288 BOTANICAL GAZETTE [APRIL 
They demonstrated an oxidizing enzyme that produces tannin 
from gallic acid, and found that the abundance of this enzyme in 
fruits is correlated with their resistant properties. BASSETT and 
THOMPSON (3) studied this enzyme still further. KNuDSON (37) 
found that Aspergillus and Penicillium can utilize tannin as a source 
of carbon, by means of the enzyme tannase. That genera and even 
species within a genus vary greatly in their sensitiveness to tannin 
was shown by Cook and Witson, (18) in studying the chestnut 
blight. In the case of Sclerotinia, VALLEAU (46) failed to find any 
correlation between tannin content and resistance in plum varieties. 
In recent years considerable attention has been given by investi- 
gators to the relation between the H-ion concentration of the soil 
and that of the plant, and between the latter and its resistance 
properties. In 1912 Comes (15) announced that the wheats that 
were more resistant to rust had more acid saps, and that fertilizers 
which would increase the acidity of the sap would convey added 
immunity to the plant. Although others have since failed to cor- 
roborate these statements, positive correlations have been estab- 
lished between the acidity of the soil and the occurrence of potato 
scab (23, 24), between the acidity of grape saps and their resistance 
to disease (2), between the H-ion concentration of soil and that 
of the plant juice (14, 45), and between spinach mosaic and the 
PH value (27). On the other hand, a lack of correlation between 
the acidity of the host and its resistance properties has been 
found in the case of potatoes toward Pythium debaryanum (32), 
Phytophthora infestans (35), and Chrysophlyctis endobiotica (50). 
WAGNER (47, 48) noticed that certain plants increased their acidity 
when infected with bacterial pathogens. The acidity returned to 
normal after a brief period, unless the plant were unable to with- 
stand the attack, in which case there took place a sudden fall in 
acidity much below normal, the death of the tissue, and then a 
post mortem rise in acidity. The relation of H-ion concentration 
to the metabolism of fungi and bacteria has received some atten- 
tion (25, 38, 49). Scummpt and Hoactanp (41) give an extensive 
bibliography on the relation of bacteria to the reaction of the 
medium. In general the fungi have been found to be less sensitive 
than bacteria to the reaction of the medium, and hence fewer 
