Extraction and Measurement Techniques 27 



the curvature from any one concentration of auxin greatly reduced, 

 but the shape of the concentration curve is very greatly altered (Lar- 

 sen, 1939). Serial dilutions of auxin solutions containing inhibitors 

 characteristically show a reduced slope, a rounded curve instead of 

 linearity, and a lower maximum curvature. A graphic comparison 

 of a pure auxin dilution series with an auxin-plus-inhibitor dilution 

 series is shown in figure 12. If the presence of an inhibitor is suspected 



0.1 0.2 



CONC. OF AUXIN 



0.3M6/L 



Fig. 12. Dilution curves of indoleacetic acid alone and in the presence of an in- 

 hibitor — parascorbic acid (Larsen, 1947). 



in a plant extract or diffusate, the establishment of a dilution curve 

 may be extremely helpful in verifying whether an inhibitor is present 

 or not. .Another technique for establishing the presence of an inhibitor 

 is to test a constant concentration of indoleacetic acid with a dilution 

 series of the extract containing the suspected inhibitor (van Overbeek 

 et al, 1945). 



The presence of moderate amounts of most salts, metals, sugars, 

 or amino acids in the agar block will have no perceptible effect on 

 the Avena test. However, it should be pointed out that the presence of 



