Blister Canker 
53 
tibility of apple trees to blister canker must be due to several 
factors. It is evident that structural differences influence to a 
great extent susceptibility to and amount of infection. That 
the degree of immunity is due in part to physiological factors 
can not be disputed and we may no doubt explain immunity in 
part by means of chemical differences. 
Norton^-^ designated anatomical or structural differences as 
the chief determiner of immunity of asparagus to rust. Ward^^ 
in a study of rust resistance in Bromes states he has con- 
cluded "that the matter has nothing to do with anatomy but 
depends entirely upon physiological reactions of the proto- 
plasm of the fungus and of the cells of the host." Jones,^^ q[^, 
dings and Lutman do not believe that in the case of potato 
late blight, immunity is determined by either chemical or ana- 
tomical factors. Cobb^ asserted that in the case of wheat 
rust anatomical hindrance plays an important part, but this is 
denied by Eriksson.^ Cook and Taubenhaus^ suggest that 
the presence of tannin in the cell sap influences to considerable 
extent susceptibility of trees to fungous diseases. 
That immunity of apple trees to Nummularia discreta does 
depend to a large extent upon cellular structure or mechanical 
resistance is indicated by the fact that certain varieties which 
have been found very resistant were found on examination to 
have a cellular structure different from that of other varieties 
which are decidedly susceptible. Table 13 gives a comparison 
of the number, size, and thickness of cell walls, of tracheae, 
wood cells, and medullary ray cells in different varieties. By 
referring to table 12 it will be seen that varieties such as 
Delicious and Ben Davis are very susceptible, Winesap and 
Jonathan moderately resistant, and Oldenburg and Wealthy 
practically resistant. The table shows no constant difference 
in the number of tracheae present in a given area but the 
walls of the tracheae were found on an average to be thicker 
than in susceptible varieties. The wood cells which surround 
the tracheae were found to be considerably larger and thinner 
walled in susceptible varieties such as Delicious and Ben Davis 
than in resistant varieties such as Oldenburg and Duchess. 
A greater proportion of the cells were also found to have 
pitted walls and, since the hyphae must depend upon utilizing 
the pits as a means of passing from one cell to another, it fol- 
lows that these varieties would be more susceptible. Perhaps 
the greatest difference in cell structure of the different vari- 
eties is in the number and size of the medullary ray cells. 
While there is no great difference in the shortest diameter of 
the cells they are considerably longer in Delicious and Ben 
