PLANT GROWTH HORMONES 63 



too. This simple view had good success in explaining the facts so long as 

 the auxin was determined by diffusion out into agar. The two-factor 

 scheme given by Went and Thimann (1937) for many seedlings, and 

 founded on diffusion experiments, worked very well. Perhaps the best 

 case is furnished by Dolk's well-known study of geotropism in the 

 coleoptile, where the excess growth of the lower side, which causes the 

 geotropic curvature, was accounted for by the demonstration that 65% 

 of the auxin (instead of 50% when vertical) went to the lower side. 

 Many other organs have given essentially the same results. 



The introduction of a new method, i.e. extraction of the auxin from 

 the plant tissue with an organic solvent, has gradually undermined all 

 this. The very first such experiments, on Avena, showed auxin all 

 through the plant, including the parts which are no longer growing. 

 This destroys the simple correlation between auxin and growth and 



TABLE I 



Auxin in Spinach Leaf Fractions 



All samples extracted with ether 6 weeks at 2° after the treatment and 

 subsequent acidification. Figures are units of auxin per mg. dry weight. 



introduces a distinction between free auxin, which diffuses readily into 

 agar, and bound auxin, which does not. As Went has shown (1942), it 

 is only the former which is redistributed under the influence of gravity. 

 Extension of the extraction method to other material has raised many 

 problems. Let us consider first the case of green leafy tissue. In Lemna, 

 for instance, auxin is set free into ether continuously but very slowly. 

 From dried material no liberation occurs, but on wetting it re-starts. 

 The auxin is apparently being set free by hydrolysis (Thimann, Skoog, 

 and Byer, 1942). Proteolytic enzymes, especially chymotrypsin, liberate 

 the auxin rapidly, so that we have here an auxin-protein. Using chymo- 



