July 21, 192 i 
Hydrogen-Ion Changes 
157 
this interesting phenomenon, data already presented must be briefly 
reviewed. It has been shown, for example, that Rhizopus tritici when 
grown on a nutrient solution makes the substrate considerably more acid. 
Rhizopus nigricans , on the other hand, renders the substrate less acid. 
In the case of the former species when growing on sweet potato decoction, 
the solution is not made acid enough to cause maceration of the cells, so 
that this can not account for any of the maceration noted. 
In order to determine if the acid produced under natural conditions 
in decaying potatoes bore any relation to the rapidity of decay, a series 
of experiments were performed in which the rate of maceration by the 
expressed juice was measured. A number of sweet potatoes were inocu¬ 
lated by the “well method” with Rhizopus tritici and R. nigricans. 
After a definite length of time the juice was pressed from the decayed 
portion and the hydrogen-ion concentration determined. The time 
required to macerate raw sweet-potato disks by the enzym suspended 
in the solution was also obtained. Control flasks in which the juice 
was boiled to inactivate the enzym were run in every case. The results 
of these experiments showed that the sweet-potato disks were decayed 
in considerably shorter time in the juice from potatoes decayed by R. 
tritici than in the juice from those decayed by R. nigricans —that is, in 
1.5, 4, 2, and 2.5, 4.75, and 5.5 hours respectively, or in an average of 
2\ and 4J hours. 
The hydrogen-ion concentrations of the expressed juice were as follows: 
R. tritici , 4.008 and 4.082; R. nigricans , 5.575 and 5.145. The P H values 
of the juice of sound sweet potatoes have been found to vary from 5.0 
to 6.0 in a considerable number of tests. The results show that R. 
tritici increases considerably the acidity of the cell sap while R. nigricans 
causes little or no change. The juice from the potatoes decayed by R. 
tritici macerated raw sweet potato disks in almost one-half the time 
required by the juice from those decayed by R. nigricans . If, however, 
enough acid was added to the juice from potatoes decayed by the latter 
to make it as acid as that from potatoes decayed by the former, the 
sweet-potato disks were macerated in the two solutions in exactly the 
same length of time. Previous investigations have shown that the 
maximum rate of maceration is obtained when the P H value of the sys¬ 
tem is between 3.0 and 4.0; that when it reaches 5 or beyond, the action 
of the enzym is considerably retarded. In other words, the acid in 
the system appears to function as a co-enzym and serves to hasten the 
reaction. Judging from these results, it seems that the feeble macerat¬ 
ing action of R. nigricans when grown in nutrient solutions may be due, 
in part at least, to its inability to produce acid; also that it would decay 
sweet potatoes equally as rapidly as Rhizopus tritici if it were not for the 
fact that it fails to provide optimum conditions by the production of 
an acid. 
LIGHT 
The studies on the effect of light on the vital activities of R. tritici 
were more or less preliminary to some of the investigations which will 
be detailed later. The original purpose of this study was to determine 
if light was in any way connected with the failure of R. tritici to produce 
pectinase in Czapek’s nutrient solution, or with the production of acids 
in this solution in sufficient quantity to cause maceration identical 
with that caused by the fungus when grown on sweet potato decoction. 
The investigations included the influences of light, on (1) the hydrogen- 
ion concentration of the substrate. (2) the rate of maceration by the 
