Apr. 7,1923 
Temperature Relations of Eleven Species of Rhizopus 37 
tubes of from three to five times as long as the diameter of the spores 
had formed on the agar containing 20 per cent dextrose. The differ¬ 
ences on the two agars seem to indicate that the presence of 20 per cent 
dextrose in Irish potato agar lowers slightly the minimum temperature 
for germination. 
To determine whether the optimum would be changed by the addition 
of dextrose, plates of each of the agars were prepared and placed at 
temperatures of 23, 26.5, 27.8, 28.7, and 30° C. Measurements of the 
diameters of the mycelial growths ^ter two days at 23, 26.5, 27.8, 28.7, 
and 30° on the unmodified agar were 42, 65.6, 61, 51.8, and o mm., 
and on agar with 20 per cent dextrose added 66.4, 87.8, 92, 92, and 83.2 
mm,, respectively. These figures show that the optimum for growth 
on the unmodified agar was 26.5°, while that on the agar containing 20 
per cent dextrose probably lay between 27.8 and 28.7°. The measure¬ 
ments of the diameters of the growths at 26.5, 27.8, and 28.7® after 
24 hours were 26, 36, and 34 mm., respectively, showing that the optimum 
was nearer 27.8° than 28.7° but nearer the latter than 26.5®, and was 
raised 1.5° to 2°. 
These experiments show rather conclusively that the presence of 
dextrose in Irish potato agar shifts the cardinal temperatures of this 
strain of Rhizopus nigricans. Thiele {21) also found that the addition of 
dextrose to glycerine and formic acid changed the maximum tempera¬ 
tures for the growth of one species of both Penicillium and Aspergillus. 
He observed that dextrose added to glycerine and to formic acid caused 
a shifting of the maximum temperatures for the growth of Penicillium 
of 5° and 4° C., respectively. The temperature maximums for the 
germination of Aspergillus spores and for mycelial growth were raised 
about 2® and 3°, respectively, by the addition of different amounts of 
dextrose. On the other hand the maximum for total growth was raised 
about 2® when growing on glycerine and lowered 3° on the higher con¬ 
centrations of fonnic acid. He concluded that the nutritive value of 
a substance is correlative to a certain extent with the temperature. 
Similarly, Bruderlein (j) found the growth of R. maydis to be retarded on 
potato, almost arrested on carrot, and prohibited entirely on agar at 
42^. Peltier (13) showed that the minimum temperature for the growth 
of Psuedomonas citri Hasse when grown in beef bouillon and on soluble 
starch agar is slightly higher than on cooked potato cylinders, its maxi¬ 
mum being slightly higher in the former than on the two latter media. 
Likewise, Goss (7) found that Fusarium trichothecioides Wollenw. made 
its best groAvth on a S3mthetic solution at 25° and no growth at 5^. On 
the other hand, the optimum lay between 15® and 20°, and a weighable 
growth was made at 5^ when this fungus grew on Link’s potato extract. 
Although it has been demonstrated by the writers and others that the 
cardinal temperatures of some fungi vary with the nature of the sub¬ 
strate, the principles underlying this phenomenon have never been 
explained. Thiele (21) attributes the shifting of the maximum tempera¬ 
ture of the fungi with which he worked to ^e different nutritive value 
of the substrate at different temperatures. It may be in the case of the 
writers’ experiments that the dextrose when present in considerable con¬ 
centration (10 per cent or over) was more available and hence supported 
growth at a higher temperature than when it was present only in small 
amounts. It is a well-known fact that increasing the molecular con¬ 
centration of a solution raises its boiling point and lowers its freezing 
point. It is not known whether or not the increased concentration of 
the dextrose in the substrate causes a sufficient increase in the molecular 
