370 
Journal of Agricultural Research 
Vol. XXVI, No. 8 
had been secreted, and, if so, to obtain an idea of the amount as indicated 
by the rate at which it caused maceration. Control flasks were carried 
in all the experiments. The rate of maceration in the steamed con¬ 
trols (steamed 15 minutes) of the solutions on which the organisms grew 
was compared with that in the solutions not steamed. 
With respect to pectinase production it may be stated that no macera¬ 
tion of raw sweet-potato disks occurred in any of the solutions at the 
end of forty-eight hours. There was no apparent difference between 
the steamed and unsteamed solutions, which seemed to indicate that no 
pectinase was secreted. The fact that the eighteen organisms all acted 
alike in this respect seemed to suggest that under these conditions R . 
nigricans characteristically produces none, or at least only a very small 
amount of pectinase. 
The hydrogen-ion concentration of the solution after the organisms 
had grown on it for four days is shown in Table IV. 
Table IV.— Hydrogen-ion concentrations of the sweet-potato decoction after the different 
strains of R. nigricans had grown on it for 4 days (average of several experiments) 
Organism No. 
Ph. 
Organism No. 
Ph. 
5061 
S°S3 
5063 
5°54 
S°S6 
S°S7 
6.853 
6.997 
6.873 
6.78O 
6-353 
5-334 
6.509 
6.157 
5.322 
4.468 
6.66l 
5-814 
5055 
4652 
5052 
5060 
5058 
4682 
Organism No. 
Ph. 
4684. 
6.560 
4787. 
6.718 
5 ° 62 . 
7.021 
6.836 
S-I 58 
Table IV shows that all the different strains, with the exception of 
5060, decreased the acidity of the solution. In the case of 5060 the 
acidity of the substrate was considerably increased by the growth of 
the fungus. 
The results of these investigations show that pectinase is not generally 
produced when a nutrient solution, such as sweet-potato decoction, is 
used as a substrate. 
Just why this is so, is not clear. Results previously published ( 1) show 
that one strain of Rhizopus nigricans (4652) produced a small amount 
of pectinase when compared with some of the other species. It is quite 
evident from the results here that the failure to do so is not peculiar to 
any one strain, but is probably characteristic of the species as a whole. 
The writers found that, if R. nigricans and R. tritici were inoculated 
into sweet potatoes, decay took place in both cases, the cells being 
separated along the line of the middle lamellae. If raw sweet-potato 
disks were suspended in the expressed juice, they were macerated in a 
shorter time in that from potatoes decayed by R. tritici than in that 
decayed by R. nigricans. The juice from R. tritici decayed potatoes 
was the most acid. If, however, the expressed juice from R. nigricans 
decayed potatoes was made as acid as the R. tritici juice, maceration 
took place in the same length of time in both cases. It would seem, 
therefore, that the composition of the substrate has much to do with the 
production of pectinase. There is probably something lacking in all 
of the artificial culture media used which is required to stimulate the 
secretion of the enzyme. 
