Apr., 1922] HOPKINS — HYDROGEN-ION CONCENTRATION I77 
phosphate, and lactic acid gave similar low points in the growth-acidity 
curve. 
Fig. 18. Soil acidity and infection — hydrogen-ion concentration in its relation to the 
percentage of seedling infection. Hydrochloric-acid series. 
Summary 
The experiments reported in this paper were undertaken to determine 
the relation of hydrogen-ion concentration to wheat scab. It was desired 
to find if possible a limiting acidity for seedling infection of wheat by the 
causal organism of this disease, Gibberella Sauhinetii. 
A study of the pathogene Gibberella Saubinetii shows that, although a 
wide range of acidity is tolerated, there is a minimum in the growth curve. 
This minimum in the curve was found to be present in three different series 
of cultures. In the first series the reaction of a liquid medium was adjusted 
by means of sulphuric acid and sodium hydroxide; in the second, primary 
and secondary potassium phosphate, phosphoric acid, and potassium hy- 
droxide were used in a liquid medium, and in the third, the acidity of potato- 
dextrose agar was varied by means of lactic acid. The minimum point in 
the curve varied from about pn 5-5 to Ph 6.0. This is similar to the results 
of Webb (13) in his work on spore germination and hydrogen-ion concentra- 
tion. The use of various substances to change the reaction shows that the 
effect on the growth is due to the hydrogen-ion concentration and not to 
other molecules or ions. 
An interesting correlation appears in the relation of soil acidity to 
seedHng infection. Here also a minimum was obtained in two cases at 
Ph 5-5- In one instance the reaction was adjusted by means of sulphuric 
acid and sodium hydroxide, and in the other hydrochloric acid and sodium 
hydroxide were used. It seems, therefore, that there is a relation between 
the effect of acidity on the growth of the pathogene and its effect on in- 
