46 North Carolina Agricultural Experiment Station 
forms as members of the colon-typhoid-dysentery group. As an increasing 
number of plant pathogenic forms on the same or closely related hosts come 
to be known, as may be illustrated by the present studies with organisms as¬ 
sociated with tobacco leafspot diseases, more emphasis will of necessity be 
placed upon the use of rare sugars. Their value for diagnostic purposes, even 
at the present stage of pliytopathological development, appears to necessitate 
their employment. 
Agitation. The oxygen relation is customarily studied in fermentation 
tubes. With certain organisms, clouding has been observed in the closed arm in 
tests, which when repeated, resulted in growth only in the open arm. The slight 
agitation resultant on handling has been noted to give an apparent growth in 
the closed arm. In consequence, inoculated fermentation tubes are left undis¬ 
turbed for from 7 to 10 clays, the period of duration of such tests. 
If cultures are continuously agitated during fermentation, a marked in¬ 
crease in rate of fermentation and under certain conditions in amount also is 
produced. 
THERMAL DEATH POINT 
This is normally studied according to an accepted procedure. It has been 
shown though that variations in thermal death point may arise dependent 
upon hydrogen ion concentration. Furthermore, it is highly probable that, 
just as in studies on tolerance to acids, the kind of acid and character of the 
medium are factors which affect thermal death points. 
NITRATE REDUCTION 
Studies by Conn and Breed (6) have pointed out that several sources of 
error are inherent in nitrate reduction tests with standard nitrate broth. The 
poor growth of the colon organisms, the inability of Psuedomonas fiuorescens 
to reduce nitrate in the presence of peptone, and the utilization of nitrate by 
B. caudatus as fast as it is formed, might lead to the erroneous conclusion that 
these organisms are unable to reduce nitrate. In view of the fact that these 
same causes may be operative in the case of many plant pathogenes which 
have been called non-nitrate reducers, exhaustive tests alone can determine 
ability to reduce nitrates. 
TOLERANCE TO ACID OR ALKALI 
The procedure usually employed in this phase of cultural studies of plant 
pathogenes yields little of value since so many factors to which no considera¬ 
tion has been given, are involved. Brightman, Meacham and Acree (7) have 
summarized this fact by stating that when measuring the limits of tolerance 
for hydrogen or other ions or molecules by organisms, care should be taken 
to state the concentration of chemicals and buffers used. When, therefore, 
data are accompanied by a statement of the exact composition of the experi¬ 
mental media which should include kind and concentration of acid or alkali, 
of buffers and of chemicals, they are reproducible and have a definite and dis¬ 
tinctive value. Whether or not, however, such data can be correlated with 
problems of immunity and resistance to disease remains to be determined. 
