Journal of Agricultural Research 
Vol. XXV, No. 3 
156 
the species might be due to the difference in the amount of acid pro¬ 
duced, which acted alone in the absence of pectinase or as a co-enzym. 
It was accordingly proposed to make a comparison of the changes in 
hydrogen-ion concentration produced by the different species. A study 
of the influence of light on some of the vital activities of R. tritici was also 
made. 
COMPARISON OP DIFFERENT SPECIES OF RHIZOPUS 
A comparison of the different species of Rhizopus growing on sweet- 
potato decoction was made. A sufficient quantity of the decoction was 
prepared at the beginning to carry out the entire investigation. Every 
precaution was taken to grow all the species under identical conditions. 
A number of flasks were prepared for each species and several held as 
controls. The inoculated flasks and controls were incubated at a tem¬ 
perature of from 23 0 to 24 0 C. in the dark. At the end of the growth 
period (7 days) the substrate was filtered through cotton to separate it 
from the mycelium. The solution on which each species had grown was 
collected into one sample and after thoroughly mixing, its hydrogen-ion 
concentration was tested. The control solution (collection from several 
inoculated flasks) was found to have a P H of 5.35. 
Table I. —Showing the hydrogen-ion concentration of the solutions on which 11 species 
of Rhizopus had grown for 7 days 
Species. 
Arto¬ 
carpi. 
Nigri¬ 
cans. 
Micros¬ 
porus. 
Reflex- 
us. 
Tri¬ 
tici. 
Dele¬ 
mar. 
Ory- 
zae. 
Nodo¬ 
sus. 
Arrhi- 
zus. 
May- 
dis. 
Chinen¬ 
sis. 
Ph. 
4* 00 
5*44 
5* 62 
3-42 
3 * 07 
3 - 07 
3 - 07 
3-07 
3-24 
3* 22 
4-31 
Table I shows some interesting data. The sweet-potato decoction did 
not reach as high a hydrogen-ion concentration as is usually obtained in 
Czapek’s nutrient solution. It will be seen that two species, R. nigricans 
and R. microsporus van Tieghem, make the solution less acid, while in 
all other cases the acidity of the solution is increased. A number of the 
fungi have changed the P H value to a little above three. Two species, 
artocarpi Racib. and chinensis Saito, have brought the P H of the solution 
to 4.0 and 4.31, respectively. It is interesting to note the exact similarity 
in the results obtained with tritici , delemar (Boid.) Wehmer and Hanzawa, 
oryzae Went and Pr. Geerlings, and nodosus Namysl. These four species 
are very similar morphologically, and in many cases it is not possible to 
distinguish between them. The writers have several times called atten¬ 
tion to the fact that these four species fall into one group and may 
eventually be found to be identical taxonomically. 
ACIDITY IN RELATION TO MACERATION 
In a previous publication (4) it was shown that the time required to 
macerate raw sweet potato disks by the enzym secreted by Rhizopzts 
nigricans in the solution on which it grew and that retained by the 
mycelium, was much longer than that required by such species as tritici , 
delemar, and others. However, Rhizopus nigricans is the species responsi¬ 
ble for most of the decay of sweet potatoes in storage. It does not, 
however, decay potatoes as rapidly as some of the other species. Thus 
far the writers have been unable to offer any explanation of why this 
species will readily decay sweet potatoes, causing a loss of coherence of 
the cells in an identical manner, but producing a much less amount of 
pectinase in artificial cultures. In attempting to offer an explanation of 
