110 L. J. Audus arid J. K. Baklisli 



directly to these lAA homologues, either with or without changes in 

 lAA-oxidase, depending on whether Galston's or Burstrom's interpre- 

 tation of adaptive growth changes are correct. 



The purpose of the present investigation therefore was to in- 

 vestigate adaptive changes in pea roots, not only to lAA but also to 

 its synthetic homologue, 2,4-clichlorophenoxyacetic acid (2,4-D), and 

 to two other compounds, both suspected of interfering with growth 

 via the auxin system. One was 2,3,5-triiodobenzoic acid (TIBA), 

 which, among other things, is claimed to increase lAA-oxidase activ- 

 ity in pea stem tissue (7), and lowers lAA concentrations in pea 

 roots (3). The other was 2,4-dichIoroanisole (DCA), which has cer- 

 tain claims to being an auxin antagonist by direct competition at 

 the growth centers (8). The changes studied ^vere of two kinds: (a) 

 growth responses to depressive concentrations not only of the adapt- 

 ing molecule but also of the other substances; and (b) changes in 

 lAA-oxidase activity during adapting treatments. 



METHODS 



Seeds of 'Meteor' pea were germinated and grown from tiie second 

 to the fourth day with their roots in a dilute solution of the chosen 

 growth substance at concentrations that induced small inhibitions 

 of elongation. Tests of the sensitivity of these roots to lAA and other 

 growth substances were made by the excised-segment technique (2). 

 Segments 1.7 to 2.0 mm. long were cut 1 mm. behind the apex of 

 both treated and normal roots of the same age and then grown in 

 aerated solutions of 0.5 per cent sucrose. Total extension of these 

 segments was determined over the subsequent 48 hrs. Complete fac- 

 torial experiments involving growth substances at different concentra- 

 tions allowed the growth responses of segments from treated roots 

 to be compared with those from normal roots. In each factorial ex- 

 periment samples consisted of 10 root segments each and were repli- 

 cated once. Furthermore, each experiment was exactly repeated a 

 number of times on different occasions and the data thus acquired 

 were subjected to an analysis of variance to determine the con- 

 sistency of the responses and of their dependence on the adaptation 

 treatment. Residual errors from these analyses were used to tleter- 

 mine the least significant differences to be used as a basis of com- 

 parison in the results to follow. 



Since the length of the meristem of adapted roots usually differs 

 somewhat from that of normal roots (e.g., with lAA adaptation it 

 is slightly shorter, as shown by Burstrom in wheat roots), it is clear 

 that by taking the same length of segment at the same distance from 

 the apex, the segments from normal and adapted roots will have 



