DISEASES. 193 



as already stated, have been maintained by different 

 writers. 



" 1st. That Black-knot is a mere disease like the can- 

 cer. — Dr. Fitch, who maintains this opinion, allows that 

 the black granules found on the Black-knot are a true fun- 

 gus, ' that the surface of tliese excrescences, when mature, 

 is always covered with this plant,' and that 'this plant 

 never grows, or at least has never been found, in any 

 other situation.' {Address N". Y. State Ag'l Soc, 1860, 

 p. 21.) * * * 



"2d. That Slack-knot is a gall. — As already stated, 

 there is no tme gall-making insect that inhabits the Black-* 

 knot, so far as I can discover on the fullest and most ex- 

 tensive investigation that I have been able to give to the 

 subject. The minute holes commonly found in the old 

 dry Black-knot, wliich are too large either for the ' Cur- 

 culio' or for the small moths bred by myself from Black- 

 knot, are of a suitable size for either of the two dipterous 

 insects which I have enumerated in a note as bred by my- 

 self from Black-knot. Consequently the argument which 

 I based upon the existence of these minute holes (Proc. 

 Eixt. Soc. Phil, m, p. 614) falls to the ground ; and al- 

 though I found on one occasion the larva of a Gall-gnat 

 embedded in a cell in a Black-knot, yet this was most 

 probably that of the Guest Gall-gnat which I actually 

 bred from-Black-knot, as stated in the note, and not of a 

 true gall-making Gall-gnat. 



" 3d. That Black knot is a fungus. — Just as Dr. Fitch, 

 having proved to his own satisfaction that Black-knot is 

 neither a gall nor a fungus, infers by the method of ex- 

 haustion that it must be a disease ; so, having proved that 

 9 



