Adaptation of Pea Roots to Auxins and Hornologucs III 



somewhat different cellular constitutions; indeed there will be some 

 variations even within groups of segments cut from the same root 

 sample. Obviously it is not possible to allow for such differences in 

 the cutting of the segments and so, to supplement observations on 

 the over-all growth, studies were made of the extension of individual 

 epidermal cells of representative segments, so that the behavior of 

 comparable cells in segments from normal and treated roots could 

 be studied. Such observations were made on 8 ^ sections cut from 

 segments fixed in Navashin's fixative and stained in hematoxylin. 

 Samples of five segments were taken for each treatment and lengths 

 of individual cells measured from basal to apical end. Taking the 

 cells in successive groups of 10 (i. e., groups of 50 cells for the sample 

 of five segments), mean lengths and their standard errors were de- 

 termined for the construction of cell length distribution curves. 



Finally lAA-oxidase content of seedling roots similar to those used 

 for the cutting of segments was determined from enzyme extracts 

 by methods described by Galston and Dalberg (7). Average rates of 

 lAA destruction, eliminating the small inconsistent initial lag and 

 the enzyme inactivation which sets in as the reaction proceeds, were 

 determined from a number of replicate samples by a statistical meth- 

 od, which also gave estimates of over-all errors for the evaluation 

 of confidence limits. 



RESULTS 



Segments From Roots Grown in 10^ G/Ml of Indole-3-acetic Acid 



Responses to inhibiting concetrations of lAA (10^^ and 10-'' gjin^)- 

 In Figure lA are plotted the mean percentage extensions of samples 

 from eight identical factorial experiments. In sucrose, lAA-grown 

 segments extend less (i.e., about 15 per cent) than normal segments. 

 On the other hand, they extend very much more than normal seg- 

 ments exposed directly to the adapting concentrations. If the re- 

 duced extension of these adapted segments is due to a carry-over of 

 lAA from the adapting solution, then either its concentration must 

 be much lower than in the adapting medium or cell sensitivity to 

 lAA has been lowered by the adaptation treatment. Inspection of 

 the cell extension graphs (Figure IC, D) shows that this reduced 

 growth is not a property of all cells in the adapted segment. In 

 normal segments in sucrose all cells extend, but the less mature cells 

 at the apical end of the segment, where extension had not com- 

 menced at the time of excision, do not attain the same final length 

 as the more mature cells at the basal end of the segment. In lAA- 

 grown segments, these more mature basal cells extend as much as 

 the corresponding cells in normal segments while the growth of the 



