Apr., 1922] 
HOPKINS — HYDROGEN-ION CONCENTRATION 
161 
and limiting acidities for the germination of the spores of Aspergillus niger, 
Penicillium cyclopium, Botrytis cinerea, and a species of Fusarium. His 
results with Fusarium sp. seem especially significant in connection with the 
data presented in this paper. Webb found with the spores of this fungus 
that at 22° after 20 hours no germination occurred at pn 2.8, slight germina- 
tion at 3.1, and increasing germination up to 5.0. From this point the 
germination falls off to 6.2, rises to a second high point at 7.4, and finally 
declines to lO.o. The curve therefore has two maxima with a minimum 
between. Webb's curve for the germination of the spores of Fusarium sp. 
at 27° also shows a distinct minimum at pn 6.2. He also obtained minimum 
points, though less marked, in the germination curves of the spores of 
Aspergillus niger at 27° C. and of Penicillium cyclopium at 27° and 31° C. 
In the present study, the effect of hydrogen-ion concentration on the 
growth of Gibberella Saubinetii, the causal organism of wheat scab, was 
determined. An authentic culture of the pathogene was secured from 
Professor J. G. Dickson of the University of Wisconsin. A single spore 
isolation from this culture was used throughout the studies here presented. 
Three experiments were performed. In the first the reaction of a liquid 
medium was adjusted with sulphuric acid and sodium hydroxide, in the 
second with phosphates, phosphoric acid, and potassium hydroxide, and 
in the third lactic acid was used with potato-dextrose agar. 
Experiment i. Growth on Liquid Media — Sulphuric-Acid- 
Sodium-Hydroxide Series 
The culture solution used had the following composition : 
KNO3 2.0 g. 
MgS04.7H20 0.5 g. 
KH2PO4 0.1 g. 
Glucose 10. o g. 
H2O 1,000 cc. 
Fifty cubic centimeters of this solution were added to each culture 
flask. Erlenmeyer Pyrex flasks of 150-cubic-centimeter capacity were 
used. The flasks containing the solution were sterilized, and the reaction 
was adjusted when they were cool by adding a varying number of drops 
of sterile solutions of N/i and N/10 H2SO4 and N/i and N/io NaOH. Five 
cubic centimeters of the solution were then withdrawn aseptically from each 
flask for the determination of the hydrogen-ion concentration. The colori- 
metric method devised by Gillespie (2) was used in obtaining the pH values. 
The cultures were inoculated with mycelium cut from potato-agar plates. 
They were incubated in a dark room at a temperature of 26° C. At the 
end of seven days, ten cubic centimeters of concentrated hydrochloric acid 
were added to each flask to stop the growth. The dry weights of the 
mycelial mats were determined by filtering into a Gooch crucible and drying 
at 110° C. It was found that the filtering through the Gooch filters was 
