THE ANATOMY AND OTHER FEATURES OF THE BLACK KNOT " II9 
than before. After this has taken place it begins to form xylem of the 
current year opposite the xylem that has been pushed outward; the 
continuation of this activity results in the production of wedge-shaped 
masses of xylem. Between these still other wedges may be formed ^ 
which are similar to those above mentioned except that they are not 
in contact with any of the xylem of the previous year's growth toward 
the center. The wedges are all well separated from each other by 
the rays, which have further broadened, so that cross sections through 
the infected part have a superficial resemblance to those of semi- 
herbaceous stems in which the fibrovascular bundles are separated from 
each other by broad rays. This is especially true in knots on young 
stems which have become changed throughout their entire circum- 
ference and into the pith. The structure of these wedges usually ap- 
pears to be quite regular in cross section, but longitudinally they 
show a considerable amount of wood parenchyma and scalariform 
tracheids to be present. It is evident that these tracheids are truly 
scalariform and not apparently so, owing to tertiary thickening, 
because none of the ordinary bordered pits are present in their walls. 
These scalariform tracheids are also very common in the cortical 
portion of the knot at this stage, but are not present in normal adult 
wood. 
Tangential sections through the wedges (see fig. 5) show that the 
elements that compose them are very much bent and twisted, a con- 
dition that is brought about by the broadening of the rays. Radially, 
the course of these elements is usually somewhat wavy. 
The development of the wedges is quite uniform for a while but 
sooner or later they may become altered in appearance by the pro- 
duction of a considerable amount of wood parenchyma along their 
sides. At about the time that this is taking place, there is a sudden 
growth of the rays in a radial direction. In this growth the position 
of the cambium opposite the rays is very much disturbed and portions 
of it are pushed outward into the cortical portion of the knot and 
more or less torn apart. It is often the case that the segments of the 
cambium opposite the rays are simply broken in two by this process 
and the free ends thus formed are pushed out into the bark, but 
otherwise remain in contact with the cambium opposite the xylem 
wedges. In the misplacement and disorganization of the cambium 
opposite the broad rays, its cells may become turned over more or 
less at right angles, so that tracheids, vessels, and fibers are commonly 
