Blister Canker 
29 
due to the presence of protoplasm in the still active tissues. 
This is shown by the fact that in the laboratory the one-year- 
old xylem of wood cylinders in which the protoplasm had been 
killed was invaded as readily as the old heartwood, but was 
not invaded in cylinders where the protoplasm was not killed. 
As shown by the data in table 5, while infection occurs in 
xylem of all ajes the rate of growth is very slow in the outer 
annual rings of growth, as compared with the older tissues. 
Infections also took place from the inoculations in the region 
of the cambium, but examination showed that the infection 
was in the xylem lying just beneath. Where the strands were 
found just beneath the cambium an extra number of small 
wood cells were found between the infection and the cambium. 
In many cases where the infection was in small limbs this 
caused a distinct ridge in the bark which extended as far as 
the end of the diseased streak. 
These inoculations were made in May, 1915, and exam- 
ined in November, 1916. The time which had elapsed was 
equal to almost two growing seasons. 
In determining the amount of growth made by the myce- 
lium, microscopic examinations were made as in the case of 
identifying infections. In fact many of the inoculations 
served the same purpose. At first sections were made and 
examined every inch above and below the inoculation until no 
discolored strands showed in the tissues. It was soon found 
that the hyphae could be found easily until the region within 
two to three inches of the end of the brown strands was 
reached where they became very few and were found only in 
the tracheae. In the majority of cases, however, on close ex- 
amination hyphae were found within a few mm. of the end of 
the discolored areas. Accordingly in measuring the distance 
covered by the hyphae the end of the discoloration was taken 
as the limit of infection. Radially hyphae were nearly always 
found to extend to within at least one or two cells of the mar- 
gin of the discolored area except in the case of the medullary 
rays where the distance was sometimes greater. 
In the long axis of the branches the active tissues are killed 
one to three or even more inches ahead of the invading hyphae. 
The whitish dry spots may often be seen in cross section with 
the naked eye. The killed area is readily determined by stain- 
ing and by plasmolysis tests. In using the Pianese stain it 
was found that the dead cells stained much less readily than 
those which were still active. For this reason the cells of the 
infected sections were much lighter in color or were only 
