Adaptation of Pea Roots to Auxins and Homologues 123 



after excision and during subsequent extension. The pattern of ad- 

 sorption and of growth-substance competition may also change from 

 meristem to extending cell, as was indicated by the 2,4-D adaptation 

 results. These great changes in cell behavior at the time when exten- 

 sion is commencing and the variety of effects of different adapting 

 molecules emphasize the great complexities of the phenomena and 

 the dependence of any phase in the growth of a cell on the growth 

 conditions of the previous phase. Future studies of adaptation phe- 

 nomena cannot ignore this. 



A word of warning must also be sounded on the dangers involved 

 in the interpretation of responses of intact organs to growth-modify- 

 ing substances. It is usual to assume that any such responses of the 

 extension growth of the cell invoked by an exogenous growth sub- 

 stance arise from direct effects on the extension processes themselves. 

 This may be true if responses can be recorded immediately after 

 application of the regulator, but observations over an extended per- 

 iod may well involve indirect effects acting via the meristem similar 

 to those just described. On the other hand, elimination of the in- 

 direct effect of the meristem by the excised segment technique also 

 has its dangers since, as we have just seen, marked differences in be- 

 havior can occur at the two ends of what is all too frequently assumed 

 to be a homogeneous segment, a situation undoubtedly complicated 

 by the traumatic effects of excision. Furthermore, the same total ex- 

 tension of differently treated segments may conceal quite different 

 behaviors of the various constituent cells and hence lead to over- 

 simplified and erroneous conclusions. To avoid such pitfalls, there- 

 fore, future research will have to concern itself more and more with 

 following the behavior and biochemical properties of the individual 

 cell through the successive stages of its growth in intact organs. 



SUMMARY 



An attempt has been made to study by direct methods adaptive 

 changes in the sensitivity of roots to auxins and auxin homologues. 

 The technique involved the measurement of the extension of 1.7 to 

 2.0 mm. segments excised from just behind the root tip and grown 

 in 0.5 per cent sucrose solution with or without auxin. The exten- 

 sion pattern of the individual epidermal cells in these segments was 

 also investigated. The behavior of segments from auxin-grown roots 

 was compared with control segments from water-grown roots in all 

 cases. 



Roots grown in inhibiting concentrations (10 " g/ml) of indole- 3- 

 acetic acid (lAA) show a greatly reduced sensitivity to lAA, which 

 varies with the age of the cell in the segment. These roots have a 



