104 



PHYSIOLOGY OF THE FUNGI 



be modified by the presence of other compounds in the medium. Among 

 these the organic acids, especially the four-carbon dicarboxylic acids, play 

 an important role. This subject has been studied by Leonian and Lilly 

 (1940), Burkholder and McVeigh (1940), Brian et at. (1947), and Bernhard 

 and Albrecht (1947). The data in Table 19 illustrate the effect of organic 

 acids on the amount of growth of four fungi. 



Succinic and fumaric acids were most uniform in their effect on nitrogen 

 assimilation. Figure 18 shows the reciprocal effect of varying amounts of 



3.0 



2.0 



1.0 



g. succinic acid/l 



Fig. 18. The reciprocal effect of varying amounts of succinic acid (ammonium 

 nitrate constant) and ammonium nitrate (succinic acid constant) on the growth of 

 Phycoviyces blakesleeanus. (Drawn from data of Leonian and Lilly, Am. Jour. Botany 

 27: 22, 1940.) 



succinic acid and ammonium nitrogen on the growth of Phycomyces 

 blakesleeanus, which does not utilize nitrate nitrogen. The amount of 

 growth, within certain limits, is directly proportional to the amount of 

 succinic acid in the medium. 



Brian et al. (1947) have suggested on the basis of studies on Myro- 

 thecium verrucaria that a definite antagonism exists between the metabolic 

 pathways involved in nitrate and ammonium utilization, and in the 

 presence of ammonium nitrogen the nitrate pathway is blocked. Ammo- 

 nium nitrogen is poorly utilized unless certain organic acids are present in 

 the medium. Malic acid has no effect on utilization of nitrate nitrogen. 



