Nov. 24, 1923 
Variations in Strains of Rhizopus nigricans 
369 
The conclusions to be drawn from these experiments are, first, 
that there is a considerable uniformity in the cardinal temperatures for 
the growth of the mycelium of these different strains; and second, that 
the optimum and maximum temperatures for growth are several degrees 
lower than those for spore germination. The authors (8) obtained 
similar results in experiments in which the cardinal temperatures for 
germination and growth of different species of Rhizopus were studied. 
It is quite evident from the results obtained in these experiments and 
elsewhere, as already cited, that the temperature at which the spores 
will germinate can not be assumed to be that at which infection will 
take place. As a matter of fact, the results suggest that infection and 
decay would occur only at temperatures somewhat lower than those 
required for the germination of the spores. 
PECTINASE PRODUCTION AND HYDROGEN-ION CONCENTRATION 
Previous investigations showed that several species of Rhizopus pro¬ 
duced pectinase ( 1 ) in considerable abundance. In the same experi¬ 
ments it was found, however, that Rhizopus nigricans produced none 
or a very small amount of the macerating principle. This was sur¬ 
prising, in view of the fact that this species was the one responsible 
for most of the loss of sweet potatoes in storage due to soft rot. It has 
been noted also that when sweet potatoes are decayed by the organism, 
both in the laboratory and in commercial storage houses, the cells are 
separated along the line of the middle lamellae just as they are when 
decayed by species which produce an abundance of the macerating 
enzym. Investigations (7) have shown that one strain of R. nigricans , 
number 4652, when grown in sweet potato decoction tends to make 
the substrate less instead of more acid, as is the case of those species 
which produce a very active macerating principle. Furthermore, it 
has been found that the expressed juice from sweet potatoes decayed by 
R. nigricans was less acid than that from potatoes decayed by R. tritici; 
also that this juice did not macerate raw disks as readily as the juice 
from potatoes rotted by R. tritici. If, however, the juice of potatoes 
rotted by R. nigricans was made as acid as that from potatoes decayed 
by R . tritici , it macerated sweet-potato disks in the same length of time. 
In view of these facts, it was proposed to determine if the inability of 
R. nigricans to produce pectinase under cultural conditions as readily 
as do some of the other species was peculiar to a single strain or charac¬ 
teristic of the species in general, as represented by eighteen different 
strains, and also whether or not the same peculiarity is true with respect 
to the acid production. 
The method employed in these experiments was as follows: Seventy- 
five cubic centimeters of sweet potato decoction were placed in 250-cc. 
Erlenmeyer flasks. Enough flasks were prepared so that duplicate 
cultures of all the strains could be grown at the same time, thus sub¬ 
jecting all the organisms to as nearly the same conditions as possible. 
After inoculation the flasks were held in an incubator room with a 
temperature which varied from 23 0 to 24 0 C. At the end of the growth 
period (four days) the contents of the flasks on which the same strain 
had grown were made into a compound sample and the hydrogen-ion 
concentration determined. Raw sweet-potato disks were placed in 
one portion of the solution in order to determine whether or not pectinase 
