V. The Fermentative Activity of Some Plant Pathogenic 
Bacteria in Relation to Hydrogen Ion Concentration 
The classification of bacteria is based, in part, upon their ability to ferment 
carbohydrates. The reliability of such fermentation tests for diagnostic pur¬ 
poses, moreover, has frequently been brought into question in physiological in¬ 
vestigations. This doubt has been largely removed, during the past few years, 
however, through a more adequate knowledge of the physico-chemical laws 
which govern fermentation and other microbial activities as well. The classi¬ 
cal works of Sorensen (19 and 20) and Micliaelis (17), basic to the develop¬ 
ment of methods for the measurement of hydrogen ion concentration and in 
consequence to the numerous researches which have demonstrated its import¬ 
ance, have contributed more than any other factor to this proper understand¬ 
ing. Singularly, in researches with bacterial plant pathogenes, the influence 
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of the hydrogen ion lias been accorded only scant attention and has never been 
emphasized although a very considerable interest exists in this group of or¬ 
ganisms as manifested by the number of published investigations. The char¬ 
acter of these investigations, furthermore appears to be extensive and de¬ 
scriptive rather than intensive and analytical, when contrasted with medico- 
bacteriological studies and no methods especially adapted to work with plant 
parasites have been developed. Reasons for this are not far to be sought 
since most of our knowledge of bacterial diseases of plants, a field containing 
a wealth of new or little known problems, has been acquired from studies dur¬ 
ing the past ten years and since in training phytopathologists too little em¬ 
phasis has been placed upon a fundamental knowledge of plant physiology and 
chemistry. The future alone will disclose to what extent anything of value 
is to be gained from studies in plant bacteriology comparable to those in med¬ 
ico-bacteriology on such problems as the carbohydrate and nitrogen metabolism, 
or any other phase of what may be included under bacterial dietetics, as their 
biochemistry or as the influence of hydrogen ion concentration in so far as it 
may affect rate of cell multiplication, production of pigment, rate of fermenta¬ 
tion or putrefaction, immunity of the host, time of infection, etc. 
The present inquiry into the relation of hydrogen ion concentration to the 
fermentative activity of some organisms from plant tissues is based upon re¬ 
cent researches in sanitary and medical bacteriology. Comparison of results 
and methods will, therefore, be freely made in establishing points of similarity 
or difference. As might be anticipated, there are many points of similarity 
between these two groups of organisms. It might be surmised, too, that a 
number of fundamental differences exist between species in either group 
which could not be brought to light by the employment of any stereotyped 
technic. That such is the case and that a knowledge of the physiology of one 
species should lead one to generalize with caution regarding the physiology of 
another becomes apparent in the course of the studies to be herein reported. 
METHODS AND TECHNIC 
Organisms. Two organisms causing leaf spot diseases of tobacco, Bac¬ 
terium tabacum and B. angulatum, two causing leafspots of soybean, B. gly- 
