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yellow tissue below. Cross sections at a little later stage than this 
reveal the condition shown in fig. 12, in which all of the outer part of 
the bark, including the bast fibers, is being sloughed ofif. This is 
apparently brought about by the formation of fan-shaped masses of 
hyphae just inside the bast fibers. These masses unite later, one 
with the other, and form a thick stroma around the outside of the 
knot. Farlow (3) states that "The bast fibers are less altered in their 
direction and appearance than the other elements of the stem." This 
is due to the fact that they are located in a region that is not much 
changed in structure, so that they retain their normal position until 
they are sloughed ofT with the rest of the outer bark. The prevalent 
idea that bast fibers are included in the mature knot is probably not 
correct. 
A. cross section of a four-year-old stem infected with a knot is 
shown under low magnification in fig. 9. This knot has reached its 
maximum size and is covered on the outside with the black stroma 
containing the perithecia. As it illustrates well the structure of a 
mature knot it seems best to give it some description. The greatest 
diameter of this knot is 11.5 mm., of which 7.5 mm. is composed of 
xylem and the remainder of bark. The stem just below the knot has a 
diameter of 3 mm., of which 2 mm. is xylem, so that in this instance 
at least the proportional increase in thickness of both wood and bark 
had been about the same. About three-fourths of the circumference 
of the third ring of growth is changed in its outer part, and shows that 
the stimulating effect of the fungus reached this particular portion of 
the stem somewhat late in the growing season. Just above the normal 
xylem, near the center of the figure, the transitional parenchyma 
forms a dark band separating the xylem from the true parenchyma 
above. The parenchyma forms a light colored crescent-shaped mass 
above the center of the figure with dark colored radial bands of hyphae, 
and bits of last year's xylem scattered through it. An arch of xylem 
wedges borders the parenchyma above and becomes connected with 
the normal xylem on the right side of the figure while on the left side 
the conditions are further complicated by the fact that a lateral 
branch is given off at the point where the section was taken. The 
xylem on the lower side of the figure has not become greatly changed 
except that some of the multiseriate rays have become very broad in 
the outer ring of growth. It should be noticed that while this ring 
has not been greatly altered structurally on this side of the stem, its 
