342 The Black Knot. [June, 
doned, in consequence of the ravages of the black knot. The 
disease is peculiar to America, and has been the bane of fruit- 
growers from early times; but although much has been written 
in agricultural papers about its injury to the fruit crop, the sub- 
ject has been almost entirely neglected: by botanists. In the 
present paper we shall consider the cause and prevention of the 
knot, and the question whether the disease is the same on plums 
and cherries. Asa preliminary step, it will be well to trace the 
development of the knot as it occurs on a single species, and for 
this purpose the choke-cherry, Prunus Virginiana L., may be 
selected. 
The size of the knots varies greatly, being found on the 
species of Prunus under consideration all the way from a few 
lines to several inches in length, with an average of two inches 
in circumference. The knot does not usually entirely surround 
the branch, but growing from one side, often causes the branch 
to bend or twist into an irregular shape. In the winter, when 
the branches are leafless, the knots are much more noticeable, 
and at this season they are often cracked and broken, worm-eaten 
and hollow. - À 
In the swollen portions of the branch above and below the 
knot, sections under the microscope show the vegetative portion 
of the fungus in the form of minute threads, .0007 mm. in 
diameter, twisted together and extending from the cambium 
towards the outer portion of the stem, where they become sep- 
arated as shown in Plate IV., Figure 1. The fungus first reaches 
the cambium either by the germination of spores on the surface 
of the branch, or by the mycelium proceeding from a neighboring 
knot. The part of the cambium free from these bundles of 
mycelial threads grows in the usual manner, and in an old branch 
a cross section shows at the end of the season one more “ rmg 
or layer of wood on the sound than on the diseased side. From 
this it is to be concluded that the growing layer of tissue of the 
plum or cherry branch is the place from which the fungus begins 
its destructive work. : 
In the spring the swollen portion of the branch, whether it be 
on either side of an old knot or at the beginning of a new OM" 
increases in size, and the mycelium soon reaches and bursts through 
the bark, so that by the time the choke-cherry is in flower fi 
knot has reached nearly its full size, though differing from an © 
one in being still greenish in color and solid or pulpy ™ eet 
sistency. 
