236 Wisconsin Academy of Sciences, Arts, and Letters. 
Cell Sap Acidity as a Factor in Resistance 
A correlation between resistance and a higher acidity of the cell 
sap of the host tissues has been pointed out by several investigators. 
Averna-Sacca (4) reports a higher acidity in grapes resistant to 
Oidium and Peronospora, and Comes (18, 19) reports a similar 
correlation for a variety of wheat (Rieti) resistant to rust. On 
the other hand, Vavilov (82) found no connection between the 
acidity of cell sap of many varieties of oats, wheat, and roses 
and their resistance to rusts and mildew. No such correlation was 
found in the potato by Jones and co-workers (44) in the case of late 
blight (PhytopJithora infestans), by Hawkins and Harvey (39) in 
the case of leak (Pythium debaryanum), nor by Weiss and 
Harvey (92) in the case of black wart (Sychitrium endobio- 
ticum). Gardner and Kendrick (35) in their work upon a tomato 
fruit spot caused by Bacterium exitiosum G. & K. find a corre¬ 
lation between the hydrogen ion concentration of the plant tissues 
and their resistance to infection. The organism did not grow in 
culture media more acid than a Ph value of 5. Examination of 
various tomato plant parts yielded Ph values as follows: seedlings 
and leaves, 6.3 to 6.5; green fruits, 5 to 5.4; ripening and mature 
fruits, about 4.6. Seedlings, leaves, and green fruits were very 
susceptible to the disease, but inoculations of ripe fruits were 
usually unsuccessful. Aside from the last instance, we have 
as yet very little convincing evidence that cell sap acidity is an 
important factor in disease resistance. 
Tannin as a Factor in Resistance 
The relation of tannin to the growth of a number of fungi was 
studied by Cook and Taubenhaus (21). They found that in 
general when tannin was added in increasing amounts to a favor¬ 
able medium, germination was inhibited and finally spores were 
killed. Considerable variation in the reaction of different fungi 
was noted, and in general, parasitic fungi were less resistant to 
the toxicity of tannin than saprophytic fungi. Cook and coworkers 
(20) later claimed that tannin as such does not exist in the host 
cell, except in small amounts. There does exist a polyatomic 
phenol, which, upon injury to the cell, results in the formation 
of a tannin or tannin-like substance. They point out that condi¬ 
tions for such a reaction prevail in normal immature pomaceous 
fruits which are injured by the invading hyphae. A germicidal 
