226 



PHYTOHORMONES 



bacterium. The apparently widespread ability of bacteria 

 and fungi to produce indole-acetic acid from tryptophane, 

 or possibly other auxins from other amino-acids (c/. IV E), 

 is doubtless of great significance in this connection. 



Fig. 61. Gall, produced in 5 months on f^liaseolus stem by decapitating 

 and smearing wound with indole-acetic acid paste (20 mg. per gram of lanoline). 

 (From Brown and Gardner, Phytopath. 26: 708-713, 1936.) 



The intumescences on leaves of Populus grandidentata, 

 which are not caused by microorganisms but arise under 

 conditions of high humidity (La Rue, 1933) may be per- 

 fectly imitated by injection of indole-acetic acid in very 

 low concentration ; if in some way the action of auxin in the 

 leaf is intensified by high humidity this would explain them 



