446 BULLETIN OF THE BUSSEY INSTITUTION. 
we shall find one more layer of wood on the sound side of the stem 
than on the side of the knot. In other words, on one side, the forma- 
tive power of the cambium has expended itself in forming a new layer 
of wood and bark (Phloem), and, on the other, irritated by the pres- 
ence of a fungus, it has produced a mass, the knot, in which all dis- 
tinction between wood and bark has been lost. In the knot we find 
bast fibres, wood cells, and dotted ducts; but the prevailing tissue con- 
sists of a collection of dotted, rectangular, parenchymatous, cells, with 
very thick walls, which closely resemble the cells of the medullary 
rays. These thick-walled cells, by their excessive growth, push the 
prosenchymatous cells out of their natural direction, parallel with the 
axis of the branch, and at intervals force their way through them, 80 
that the latter seem to form a series of arcs of circles with the con- 
cavities outward. The dotted ducts are numerous, shorter than in the 
healthy part of the stem, and, owing to the abnormal position into 
which they are forced, cross sections of the stem frequently show them 
in lateral view, rather than in section. The separate dotted ducts, 
instead of lying side by side as usual, are closely twisted or braided 
together. The bast fibres are less altered in their direction and 
appearance than the other elements of the stem. The mycelial threads 
of the fungus form bundles, which are imbedded in the parenchyma of 
the knot. 
The condition of the interior of the knot, in its later stages, is modi- 
fied very much by the depredations of insects, as well as by the drying 
and crumbling of the tissue itself. On the wild cherries, the outer part 
is generally a mere shell, and the internal part is nearly empty. On 
the plum, the interior is more apt to be honeycombed; and we not 
unfrequently have a very hard layer next the wood, composed of thick- 
walled, dotted cells. The knot on cultivated cherries is intermediate 
between that on the wild cherries and that on plums. ‘The condition 
of the interior of the knot has an important bearing on the propagation 
of the disease. Where it is hollow, and the outer part brittle, as in 
the choke cherry, it is easy to see that the spore-bearing portion will 
readily be broken up, and the spores blown about without difficulty. 
Where it is more solid, as in the plum, the spores will be dispersed 
with much less ease; and we can see how, by the action of the eureulio 
in boring into the knot and making its tissue more spongy, it is facili- 
tating the dispersion of the spores of the fungus when they shall ripen; 
