8 Fundamentals of Auxin Action 



After some time it was found that not only could auxins a and b 

 not be found in plant materials, but also synthetic analogues of these 

 acids have failed to show auxin activity in conventional growth tests 

 (Kogl and de Bruin, 1950). Consequently it seems reasonable to dis- 

 card these early substances from the list of known auxins. 



Further research by Kogl's group (1934) identified another auxin 

 compound. Some of the urine samples they worked with exhibited 

 considerably greater auxin activity than could be accounted for as the 

 supposed auxins a and b. They set about isolating this compound and 

 foiuid it to be identical with indole-3-acetic acid. The presence of this 

 auxin was demonstrated in Rhizopus cultures by Thimann (1935), 

 and he showed that it can be differentiated from auxins a and b by its 

 instability in warm acid and stability in warm alkali. This is the 

 opposite reaction to that of auxin a. Auxin b is unstable in either acid 

 or alkali. In the subsequent twenty years indoleacetic acid has been 

 conclusively demonstrated to be the principal growth hormone in 

 seven different species of higher plants, and it is generally accepted 

 now as being the commonest growth hormone in higher plants. 



J\ 



v-\./ 



CHgCOOH 



H 



Indoleacetic acid 



The quantitative relationship of the growth hormone to tropisms 

 was essentially clarified soon after the Avena test was made available. 

 Blaauw (1918) had shown that phototropic movement in Avena was 

 owing to a retardation of growth on the lighted side. Went (1928) now 

 associated this retardation with a decrease in the relative hormone 

 content of the lighted as compared to the unlighted side. Dolk (1929) 

 showed that geotropic movement in Avena was caused by a simple 

 redistribution of the hormone present. His measurements of diffusible 

 auxin from geotropically stimulated coleoptiles showed that 62.5 per 

 cent was in the lower half. Geotropism in roots was found to be 

 brought about by very similar changes in relative hormone contents 

 of the upper and lower root halves (Hawker, 1932). 



With the advent of Went's Avena test, it was quickly found that 

 different concentrations of auxins may have opposite effects on growth. 

 Where low concentrations of auxin promote growth, higher concen- 



