Walker—Nature of Disease Resistance in Plants. 
241 
found in the cell membranes or in the cell contents. In the first 
class are included (1) the internal cell walls as a factor in limiting 
the potato-rotting fungus (Pythium deharyanum), (2) the forma¬ 
tion of a layer of suberized cell walls in the cortex of flax plants 
resistant to wilt, and (3) evidence of differences in cell wall 
composition in plums resistant or susceptible to attack by Sclero- 
tinia cinerea. 
In the second case—that of internal differences in the cell con¬ 
tents—^we have first the examples in the various cases of rust re¬ 
sistance. While the physiology of the process is not clear we know 
that invasion of the cells of the resistant plant occurs but that 
progress is stopped either by lack of adjustment on the part of the 
fungus or by the presence of toxic substances in the host protoplasm. 
A theory of chemotropism to explain resistance has been offered 
by Massee (53) but it has never been well substantiated. The 
correlation between cell sap acidity and resistance was brought 
out by certain earlier workers; more recent investigations, how¬ 
ever, with other diseases has failed to show such a relation. 
Tannins have been shown to be more or less toxic to fungi and there 
are suggestions that in some cases this may contribute to resist¬ 
ance. Correlation between the anthocyan or flavone pigments and 
resistance has been pointed out in a number of instances. In only 
one case, that of onion, is the evidence sufficient to justify the sup¬ 
position that the coloring matter is the resistant principle, and 
even there further investigation is necessary to ascertain definitely 
whether it is the coloring matter or some other closely associated 
substance. 
The presence within the host tissues of a volatile substance 
more or less toxic to parasites of the same tissue has been shown 
in the case of onion. Curiously enough this substance has not been 
correlated with resistance although it may play a role in limiting 
the parasitism of the smudge fungus. 
The above summary is sufficient to show that the true nature of 
resistance has been exceedingly difficult to determine and, more¬ 
over, it is quite obvious that almost every case of resistance very 
probably presents a specific problem. In certain cases, no doubt, 
external factors, such as size of stomata, hairiness of leaf surface, 
or presence of bloom are factors contributing to resistance. In 
a larger number of cases, however, the underlying causes will be 
found in the interrelations between the parasite and the host cell 
membranes or the host cell contents. We can conceive of several 
