440 PLANT GROWTH SUBSTANCES 



This result is of considerable interest, for it illustrates how a substance 

 which is entirely foreign to metabolic systems may simulate the action 

 of a true growth factor. Indeed, if these effects had been first observed 

 in a natural medium, and if the effect of vitamin Be had not been known, 

 neopyrithiamine might well have been considered as a true growth 

 factor. It should not be considered as such despite its growth-promoting 

 properties under these conditions, since it plays no role in the normal 

 metabolism of this or other organisms. 



Other somewhat similar examples are known. Lactobacillus bulgaricus 

 for example, requires oleic acid (or other unsaturated fatty acids) for 

 growth, but these acids are highly inhibitory to growth at higher con- 

 centrations (41). Thus in a medium which contains a considerable 

 quantity of free fatty acid no growth occurs. Addition of a synthetic 

 wetting agent and surface-tension depressant, Tween 40, will now permit 

 growth (Fig. 4) simply because it eliminates the toxicity of oleic acid 

 without eliminating its growth-promoting properties (41). Again, if the 

 mechanism of the effect were not known Tween 40 might be considered 

 a true growth factor, although it obviously should not be so considered. 

 These examples are from natural populations. That entirely similar 

 phenomena occur in artificially induced mutants is shown by the isola- 

 tion of a mutant culture of Neurospora which appeared to require 

 sulfonamides for growth. The mutant apparently synthesized /J-amino- 

 benzoic acid in amounts which indirectly inhibited growth; and by 

 counteracting this action of /J-aminobenzoic acid the sulfonamides pro- 

 moted growth of this organism (47). Although a sulfonamide is thus 

 required for growth of the organism, it can scarcely be considered as a 

 true growth factor in the sense that the various compounds of Table 1 

 are growth factors. Recent reports that certain streptomycin-resistant 

 mutants of bacteria come to require streptomycin for growth may have 

 a similar explanation. 



These examples of substances foreign to normal growth processes 

 which come to behave as growth factors can be readily explained once 

 the view is accepted that substances normally present in growing cells, 

 and themselves essential in small amounts for growth, may under certain 

 conditions act as inhibitors of other essential metabolic reactions. It then 

 becomes understandable how such inhibitions can be alleviated either 

 by appropriate normal metabolites (for example, the products of the 

 inhibited reactions within the cell), or by appropriately fashioned anti- 



