45^ PLANT GROWTH SUBSTANCES 



in response to optimal concentrations of the required substance, is 

 completely independent of an exogenous supply of the growth factor 

 since it can grow indefinitely at maximal rate on minimal medium from 

 myceUal transfers. The adaptive behavior of these mutants is not carried 

 through the conidia, and the cultures must again pass through a lag 

 phase on minimal medium. Although a differential storage in the conidia 

 of the required factor may differentiate the mutant and wild-type, this 

 does not seem likely. In contrast to wild-type conidia or ascospores 

 which germinate and grow immediately on minimal medium, ascospores 

 of the leucineless mutant send out germ tubes but fail to grow further 

 on minimal medium. Ascospores of the tyrosineless strain initially grow 

 slowly even on supplemented medium. Although much additional in- 

 formation is needed before a final conclusion can be reached, existing 

 information suggests that as the result of mutation the enzyme systems 

 involved in biosynthesis of these factors are lost in these mutants during 

 the return of the cytoplasm to a resting state in conidiation or ascospore 

 production, and that the reconstitution of these systems depends upon 

 and always accompanies growth. We may therefore have in these adapting 

 strains examples of gene control of a biochemical reaction by altering 

 the efficiency of initiation of the synthesis of an enzyme rather than 

 by affecting enzyme production either quantitatively or qualitatively 

 as suggested in the cases of the complete or partial blocks as discussed 

 above. Strains which behave in this manner may represent the closest 

 approximation to an extra-nuclear control of enzyme production yet 

 found in Neurospora, since the maintenance of the enzyme activity in 

 the cytoplasm would seem to be to a certain extent independent of the 

 gene. 



Studies of the nutrition of fungi have gone through two general 

 phases. First, the identification of specific growth requirements leading 

 to culture of the organisms on media of known chemical constitution, 

 with the substantiation of the tenets of comparative biochemistry that 

 fungi require the same factors, amino acids and vitamins, as do other 

 organisms. Second, the experimental demonstration of the gene control 

 of biosynthesis of these factors by means of experimental gene mutation 

 with consequent biosynthetic deficiencies. Detailed biochemical study 

 of strains with such induced deficiencies has added significantly to 

 biochemical knowledge of vitamin and amino acid syntheses and inter- 

 relations. It may be predicted that future investigations will continue 



