56 Fundamentals of Auxin Action 



in figure 27. For example, if in the top block (the donor block) one 

 found 32 per cent of the auxin after the diffusion period, the x value 

 for that block would be 0.088, as derived in figure 27. The x values 

 obtained in this way for each of the four blocks are then averaged to- 

 gether, and the mean x value is used to solve for the coefficient of 

 diffusion (D) in the following formula: 



D 



Axt 



(1) 



where h is the thickness of the block in cm. (0.142 cm.), and t is the 

 time of diffusion in days (1.5/24 or 0.0625 days). The value of D 

 varies with temperature, so the temperature should be kept constant 

 during the diffusion time. Knowing Z) at a given temperature, the 

 molecular weight (M) can be approximated by the formula: 



M 



-i^' 



(2) 



where ^ is a constant for the temperature. When diffusion is carried 

 on at 26° C, k = 8.8 (Kramer and Went, 1949). It is best to calculate 

 k under the actual experimental conditions by determining D for an 

 auxin of known molecular weight, and then solving the second formula 

 for k. Using the experimentally determined k, the formula (2) can 

 be solved for the molecular weight of the unknown auxin. 



A sample determination of molecular weight may serve to illus- 

 trate the method. Using agar blocks of 0.127 cm. thickness, an auxin 



TABLE 3 



Sample Data for Determination of Molecular Weight by the Diffusion Method 

 (Curvature data from Kramer and Went, 1949) 



