Blister Canker 
55 
Davis than in the other varieties mentioned and have much 
thinner walls. 
There is also decided difference in the relative size and 
number of cells in the tissues formed early in the season and 
those formed rather late in the fall. The earliest and most 
abundant infection is almost always found in the region of 
the larger cells, formed in the early spring growth. The com- 
parative number of late thick-walled cells is much greater in 
the before mentioned resistant varieties. 
In making cell measurements and counting cells the area 
covered under a high power objective was taken as standard. 
This measured 0.36 mm. in diameter. The tissues formed early 
in the spring and late in the fall of each annual ring formed 
during the last five seasons were examined in each variety. 
Five specimens of each variety were examined in each region, 
making a total of 50 examinations for each variety. The data 
shown in table 13 represent an average for all these measure- 
ments. 
The number of medullary ray cells was determined from 
tangential wood sections. The different dimensions of the cells 
were determined by using both cross and tangential sections. 
These data while of a preliminary nature show clearly that 
structural differences play no small part in influencing im- 
munity of apple trees to Niimmidaria discreta. 
It is also quite evident that immunity is influenced to 
some extent by physiological factors. The fungus progresses 
much more slovdy in tissues where the protoplasm is still 
active. In fact the regions of the cambium and phloem are 
rarely if ever invaded until the tissues have been killed. As 
soon as the tissues are killed they are readily invaded as shown 
by both laboratory and field inoculations. This in itself does 
not account for the difference in degree of immunity of differ- 
ent varieties unless the age at which the cell content becomes 
inactive varies. There is a great variation in the age at which 
xylem tissues remain active in different species of trees but 
how much difference there may be in different species or sub- 
species and varieties of Malus has not been determined but is 
being studied by the writer at the present time. 
By comparing the rate of growth of mycelium in tissues 
which were known to be active and those known to contain no 
protoplasm some interesting results were obtained. In secur- 
ing these data 20 inoculations in one-year-old xylem of Ben 
Davis, Delicious, Jonathan, Winesap, Oldenburg, and Wealthy 
were examined. Ben Davis and Delicious were grouped to- 
