72 PHYTOHORMONES 



culture. Certain amino-acids are particularly favorable for 

 the production of auxin (Boysen Jensen, 1932), but the 

 most important, particularly as a constituent of peptones, 

 is tryptophane (Thimann, 1935a). The yield of auxin pro- 

 duced by Rhizopus under standard conditions on peptone 

 media is proportional to the amount of aeration which the 

 culture received (Bonner, 1932; Thimann and Dolk, 1933); 

 this has been explained as due to the oxidative deamination 

 of tryptophane to indole-acetic acid (Thimann, 1935). The 

 presence of carbohydrate with the peptone facilitates the 

 production of auxin. The bulk of the auxin is produced 

 after the growth of the fungus {Rhizopus suinus) has ceased, 

 and its concentration, after reaching a maximum, decreases' 

 again on account of destruction. Auxin production by 

 yeast cultures follows similar lines, most of the auxin appear- 

 ing after growth has ceased (Robinson, u). However, with 

 yeast the yield of auxin appears to vary inversely with the 

 amount of growth. The auxin does not appear to play any 

 role in the growth of either of these fungi. Auxin production 

 by pathogenic microorganisms may be of great importance 

 in plant diseases (c/. IX C and XIII C). 



