Apr. 7,1923 
Temperature Relations of Eleven Species of Rhizopus 7 
incorrect identification or to mixed cultures, to the influence of the 
culture media, etc. A comparative study of different strains of nigricans 
which the writers have under way may throw some light upon this 
subject. The variation in temperature between different parts of an 
artificially heated incubator is a factor which can not be overlooked. 
The upper strata of air in an incubator whose temperature is measured by 
means of a thermometer inserted through an opening at the top has been 
found to be in some cases from i to several degrees warmer than that at 
the center or bottom. In chambers heated by a water jacket the writers 
have recorded a difference of 5° between the temperature of the air at 
the top and bottom of a chamber, a distance of only 16 inches. In the 
present investigations the thermometers were compared with a standard¬ 
ized thermometer and then the bulbs were lowered to the immediate 
vicinity of the cultures in order to obviate as nearly as possible this 
source of error. 
Dunn (5) found that the plus and minus strains of nigricans with which 
she worked differed somewhat in their temperature relations. The 
minus strain seemed to be more vigorous, since it had a slightly higher 
optimum than the plus and grew at both a slightly higher and a lower 
temperature. The optimum for growth by the minus strain lay between 
25° and 28® C., which agrees closely with that obtained by the writers— 
approximately 25®. The maximum temperature for the growth of the 
minus strain was about 31°, which also agrees closely with that obtained 
for the strain studied by the writers. Dunn found a considerable differ¬ 
ence in the optimums for different strains of Rhizopus from strawberries. 
The optimum temperature for one strain was about 36°, while that for 
other strains was approximately 27® or 28°. However, she expresses 
doubt as to whether she was working with strains or species, and since 
several species of Rhizopus are capable of decaying strawberries (rr) 
she may have worked with a species which thrived best at a higher 
temperature rather than with a strain. 
INFLUENCIS OF TEMPERATURE ON GROWTH 
Having determined the effect of temperature on germination, its 
influence on the continued growth of the germ tubes and resulting 
mycelium was next studied. This was done by measuring the daily 
increment of growth on agar in Petri dishes. The results are shown 
graphically in figures 2 to 12, in which the base line shows the tempera¬ 
ture in degrees centigrade and the perpendicular shows the diameter of 
the mycelial disks at stated intervals of time. 
An examination of these figures shows that they posses some common 
characteristics. Each series, for example, originates at scattered points 
to the left (the, lowest temperatures tried at which growth did not take 
place within the given time), then rises in the direction of the optimum, and 
finally falls to a point at 4 ie right (the temperature at which no growth 
took place). From these figures it is evident that some growth took place 
at the m axim u m temperatiures during the first 24 hours, while at the 
minimum temperatures growth did not begin until sometime later. 
The optimum temperature in all cases remains the same within the time 
shown in these graphs. 
The lowest temperatinre at which growth of maydis and arrhizus was 
observed was 7.4® C. after 7 and 12 days, respectively. Hanzawa (9) 
found that arrhizus could make some growth at 6®. The writers did not 
