116 PHYTOHORMONES 



verted to indole-3-acetic acid by fungi and bacteria, and 

 the same conversion by the plant is therefore extremely 

 probable. Tryptophane and tryptamine are of special 

 interest as constituting models of the auxin precursor in 

 the coleoptile: they are substances which, inactive in them- 

 selves, are readily converted to the active form by plant 

 enzymes. It may also be pointed out that tryptamine is a 

 base and therefore, according to the electrical polarity 

 theory outlined in VI E, would travel upwards in the plant. 



Not only variations in the side chain, but variations in 

 the nucleus itself, may be effected without wholly losing 

 the activity. Thus, Thimann (19356) has shown that indene- 

 3-acetic acid, which contains a carbon atom instead of the 

 nitrogen of the indole, has moderate activity on Avena. 

 Cumaryl-acetic, on the other hand, with oxygen instead 

 of the nitrogen atom, was inactive in producing curvature 

 (c/. VIII G). 



Zimmerman and Wilcoxon (1935), Zimmerman, Hitch- 

 cock, and Wilcoxon (1936), and Manske and Leitch (1936) 

 tested several substances for their activity in causing epi- 

 nasty on Nicotiaria. Their results are therefore not com- 

 parable with the above, and will be considered again in 

 VIII G. Haagen Smit and Went (1935) have tested a large 

 number of compounds by different methods; this work 

 will also be dealt with in VIII G. They have also listed a 

 number of substances which are completely inactive in all 

 test methods. Glover (1936) reported — erroneously (u) — 

 that skatole had growth-promoting activity on Avena. 



In connection with the activity of esters, it is of interest 

 that the esters of auxin a are inactive. There is good e\d- 

 dence that auxin a occurs in a number of natural products 

 in the inactive ester form. Thus Kogl, Erxleben, and Haagen 

 Smit (1934) were able to obtain considerable activity from 

 Arachis oil and some other vegetable oils by hydrolyzing 

 the oils with a lipase preparation or with sodium ethylate. 

 Some oils, such as corn germ oil, contain auxin in free form, 

 as we have seen. Auxin esters in edible fats and oils may 



