642 BULIvETiN 81. 



by various natural agencies. The knot then assumes a darker 

 color and presents its characteristic hard, almost metallic ap- 

 pearance. 



Another crop of spores is still to be borne before the knot has 

 finished its work. These spores are formed in the small pimple- 

 shaped elevations which appear on the surface of the knots in au- 

 tumn and winter. They mature during midwinter or early spring, 

 the time appearing to vary in different localities. As soon as they 

 have ripened they leave the minute elevations in which they were 

 formed, and are scattered to propagate the parasite in some other 

 spot. The central portion of the knot has then finished its round 

 of existence, but the outer portions continue to grow and form new 

 knots abt>ve and below the old one. In this manner a single knot 

 may in time extend a long distance on a branch. 



Many points regarding the development of the black knot fun- 

 gus are still imperfectly known. Two of the more important of 

 these are the time and the manner in which the spores succeed in 

 gaining an entrance into the host plant. 



At present it can only be assumed that the spores germinate and 

 penetrate the tissues soon after becoming ripe. The principal 

 periods of infection would then be early spring, as saon as vege- 

 tation begins, or even earlier ; the other period would be from 

 the middle of May to the middle of July, the time varying with 

 the character of the season and with the locality. This subject 

 is of the greatest importance when considered in its relation to the 

 most favorable times for making applications designed to prevent 

 the entrance of the fungus into healthy tissues. 



From the fact that new knots show the first external indica- 

 tions of their presence only to a very limited extent in the fall, while 

 the great majority of them do not form until spring, it would seem 

 that infection takes place one year, but the knot does not develop 

 until the following season. The manner in which the spores that 

 mature in winter invest healthy tissues is not clear. There may 

 be some relation between them and the formation of knots upon 

 older wood, but the production of the spores in summer prevents 

 the making of any definite statements which are not based upon 

 careful artificial inoculations. As already stated, a great many 

 knots are found in the forks of young branches, Fig. 3, i, and at 



