346 



5. M. Sicgcl and F. Porto 



electron availability at the heteroatom, as in indole > benzthiophene> 

 benzof uran > indene. 



Other examples of chemically meaningful effects are deactivation 

 of phenylhydrazine by the p-nitro group and enhancement of the 

 activity of pyrrole by methylation at positions 2 and 5. Hydrogen- 

 ation of pyrrole (yielding pyrrolidine) reduces its efficiency to the 

 level of aliphatic amines which usually are weak reducing agents, 

 whereas indole, after hydrogenation to indoline, retains its funda- 

 mental aromatic character (as an A'^-substituted aniline), i.e. remains 

 highly active. 



Of approximately 80 compounds studied, only one-fourth have 

 been tested as growth regulators, although most are known to have 

 some form of biological activity. Growth of the turnip radicle in 

 IQ-^M test solutions was stimulated by indole, methylindoles; isoni- 



Table 1 . Growth promoting activity of antioxidants. 



