Ii6 
ALBAN STEWART 
mately a year after infection, is shown in fig. 8. In this section the 
cambium is present opposite the projecting segments of xylem but it 
is not present opposite the broad rays. Figure 9 is a photograph of a 
similar section taken some two months later, after the knot has fully 
formed and has become surrounded by the hard carbonaceous layer 
containing the perithecia. In this section the cambium is seen as a 
somewhat broken line just outside the xylem wedges, which are formed 
at this stage, so that the prevailing idea that the cambium is broken 
up and scattered during the development of the knot is incorrect. 
Usually the cambium is first stimulated to produce a greater or lesser 
amount of parenchyma of a somewhat transitional nature. With the 
exception of the early spring growth, normal xylem always contains a 
large number of fibers which have the usual angular appearance, with 
thick walls, and small lumina when seen in a cross section. When 
the results of infection first begin to appear, there is a change in the 
character of the cells formed, and those which would probably have 
been fibers under normal conditions, lose their angular appearance 
somewhat, the walls become thinner, and the lumina correspondingly 
larger. The change maybe but slight next to the normal xylem, but 
farther away it becomes greater and can be studied best by following 
out a radial row of cells from the normal into the abnormal part. 
The parenchymatous character of these cells is shown by the fact that 
they contain an abundance of starch, nuclei are often present in them, 
and the pitting in their walls is distinctly plain. A considerable zone 
of these cells may be produced, there may be but few of them, or, 
occasionally they may be entirely absent. Since cross sections of 
these cells have more the appearance of fibers next to normal xylem 
than they do farther away, it suggests that the stimulating effect of 
the fungus might have become effective in their vicinity while they 
were still in a plastic condition and that they were further altered by 
its influence. These transitional parenchyma cells react more strongly 
to safranin stain than do the fibers near by, so there can be no difficulty 
in determining whether they are present or not by this alone. 
After the production of the transitional parenchyma, the cells 
formed are usually more distinctly parenchymatous in character. 
These cells are larger and their walls thin and unlignified. Occasional 
fibers or groups of fibers are often found scattered through them. 
This condition is usually more pronounced towards the center of the 
infected area, when seen in cross section, while towards the sides, the 
