A. J. HAAGEN-SMIT 7 



a somewhat lower sensitivity than the other two quarters (52). With 

 this method indoleacetic acid can be measured in concentrations of one 

 gamma per hter. The curvature observed is proportional to the logarithm 

 of the concentration. 



For special purposes other test methods have been developed in which 

 the auxins are taken up in lanoHn or other water-insoluble material and 

 applied to different regions in the plant. These methods are especially 

 useful for work outside the laboratory where the conditions cannot be 

 as strictly controlled as is necessary for the Avena test. 



In principle, any reaction of plants to the auxins can be used for 

 measuring auxin activity. For example, the sweUing of decapitated 

 stems of Viciafaba has been utiHzed for an assay method (33). Hitch- 

 cock and Zimmerman (22) have made use of the fact that application 

 of auxins changes the angle between the stem and the petiole in a 

 number of plants. 



In carrying out measurements of auxin activity all experimenters are 

 aware of fluctuations in the sensitivity of the test object, although the 

 temperature, humidity, and Hght conditions are kept constant. Measure- 

 ments of the same amount of indoleacetic acid at different hours of the 

 day gave values which may differ considerably from the average value. 

 No explanation of this interesting phenomenon has yet been given, and 

 attempts to prevent these variations have not been successful. In de- 

 terminations where results have to be compared with those previously 

 obtained, it is therefore general practice to test known concentrations 

 of indoleacetic acid at the same time and express the results as indole- 

 acetic acid equivalents. With proper technique the measurement of 

 the over-all growth activity of active material has reached a satisfactory 

 degree of accuracy, and the difficulties at the present time are found in 

 the interpretation of these results. 



Quantitative determination of the growth-hormone content of ex- 

 tracts can be seriously influenced by the presence of growth inhibitors. 

 While the study of these inhibiting substances is of importance for 

 understanding the mechanism of plant growth, it also has a direct 

 bearing on the evaluation of the auxin analysis. In low concentrations 

 of the growth substances these interfering agents may cause a positive 

 curvature. In higher concentrations the activity curve may be con- 

 siderably depressed and consequently give low values for the amounts 

 actually present. Lipoid-soluble inhibitors have been found in extracts 



