Extraction and Measurement Techniques 57 



preparation was diffused for 1.5 hours and then assayed for the activ- 

 ity in each block as shown in table 3. Block #1, which originally 

 contained all of the auxin, contained 33.4 per cent of it after the dif- 

 fusion time. An x value for 33.4 per cent can be taken from the 

 graphic form of Kawalki's table given in figure 27. The x value is 

 found to be 0.097. Block #2, which contained 28.1 per cent of the 

 auxin, has an x value of 0.091. Similar values are established for each 

 of the other blocks, and their sum is taken to be 4x, one of the values 

 needed for solution of formula (1). Solving the formula for D, one 

 obtains: 



(■3«6)(1|) 



For the purpose of this example, we will take tlie value of k in formula 

 (2) to be 8.8. We can now solve lor the molecular weight as follows: 



- = (ij = m - -- 



It would appear that the auxin being tested was indoleacetic acid, 

 which has a molecular weight of 175. 



This technique can be very useful in obtaining evidence of the 

 identity of an auxin or for determining whether chemical reactions 

 altering auxins have occurred. However one should be always aware 

 that the coefficient of diffusion obtained in this way will be greatly 

 altered by the acidity of the agar, the concentration of the auxin, and 

 the presence of electrolytes. Also the assay of the agar blocks may be 

 misleading because of the presence of mixtures of different auxins 

 and of auxins and inhibitors. Some of these variables may account for 

 the conflicting results obtained by physiologists in the past. For more 

 precise determinations of molecular weight, a description of methods 

 in The Enzymes (Sumner and Myrback, 1952, vol. 1, p. 29) may be 

 useful. 



SUMMARY 



The extraction method and the auxin assay method used for any 

 given study should be selected to yield sufficient accuracy to solve the 

 problem involved with the greatest convenience. To assist in selecting 

 the test to be used, table 4 presents a comparison of several auxin assay 

 methods in their approximate order of sensitivity. It should be pointed 

 out that the Avena test permits the best quantitative accuracy for 

 establishing small differences in auxin content. The pea root test is 



