450 BULLETIN OF THE BUSSEY INSTITUTION. 
The reasons for believing that the knot is not caused by insects may 
be summed up as follows: First, the knots do not resemble the galls 
made by any known insect. Secondly, although insects or remains of 
insects are generally found in old knots, in most cases no insects at all 
are found in them when young. ‘Thirdly, the insects that have been 
found by entomologists in the knots are not all of one species, but of 
several different species, which are also found on trees which are never 
affected by the knot. On the other hand, we never have the black 
knot without the Spheria morbosa, as was admitted by Harris;* and 
the mycelium of that fungus is found in the slightly swollen stem long 
before any thing which could be called a knot has made its appear- 
ance. Furthermore, the Spheria morbosa is not known to occur any- 
where except in connection with the knots. 
That the fungus on the cherry and plum is the same, as far as all its 
microscopic characters are concerned, is certain beyond a doubt. ‘The 
only reason why there has appeared to be any doubt on this subject is, 
that some kinds of cherries are susceptible to the knot, and others are 
not. ‘Those who believe that there are two different fungi which pro- 
duce the knot, have started with the assumption that what is true of 
one kind of cherry, as far as susceptibility to disease is concerned, is 
true of all cherries. Having seen some cherries free from the knot, 
although growing near diseased plum-trees, and others, perhaps not 
near any plum-trees, covered with knots, they have jumped at the 
conclusion, that there must be two different fungi producing the knot: 
one on the cherry, derived from the wild cherry; another on the 
plum, derived from the wild plum. Now, we have no right whatever 
to infer that.a disease of fungoid origin will be found on plants closely 
related botanically. In fact, we have no right to infer any thing; we 
can only examine and find out what the fact is. Let us see, first, how 
the case stands with our wild cherries, as the species are well marked, 
and we know more exactly what we are talking about than when we 
speak of the confused cultivated varieties. Prunus serotina, Ehrhart, 
the rum cherry, and Prunus Virginiana, L., the choke cherry, are 
about equally common near Boston. The latter is very frequently 
attacked by the knot; the former never, as far as our experience goes, 
and we have examined hundreds of trees. Walsh and others also 
testify to the same effect. The following is a striking illustration: In 
* Vide quotation on page 449. 
