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AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
fection. Furthermore, it appears from results obtained on the effect of 
acidity on the rate of germination of wheat in the control flats, which also 
shows a minimum, that there is as well an effect of the hydrogen ion on the 
host, which causes a depression in the infection curve. How this comes 
about is not certain, but it seems plausible that both these phenomena 
affect the severity of the infection. Nor is it at all clear at present what 
the cause of the depression in these curves is. In the opinion of the writer 
this will be solved only by further study of hydrogen-ion concentration 
in its relation to other factors. 
Because of the great importance not only of wheat scab, but of fusarial 
diseases as a whole, it is hoped that the results obtained in this study may 
have a practical bearing in the control of such soil-borne pathogens. Atten- 
tion is directed to the fact that the minimum in the infection curve occurs 
at Ph 5-5, a not unusual soil reaction to which the soil could easily be adjusted. 
The writer wishes to express his thanks to Dr. W. J. Robbins for material 
assistance and helpful advice during the progress of the work. 
Department of Botany, 
University of Missouri 
LITERATURE CITED 
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Jour. Wash. Acad. Sci. 6: 7-16. 1916. 
2. . Colorimetric determination of hydrogen-ion concentration without buffer 
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3. , and Hurst, L. A. Hydrogen-ion concentration measurements of soils of two 
types: Caribou loam and Washburn loam. Soil Sci. 4: 313-319. 1917. 
4. . Hydrogen-ion concentration — soil type — common potato scab. Soil 
Sci. 6: 219-236. 1918. 
5. Clark, W. M., and Lubs, H. A. The colorimetric determination of hydrogen-ion con- 
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236. 1917. 
6. Duggar, B. M., Severy, J. W., and Schmitz, H. Studies in the physiology of the fungi 
IV. The growth of certain fungi in plant decoctions, preliminary account. Ann. 
Mo. Bot. Card. 4: 165-173. 1917. 
7. Meacham, M. R. Note upon the hydrogen ion concentration necessary to inhibit 
the growth of four wood-destroying fungi. Science n. s. 48: 499, 500. 1918. 
8. Brightman, C. L., Meacham, M. R., and Acree, S. F. A spectrophotometric study of 
the " salt effects " of phosphates upon the color of phenolsulphonphthalein salts 
and some biological applications. Jour. Bact. 5: 169-180. 1920. 
9. Meacham, M. R., Hopfield, J. H., and Acree, S. F. The relative effect of phosphate- 
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parasitica on malt extract and corn meal media. Jour. Bact. 5: 305-308. 1920. 
10. Gillespie, L. J. The growth of the potato scab organism at various hydrogen-ion con- 
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11. Waksman, S. A. Cultural studies of species of Actinomyces. Soil Sci. 8: 71-207. 
1919. 
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VII. Growth of wood destroying fungi on liquid media. Ann. Mo. Bot. Card. 6: 
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