Nov. 5,1917 Diagnosing White-Pine Blister-Rust from Its Mycelium 285 
haustoria which are decidedly different in appearance 1 from those of the 
blister-rust, rules out any possibility of confusing the mycelium of mem¬ 
bers of either of the two groups with the mycelium which has been 
described. 
METHOD OF PARASITISM 
The manner in which the parasite attacks the host is also important 
as a distinguishing character. The hyphae are intercellular in position. 
As stated above, practically every cell of the bark may be pierced by 
one or more haustoria. Parenchyma cells of the cortex and in the neigh¬ 
borhood of the resin ducts, sieve cells, medullary-ray cells, and even 
resin cells are all subject to attack. Only the cork and schlerenchyma 
Fig. i .—Cranartium ribicola : c, A moderately straight haustorium which has dented the side of the host 
nucleus. Note the manner in which the haustorium is constricted in passing through the cell wall. 6, A 
branched haustorium in a young medullary-ray cell. The figures were drawn with the aid of a camera 
lucida. 
cells of old bark and, of course, dead bark are apparently immune. The 
hyphse may follow the medullary rays past the cambium and into the 
xylem for at least three annual rings, and in these medullary-ray cells 
the haustoria are quite as prominent as in the ray cells of the phloem, 
Haustoria do not enter the wood cells. The parenchyma cells surround¬ 
ing the resin ducts in the last two annual rings of xylem are also heavily 
attacked. In tangential or radial sections one sees that the hyphse are 
frequently massed in strands (PI. 31, A). These strands are often formed 
in the region of the thin-walled sieve tubes, but they are also scattered 
through the parenchyma. Generally the medullary-ray cells are forced 
apart by single layers of hyphae. Probably the swelling of the bark is 
1 Smith, Grant, the haustoria or the ERYsiphe^E. In Bot. Gaz., v. 29, no. 3* p. 153-184, pi. n-12. 
1900. 
