Apr. 7, 1923 
Temperature Relations of Eleven Species of Rhizopus 3 
The rate of growth of these fungi at various temperatures was studied 
by growing them in Petri dishes containing 10 cc. of a 2 per cent Irish 
potato agar without additional sugar. A small drop of a suspension of 
spores in water was placed in the center of each plate with a 2-mm. 
platinum loop. The plates were placed in the incubators as soon as they 
were prepar^, and the rate of growth was determined by measuring 
twice daily the diameter of the mycelial felt formed. Five Petri dishes 
were placed at each temperature. When the results were not entirely 
satisfactory the experiment was repeated. 
The effect of temperature on sporangia formation was determined by 
growing the fungi at different temperatures in loo-cc. Erlenmeyer flasks 
on 30 cc. of 2 per cent Irish potato agar and observing the temperatures 
at which the sporangia were produced and the time required for their 
formation. 
INFLUENCE OF TEMPERATURE ON SPORE GERMINATION 
The effect of temperature upon spore germination has been studied 
by a number of investigators and by several different methods. The per¬ 
centage of germination at different temperatures has been used as a meas¬ 
ure of the influence of temperature upon spore germination by Melhus 
(r^), Doran, {/j ), Reed and Crabill {18 ), and oUiers. Naturally the method 
used in any particular investigation must depend upon the type of data 
desired. Preliminary tests showed that practically 100 per cent of the 
spores of Rhizopus spp. germinated except at extreme temperatures; 
hence, the percentage of germination could be used as a measure of the 
effect of temperature only by taking into consideration the time factor. 
The number of spores which germinate and cause infection in the case 
of decay-producing fungi of this type is not of as much importance as it 
is in the case of organisms producing other types of diseases, such as 
leafspots. In the former case a host may be completely destroyed as a 
result of a single infection, while in the latter the total amount of damage 
done often depends upon the number of individual infections. In view 
of these facts the writers decided to use the time required for germina¬ 
tion to begin as the measure of the influence of temperature upon this 
phenomenon. Anderson (2), Melhus (14), Ames (i), Rands (j 6), 
Ravaz and Verge (77), Shapovalov (jp), and others have used this 
same criterion in their investigations. However, in no case is it made 
clear when these writers considered germination to have taken place. It 
is possible to use as a criterion the time when the maximum percentage 
of the spores have germinated or when the spores which germinate first 
have produced germ tubes. With Rhizopus a few spores in each drop 
produce a germ tube first, followed very soon by others, the number 
gradually increasing so that in a short time nearly 100 per cent of the 
spores have germinated. The period intervening between the time 
when the germ tubes appear on the spores which are the first to ger¬ 
minate and the time when all the spores have germinated varies with the 
temperature, being shortest at the optimum and gradually increasing as 
the upper and lower temperature limits are approached. As a criterion 
of germination the writers decided to use the time necessary for the 
spores which germinate first to produce a germ tube equal in length to 
the diameter of the spores. Using the figures thus obtained, curves were 
plotted which show the variation in time due to the difference in tem¬ 
perature. The time necessary for the germ tubes to reach some specific 
