F. W. WENT 75 



chemically very attractive to study its production, source, and fate. 

 This is now being carried out by many groups of investigators, notably 

 Bonner and Wildman, and Thimann and co-workers. It is now evident 

 that tryptophan can act as a precursor for indoleacetic acid in the 

 plant and that enzyme systems exist which carry out this transformation. 

 Thus we can see how in particular instances (for example, the corn 

 endosperm) large amounts of indoleacetic acid can be produced. Another 

 enzyme system widely occurring in plants inactivates indoleacetic acid. 

 Thus we have the possibility of a complicated interplay between these 

 enzyme systems which gives us a fine opportunity to explain physio- 

 logical phenomena. This has not been done as yet, so we do not know to 

 what extent the new biochemical intelligence is able to explain the 

 physiological phenomena of growth and correlation. 



There is a basic objection against these biochemical studies of indole- 

 acetic acid inside the plant. It is usually tacitly assumed that indoleacetic 

 acid is the one auxin in plants, ignoring all the evidence for the role of 

 substances hke auxins-a and -b. Thus a necessarily one-sided and incom- 

 plete picture is obtained of the biochemistry of auxin. 



Another development has had a decisive influence on the auxin field. 

 That is the use of organic solvents for extraction of auxin. In the earlier 

 work auxin was obtained only by diffusion from the producing or 

 transporting tissues. This had the advantage that only the auxin on its 

 way as correlation carrier was caught and measured. But it was impossible 

 to make a balance of the source and fate of auxin. Thimann (22) suc- 

 ceeded in extracting auxin from Avena coleoptiles with chloroform, 

 which later was replaced with peroxide-free ether. First this method gave 

 most interesting results, showing that as a coleoptile grew the extractable 

 auxin disappeared proportionately with the amount of growth. It also 

 could be shown that less auxin could be extracted at any one time than 

 could be obtained by exhaustive diffusion, indicating continuous produc- 

 tion and utilization of auxin. It could also be shown that the diffusible 

 and extractable auxin were correlated with completely different phe- 

 nomena inside the coleoptile. The diffusible auxin bore a direct relation- 

 ship to tropisms, whereas the correlation between extractable auxm 

 and growth was as direct as Thimann and Bonner (24) had found m 

 their earlier studies. 



The extraction method, however, soon degenerated into a race for 

 the most auxin that could be extracted from any one object. The 



