1915] COOK & WILSON—ENDOTHIA PARASITICA 359 
but we can put known amounts into those with which we are 
working. 
4. The quantity and form of tannin compounds present in the 
substratum each exerts an influence on the growth of the fungus; 
but when the fungus attacks a plant, we have no way of knowing the 
form of the tannin with which it comes in contact. However, it 
is quite evident that the tannin of the plant is associated with 
coloring materials and other substances, some of which are toxic. 
Furthermore, the fungus may be selective during either a part of 
or during its entire existence, and send its mycelium into certain 
tissues containing little or no tannin. £. parasitica is especially 
destructive because it works in the cambium cells, but later in life 
it pushes through the outer tannin-bearing cells of the bark for the 
production of its spores. 
5. The character of the food supply influences the vigor of 
the fungus, and therefore its power to resist the toxicity of the tannin 
and other materials with which it comes in contact. The amount 
and character both of the food supply and of the tannin and other 
materials no doubt vary with the seasons and the growing periods 
of the host plant. 
6. In almost every instance, without regard to the form of 
tannin used or the fungus grown, a high percentage (0.8 per cent 
or more) of tannin caused a retardation of germination, fre- 
quently followed by an abnormal stimulation to growth of aerial 
mycelium. 
7. Species of Endothia show a marked response to the presence 
of tannin and related substances in the culture medium. 
8. The species of Endothia, and to a certain extent strains of the 
same species, show a considerable variation in their response to 
tannin and other substances. (a) E. radicalis mississippiensis was 
unaffected by the tannin, but did not use it for food and did not 
produce pycnospores in cultures containing tannin. (6) E. para- 
sitica was slightly retarded in its germination and early growth, 
but later was able to use as much as 2 per cent tannin as food. It 
was the only species studied that was able to utilize any considerable 
amount of tannin forfood. The American strains were more resistant 
to tannin and associated toxic materials than the Chinese strains. 
