"Three radically different theories have been broached as to the nature and origin of Black-knot: 1st, that it is a mere disease of the tree, like the cancer or the gout in the human race, which is the view maintained by Dr. Fitch, the State Entomologist of New York; 2d, that it is what naturalists term a "gall," produced by some unknown insect depositing its eggs in the twig--just as the well-known "oak-apples" are produced by a Gall-fly, (_Cynips_), depositing its egg in the bud of the oak--which is the opinion that I myself formerly held and maintained, before I had fully examined into the subject; (_Proceedings Ent. Soc., Phil._, III, p.p. 613-618;) and 3d, that it is what botanists term an epiphytous fungus, growing on the tree as a mushroom or toad-stool grows on the ground, which is the opinion of the botanist Schweinitz, and which has recently been re-asserted by Mr. Glover, the Entomologist of the Bureau of Agriculture at Washington, (_Ag'l Rep._, 1863, p. 572.) This last is the opinion which, upon full inquiry, I have now adopted. "Before discussing these theories, the facts arrived at by myself in the summer of 1865, must first be briefly noticed. It should be premised that the old, dry Black-knot remains on the tree for many years, and that the place to look for the new Black-knot is on such trees as have been already attacked and are loaded with old Black-knot, without being as yet completely killed by it. "1st. By the middle of June the new Black-knot is pretty well developed, and may then be readily distinguished from the old by its dull, opaque, brown-black color, while the old is coal-black and more or less glossy. When cut into, it is found to be fleshy inside, like an apple, but not juicy, and of a pale greenish-yellow color, with fibres radiating from the axis of the twig, while the old Black-knot is internally hard and woody, and of a reddish-brown or rust-red color. The brown-black color of the external surface is retained till the last week in July, when the surface of the new Black-knot becomes gradually covered all over with little, coal-black, hemispherical plates, appearing when viewed through a pocket glass, about the size of the head of a pin, each of these is a distinct fungus, named long ago by Schweinitz '_Sphæria morbosa_.' Even on the old Black-knot this fungus may be readily seen, at any time of the year, covering its entire surface. So far I have added little to the information already published on this subject, except by the specification of dates. But in addition to these facts, I discovered that about the last of July or the first week in August, there grows from each fungus on the surface of the Black-knot a little cylindrical filament about one-eighth of an inch long, which no doubt bears the seed or "spores," as they are technically termed, of the fungus, and that these filaments very shortly afterwards fall off and disappear, leaving