Extraction and Measurement Techniques 33 



Stem-reference reading 



If it is desired to extend the readings to the lowest ranges of auxin concen- 

 trations, the negative curvatures may be read in addition to the conventional posi- 

 tive curvature readings (Thimann and Schneider, 1938). To do this, place the 

 cut stem so that the intact base is vertical on the protractor and measure the 

 negative curvature of each arm, viz., the angle formed between (c) the vertical 

 axis of the intact base, and (d) the tangent at the point where curvature ceases 

 (i.e. the apical tip) as shown in figure 15 C. The results are expressed as the mean 

 plus or minus the standard error. 



Although in the Avena test the curvature obtained is proportional 

 to the concentration of auxin, in the slit pea test cmvature is more 

 nearly proportional to the log of the concentration. Although the 

 sensitivity to low concentrations of auxin is much less in the slit pea 

 than in the Avena test the range of auxin activity which can be tested 



-200 



10 100 MG/L 



CONC. OF AUXIN 



Fig. 16. Sample results of slit pea tests showing the relationship between curva- 

 ture obtained and the concentration of three auxins; lAA: indoleacetic acid; NAA: 

 naphthaleneacetic acid; and IBA: indolebutyric acid (Thimann and Schneider, 

 1939). 



