Juae 9, 1923 
Substrate and Hydrogen-Ion Concentration 
873 
as it was not alkaline enough to inhibit growth. Growth w^as better 
in sweet-potato decoction than in Czapek’s nutrient solution at approxi¬ 
mately the same hydrogen-ion concentration. 
The fungus tends in all cases to make the solution more acid. The 
degree of acidity reached was greater in Czapek’s solution than in sweet- 
potato decoction. In experiment i (Table III) no growth took place 
at a Ph of 1.04 and i.71 and the hydrogen-ion concentration was there¬ 
fore not much changed. As the hydrogen-ion concentrations of the 
control solutions decrease, however, the change in the Ph value of the 
inoculated solutions becomes greater, as, for example, Ph 8.53 (Table III) 
was changed to 2.66. It will be seen from Table III and the other 
tables also that the Ph of the inoculated solutions gradually increase 
with the decrease in the hydrogen-ion concentration of the control 
solutions. 
That many fungi and bacteria make the substrate acid or alkaline 
is well known. Currie (4) showed that citric acid was produced by 
Aspergillus niger in a nutrient solution and Wehmer {20) gave the 
generic name Citromyces to a group of fungi which he believed was 
characterized by its ability to produce the same acid. Lafar (17), and 
Lind {18) found that oxalic acid was produced by certain species of 
Penicillium, Botrytis, and Citromyces. Weimer and Harter {21) showed 
that Rhizopus triticiy Diplodia tuhericolOy Mucor racemosusy Penicillium sp. 
and Botrytis cinerea increased the acidity of Czapek’s nutrient solution 
when glucose was used as the source of carbon. Young and Bennett 
(25) found that Fusarium oxysporum when grown on Richard’s solution 
with a Ph 5 first made the solution acid and then alkaline so that in 40 
days a Ph of 7.4 was reached. Bacteria appear to produce acid in the 
solution, though not to so marked a degree. For example, Jones (u) 
found that a strain of Pneumococcus when inoculated into a medium 
with an initial reaction of Ph 7.0 grew poorly and developed a Ph of 
6.2. If, on the other hand, the initial reaction was 7.6 a good growth 
and a final Ph of 5.4 resulted. Streptococcus viridans was found by 
Grace and Highberger (7) to change a broth with an initial hydrogen-ion 
concentration of between Ph 7.1 and 7.3 to a Ph 6.6 in 6 days. Similar 
changes in the hydrogen-ion concentrations were noted by Wolf and 
Harris (2j) and by Karrer (12) with certain bacteria and fungi, respec¬ 
tively. A comparison of the results of other investigators with those 
obtained by the authors seems to indicate that Rhizopus tritici renders 
the substrate more acid than most fungi or bacteria. As a matter of 
fact, it will be seen that the degree of acidity produced (Ph 2.09, Table IV) 
while not prohibitive of growth, which is practically stopped in a Ph i .7» 
closely approximates it. 
From Tables III and IV it is seen that pectinase is not produced in 
Czapek’s nutrient solution when adjusted to any of the hydrogen-ion 
concentrations tested. This conclusion is drawn from the fact that, al¬ 
though a certain amount of maceration took place in certain solutions, 
especially in those with the highest hydrogen-ion concentrations, there 
was just as much in the steamed as in the unsteamed solutions. In the 
solution with an initial Ph of i.oi (Table III) no growth of the fungus 
took place and yet maceration was complete in 5 hours. When the 
initial Ph was 1.76, 24 hours were required to complete maceration. At 
this concentration no growth occurred. At all other concentrations there 
was a normal amount of mycelium produced and the final hydrogen-ion 
concentration of the inoculated solutions varied from Ph 2.15 to 2.66 
