BULLETIN OF THE BUSSEY INSTITUTION, 445 
The knot on the plum is, moreover, attacked early in the season by 
insects, which modifies its development to a certain extent. The cur- 
culio, which makes its appearance early in July, stings the knots as 
well as the plums, and in the middle of July there exudes from both 
a gummy substance in great abundance. This gummy substance in a 
few days becomes covered with the common mould, Z7ricothecitum 
roseum, Lk., which gives its pinkish tinge to a large part of the sur- 
face of the knot. On some plum-trees, one can hardly find a knot 
which has not some of the pinkish color caused by the presence of the 
Tricothecitum, which has no organic connection whatever with the 
fungus which causes the knot, but is simply parasitic on it. It is 
probably owing to the fact that the curculio stings the knots, that so 
many persons have been led to believe that the knots themselves are 
of insect origin. We were greatly astonished last summer, while study- 
ing the development of the disease on a certain plum-tree, which, until 
the first of July, had progressed just as in the neighboring choke 
cherries, to find, after an absence from town of about ten days, that the 
whole aspect of the plum knots had been changed by the attacks of 
the curculio, so that they were hardly recognizable, owing to the 
gummy masses upon them; while the knot on the cherries remained 
unchanged. The 7Tricothecium almost immediately made its appear- 
ance, and remained on some of the knots until winter. 
ANATOMY OF THE Knot. — The histological character of the knot, 
at its perfection, is very much the same no matter upon what species 
of Prunus it is growing. The granulated surface is composed of the 
stroma of the fungus we have described, and the mycelium passes 
inwards in intricately wound bundles, which run in the direction of the 
medullary rays, and which vary in extent at different stages of growth. 
The fungus begins to grow in the cambium, either by prolongation from 
the mycelium of a knot directly above, or by the germination and growth 
of spores which have fallen on the bark. When the mycelium occupies 
the cambium of the greater part of the circumference of the stem, the 
branch above the knot dies, just as though it were girdled. More fre- 
quently, the mycelium is found in the cambium of only a part of the cir- 
cumference of the stem, and that which is free from the mycelium goes 
on producing wood and bark as usual ; and, if in midsummer or winter 
we make a cross section of a knot on a branch more than a year old, 
