Feb. 3,1923 Hydrogen-Ion Concentration and Varietal Resistance 383 
due to the fact that liming sometimes injures this species. Also, some 
irregular results obtained by Haas ( 11 ) with plants grown in Plainsfield 
sand, in which the growth of some plants was found to be unfavorably 
affected by lime, might well be due to the fact that poor growth is associ¬ 
ated with acid accumulation. 
In the last column of Table II are given the P H values of a number of 
varieties grown in the virgin soil of a cleared plot in a woodland (Cabin 
John, Md.). The weather during their growth was very unfavorable, 
being warm and humid, and the plants when 6 weeks old were far from 
vigorous. The winter varieties, Kanred, Turkey, and Kharkov, grew so 
poorly and were so badly infected by Erysiphe that no determinations 
were made on them. The high acidity of these samples was very interest¬ 
ing because the P H values agreed closely with the results obtained from 
unhealthy greenhouse plants. It is interesting to note that .Spriester- 
bach (unpublished report) found that frost caused a definite increase in 
the hydrogen-ion concentration of wheat plants. 
Fungous infection, often present in these cases, was mostly mildew and 
leaf rust, but the acidity measurements bore no relation, as a rule, to the 
presence or the degree of infection, so that the acid accumulation was 
attributed usually to physiological derangements of some of the meta¬ 
bolic processes. Wagner (32), however, claims that the diminution of 
hydrogen-ion concentration immediately following the injection of 
phytopathogenic bacteria is followed by a steadily increasing acidity 
until the end of the incubation period, falling back to normal if the 
plant successfully resists infection. Maze (ig) says that fungi are. active 
destroyers of organic acid, and Boas (3) says that the acidity of potatoes 
affected by the leaf-roll disease is less than that of healthy plants. How¬ 
ever, Harvey (13) found that the hydrogen-ion concentration of tobacco 
plants affected with the mosaic disease was somewhat greater than that 
of the healthy plants, and Weiss and Harvey (33) state that the potato- 
wart organism caused an increased acidity (P H 6.00) of the diseased 
tissue as compared to the healthy tissue (P H 6.49). 
In some of the writer’s sowings it was found that wheat plants badly 
infected with Erysiphe graminis were abnormally acid, and it seemed pos¬ 
sible that pathological trouble was causal. This was quite clearly indi¬ 
cated by Little Club, which was most susceptible to Erysiphe, yet which 
grew vigorously at greenhouse temperatures when free from disease. In 
one series, in which Kota, Preston, Pentad (D-5), Mindum, Arnautka, 
and Khapli, apparently clean, had P H values averaging 5.80, Little Club, 
Kanred, and Turkey, badly infected, had P H values of 5.63, 5.64, and 
5.67, respectively. In such cases, however, it is impossible to say that 
the acid accumulation was directly due to the activities of the fungus, as 
it may well have been the result of decreased vigor attendant upon 
infection. 
CONCLUSIONS 
(1) It appears from the data presented in this paper that there is no 
correlation between the hydrogen-ion concentration of the expressed 
juice and the resistance or susceptibility of wheat varieties to disease. 
(2) Environmental factors produce much greater differences in the 
hydrogen-ion concentration of the expressed juice than were ever found 
between varieties or between plants of different ages grown under iden¬ 
tical conditions. 
