New Auxins from 'Maryland Mammoth' Tobacco 63 



show significant growth promotion. Figure 5 indicates the activity of 

 the tobacco compound, although the values are only relatively quan- 

 titative due to the probable presence of impurities in the sample. 



The identity of the substance which lay behind all this effort — the 

 elusive indolic material similar to IAN — has not yet been determined. 

 So far, all attempts to isolate it have failed, and, in fact, it frequently 

 has even escaped detection. The continued search for this substance 

 may yield an exciting story in itself some day. 



DISCUSSION 



A number of instances have been reported (1) in which the pres- 

 ence of lAA and IAN was suspected on the basis of bioassay and 

 chromatographic data, but in which the characteristic color reactions 

 of the indole nucleus could not be detected. Undoubtedly, many more 

 such cases have gone unreported. The fact that the substances re- 

 sponsible for growth promotion on these occasions were not exten- 

 sively investigated indicates the dominating influence which knowl- 

 edge of the naturally occurring indoles has had on auxin studies. It 

 is quite possible that the substances detected in previous examples are 

 similar to, or perhaps even identical with, the growth promoters in 

 'Maryland Mammoth' tobacco. 



The similarity of the chromatographic characteristics of these non- 

 indolic compoimds to those of lAA and IAN certainly is coincidental. 

 However, such similarity cannot be considered unlikely, since the pre- 

 cision of resolution in the usual hormone profile is only 0.1 to 0.2 Rf 

 units. Despite the great value of paper chromatography to the plant 

 hormone field, other severe limitations of this method for the deter- 

 mination of hormone profiles have become apparent in the present 

 investigation. Since a biologically active area may contain more than 

 one growth-stimulating substance, extensive efforts must be made to 

 separate the chemical factors in each case. Certainly, Rf data cannot 

 be used as proof, nor generally even as critical evidence of the chemi- 

 cal structures involved. 



"Specific" color tests also may be deceiving. For example, an in- 

 active indole may have the same Rf as a growth-promoting nonindolic 

 compound, or, in the case of an active indole, the two biological 

 effects may be superimposed. Although a great deal of valuable in- 

 formation obviously can be obtained by thorough chromatographic 

 analysis of plant materials, rigorous proof of the structure of the com- 

 pounds present probably can come only through actual isolation and 

 chemical study. 



It is apparent from the examination of 'Maryland Mammoth' to- 

 bacco that nonindolic growth-promoting compounds may be iso- 



