (Table 1). 

 TIBA in low concentrations appears to have a direct activating 

 effect on lAA oxidase in vitro. But such small effects (12 per cent at 

 10*5 g/ml) of TIBA in the extracts of treated roots could hardly ex- 

 plain these large activity increases. It would seem that here ^se are 

 dealing with lAA-oxidase induction by TIBA. 



The lowered sensitivity of TIBA-treated segments to ai:)plied lAA 



