PLANT HORMONES AND GROWTH 



and from corn germ. Later, "heteroauxin" (or indole-3-acetic acid) 

 was isolated by the same workers (3) from urine and from yeast, and 

 by Thimann (10) from cultures of the fungus Rhizopus suinus. More 

 recently it has been isolated from wheat germ by Haagen-Smit. The 

 growth-promoting activity of these three substances is quantitatively 

 and qualitatively very similar. 



CHs 



1 

 CHC2H5 



CjHbCH- 



I 



CH, 



CzHsCH- 



CH, 



^— CHOHCH.CHOHCHOHCOOH 



Auxentriolic acid ("auxin a") 



/\ 



CH, 

 CHC2H6 



V\ta/ 



-CH2COOH 



Indoleacetic acid 



3- 



CHOHCH2COCH2COOH 



Auxenolonic acid ("auxin b") 



In addition to the three known naturally occurring substances, 

 a large number of unsaturated ring-containing acids have growth- 

 promoting activity. These include the propionic, butyric, and valeric 

 derivatives of indole, the acetic derivatives of indene, naphthalene, 

 anthracene, thiophene, and even (weakly) benzene. m-Cinnamic 

 acid is active, but the /rani--derivative is not. The simple esters 

 and amides, as well as methyl and chloro derivatives of some of these 

 acids, also have activity. Indeneacetic acid is of particular physio- 

 logical interest because its activity, though high, is localized, and unlike 

 the natural auxins it is not readily transported through plant tissue. 

 a-Naphthaleneacetic acid is also of importance because of its stability, 

 which has led to its wide adoption for certain practical uses in horti- 

 culture. 



Since the extensive work on the relation between chemical 

 structure and growth-promoting activity has been fully reviewed 

 (7,10), there is no advantage in considering it here. The essential 

 groups are: (a) an unsaturated or an aromatic ring, and (b) a car- 

 boxyl or a group readily converted by the plant to a carboxyl. Ihe 



323 



