148 
Journal of Agricultural Research 
Vol. XXI, No. 9 
Thus, it is readily seen that only on cotyledons and young leaves on 
young plants are there numerous stomata in the upper epidermis, whereas 
the stomata are abundant in the lower epidermis of leaves of all ages. 
Thus the distribution of stomata is correlated with the incidence of in¬ 
fection; and on the basis of stomatal entry, the greater abundance of 
lesions resulting from inoculation of the lower epidermis and the greater 
susceptibility of young leaves to inoculation of the upper epidermis is 
explained. The more or less equal distribution of the stomata on the 
cotyledons also explains the fact that cotyledon lesions occur with equal 
frequency on either surface. Petioles were found to have about 34 sto¬ 
mata per square millimeter. 
Sections for microscopic study were cut from infected cotyledons fixed 
nine days after inoculation. These were stained in carbol fuchsin. 
Plate 28, C, is a photomicrograph showing a mass of the bacteria in a 
substomatal chamber directly under a stoma in the upper epidermis of 
the cotyledon. This affords evidence of stomatal entry. 
■RELATION OF HYDROGEN-ION CONCENTRATION TO INFECTION 
An approximate determination of the hvdrogen-ion concentration of 
different parts of greenhouse tomato plants was made by the colori¬ 
metric method described by Clark and Lubs (j). The plant juice w^as 
extracted by crushing the leaves or fruit in a mortar. This juice was 
diluted 1 to 5 or 1 to 20 with distilled water of a tested P H value and 
titrated by the addition of the indicators brom thymol blue, brom cresol 
purple, and methyl red. 
Seedlings and leaves yielded a P n value between 6.3 and 6.5. Green 
fruits showed P H values between 5 and 5.4, and in all cases the pericarp 
was slightly less acid than the seed pulp. Ripening and mature fruits 
showed P H values of about 4.6. According to Clark and Lubs (j, p. 221 ), 
Patten and Mains found the P H value of ripe tomatoes to be 4.2, and 
other workers have given it as 4. Thus it is evident the hydrogen-ion 
concentration in fruits is higher than in the foliage and is much higher 
in ripe fruit than in green fruit. 
There is an interesting correlation between these P n values, the 
hydrogen-ion tolerance of the bacterial spot organism in culture, and 
the observed susceptibility of the host parts to infection. In the colori¬ 
metric determination of the hydrogen-ion tolerance in culture, the upper 
limit of tolerance was a P n value of 5.3 for ordinary inoculation, although 
reinoculation resulted in raising this limit to 5. The seedlings and leaves 
are, therefore, well within the hydrogen-ion tolerance of the organism, 
the green fruit or at least the pericarp is just within the limit of toler¬ 
ance, while ripening and mature fruit has a much higher hydrogen-ion 
concentration than the organism will endure in culture. Seedlings, 
leaves, and green fruit are very susceptible to the disease, whereas, as a 
