THE ANATOMY AND OTHER FEATURES OF THE " BLACK KNOT" II7 
xylem may differ but little in appearance from that ordinarily formed 
except that it occurs in long radial segments separated from similar 
adjacent segments by the broad rays which are formed at this stage 
(see fig. 8). Parenchyma may continue to be formed until the end of 
the growing season, but usually small segments of summer wood are 
produced late in the season which appear as small islands, just inside 
the cambium, surrounded on all sides by parenchyma. 
The presence of the fungus in the stem also stimulates the formation 
of an abnormal amount of ray tissue, shown to best advantage in areas 
where the remainder of the xylem has not been too greatly changed 
into parenchyma. A photograph of a tangential section of the normal 
wood of the choke cherry is shown in fig. 3, in which none of the 
multiseriate rays are over four cells wide. Fig. 6 is a photograph of a 
similar section from an infected stem, taken on February 22, in which 
the broad rays have become many cells wide, simulating the structure 
of compound rays. This section really appears more like one through 
the inner bark, but that it is through the xylem is shown under high 
magnification, because the rays are bordered by the usual xylem 
elements. Similar sections from specimens collected on April 29, at 
about the time the growth begins, show the same condition, but 
specimens collected some three weeks later, after the infected branches 
have become greatly swollen, show that these rays have become 
further altered by the growth of the knot during the second year of 
its development. A photograph of such a section is shown in fig. 4. 
Near the center of the figure are two small rays surrounded by tracheids 
and fibers, while to the right and left are bands of similar elements 
bordering adjacent rays. The small ray to the right of the center is 
one that under normal conditions probably would not have had great 
vertical depth. It has become so broad here that its tangential 
diameter is fully as great as its vertical. Instances were noticed where 
the tangential diameter was even greater than the vertical. A photo- 
graph of a similar section taken from a specimen collected on June ii, 
after the knot has reached its maximum size, is shown in fig. 5, in 
which the rays are still broader, and it will be noticed that some of 
the elements bordering them have been pushed fully 45° from their 
usual vertical position. There has also been a considerable increase 
in the size of the cells which constitute the rays as the magnifications 
in figs. 3, 4, 5, and 6 is the same. 
The uniseriate rays are not altered as far as could be determined, 
