ief2 



PHYSIOLOGY OF THE FUNGI 



between these limits. The effects of pH upon the activity of urease 

 (Van Slyke, 1942) and upon the rate of growth of Neurospora crassa 

 (Ryan et al., 1943) are shown in Figs. 25 and 2G. From the general 

 similarity of these two curves it appears probable that pH affects the rate 

 of growth of fungi, at least in part, by modifying the rate of certain 

 enzymatic reactions. 



pH changes in media during growth. Fungi, as a result of their 

 metabolic activities, ordinarily change the pH of the media in which 

 they grow. These changes cannot be studied by making a single deter- 

 8.0r 



8 



10 



2 3 4 5 6 7 



Doys of incubation 



Fig. 27. Changes in pH during incubation of Sordaria fimicola in different volumes of 

 liquid glucose-casein hydrolysate medium at 25°C. 



mination of pH at any fixed time. Just as it is necessary to study growth 

 as a function of time of incubation (growth curve), it is necessary to deter- 

 mine the pH changes in an inoculated medium day after day to obtain a 

 complete representation of these changes (pH curve). The pH of the 

 medium should be followed in connection with the other functions being 

 studied. Since fungi differ in their metabolic activity and rate of growth, 

 the pH changes produced in the culture medium will differ. The pat- 

 terns of pH changes for the same fungus will depend upon the composition 

 and concentration of the media used. 



As an illustration of the effect of the composition of the medium upon 

 the pH changes, some of our data for Sordaria fimicola are given in Fig. 

 24. The correlation of the pH changes with the rate and amount of 

 growth of this fungus may be obtained by comparing the growth curves 

 obtained at the same time. From Fig. 27 it is evident that the hydrogen- 



