Occurrence and Role of Growth Hormones 71 



identified in plants, but the possibility of their actually acting as 

 growth hormones is obscure. The first of these is ciVcinnamic acid, 

 which occurs in plants both as the free acid and as its esters. It is 

 found in notably high concentrations in the oil of balsam. Another 

 substance, phenylacetic acid, which is weakly active as an auxin (Thi- 

 mann and Schneider, 1939), is a constituent of peppermint oil and oil 

 of Neroli. Phenylacetic acid is unable to move in the active transport 

 system and so it seems very unlikely that it serves as a growth hormone. 

 Another compound, indoleacetonitrile, has been identified in several 

 plants and shows auxin activity in the Avena test (Jones et al, 1952). 

 The lack of activity in the pea test and the ready conversion to 

 indoleacetic acid suggest that this compound may be more important 

 as a precursor of auxin than as an auxin itself. 



It should be emphasized that there must surely be growth hor- 

 mones other than indoleacetic acid in plants. To cite only a few cases, 

 of the auxins obtained from Avena coleoptiles (von Guttenberg and 

 Lehle-Joerges, 1947), from wheat seeds (Gordon, 1946) and from 

 cabbage (Holley et al, 1951), sizeable portions could not be identified 

 as indoleacetic acid. Also, a crude auxin material has been extracted 

 from bean seeds, a material which has the remarkable property of 

 inducing prolonged growth increases in bean seedlings when applied 

 to the stems or foliage, a property quite foreign to indoleacetic acid 

 (Mitchell et al, 1951). With the advent of the new and more effective 

 chromatographic methods of identifying auxins, it is probable that 

 the identity of other growth hormones will be soon established. 



From this discussion it can be seen that the identity of the growth 

 hormones in plants has not yet been clarified. From the evidence at 

 hand it would appear that indoleacetic acid is the most common 

 growth hormone, but it seems clear that it is not the only growth 

 hormone. Also, auxin may exist in several different types of complexes 

 such as complexes with protein, complexes with precursors, and com- 

 plexes with inhibitors. 



FORMATION OF AUXIN 



Factors in Formation 



The apparent role of the apex in auxin formation and the ap- 

 parent frequent correlation of auxin formation with growth have 

 been pointed out. Several factors, both internal and environmental, 

 are known to play a part in auxin formation. 



In the first place, the amount of auxin produced per apex has 

 been found to be closely correlated with the age of the plant in 



