1891.] PUBLIC DOCUMENT — No. 33. 205 



The knot may first be observed in the fall as a slight 

 swelling of the branch, arising near an old knot, or inde- 

 pendently. If near a knot, it is probably caused by the 

 extension of vegetative threads from the latter ; but, if 

 on a branch not before attacked, it is probably due to 

 infection by spores from an earlier knot. A section across 

 such a young knot shows that the swelling has taken place 

 wholly in the bark and largely in the inner bark (Phloem), 

 in which may be seen radially placed bundles of the fine, 

 intertwined threads of the fungus. The swelling continues 

 in the spring, and the epidermis ruptures, allowing the 

 protrusion of the dark, greenish-brown mass of tissue, 

 presumably due to abnormal growth induced by the irrita- 

 tion caused by the presence of the fungus. This mass is 

 firm and succulent, and its surface is usually irregularly 

 cracked and granular. In May there are developed over 

 this surface numerous short, erect threads, standing rather 

 closely together, like the threads in the " pile" of velvet, 

 and giving to the surface a dark-brown, velvety appear- 

 ance. On these threads are borne, at and near their tips, 

 the summer spores of the fungus, obovate bodies of a 

 brownish tint. (Fig. 6.) 



Toward midsummer these threads and spores disappear, 

 and, in consequence, the knot loses its velvety appearance 

 and brownish shade, and becomes dead black in color. It 

 also becomes hard and dry, and the larvae of insects whose 

 eggs were laid in it when it was young and juicy have 

 already begun to destroy its interior, so that very often 

 only the outer crust finally remains. In this fact is to be 

 found the basis of the stoutly defended theories of the 

 insect origin of the knot already alluded to. The surface 

 of the knot may now be seen to be checkered off into little 

 rounded areas, each of which has a slight depression at 

 the centre. In some cases, these black areas do not com- 

 pletely cover the surface, but leave intervening brown 

 spaces. If a section be made across the knot in the fall, 

 a year after its first appearance, under each of the black 

 areas described may be seen with the unaided eye a white 

 spot, which marks the position of a cavity in the dense, 

 black fungus-tissue. This is filled with slender, colorless 



