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 these 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 JST. T. State Ag'l Soc., 1860, 

 p. 21.) * * * 



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

 there is no true 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, which 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. 

 Ent. Soe. Phil. Ill, 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 

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