BivACk-Knot of Plums and Cherries. 639 



changes color ; and eventually it is perfectl}^ black and presents 

 the appearance which is so commonly seen, Figs. 3 and 4. 



There is much variation in the size, shape, and location of the 

 knots. Sometimes they do not grow to be more than a quarter 

 of an inch in length, and then they do no particular damage. 

 Such small knots are seldom seen, for their growth in spring is 

 generally quite rapid and the}^ frequently extend several inches 

 along a twig. The largest knot which has come to my notice was 

 on Morello cherries and was nearly five inches in length, it being 

 all one year's growth. If the growth of other years is considered, 

 the diseased portion will be considerably extended. It is a pecu- 

 liar circumstance that the knots as a rule do not encircle a branch, 

 but nearly all extend along the branch upon one side while the 

 portion opposite remains healthy and permits the passage of the 

 sap to the living portions beyond. Small twigs are frequently 

 killed outright, for in them the disease soon cuts off all communica- 

 tion between the parts above and those below the affected portion. 

 L,arge limbs do not succumb so rapidly. The disease gradually 

 extends from year to year and it may be a long time before the 

 death of the parts beyond takes place, for such, with probably 

 .rare exceptions, is the result which eventually follows the ap- 

 pearance of the trouble. 



The form of the knots varies considerably. Upon the smaller 

 branches, those two or three years of age, they are generally 

 straight, or nearly so. The swelling is often more marked at the 

 center of the diseased portion, and gradually diminishes towards 

 the ends ; but the swelling may have nearly the same diameter 

 throughout its length, and in such cases the knot terminates 

 abruptly. If the twigs are but a year or two old the presence of 

 the knot ma^' cause considerable bending of the wood, so that fre- 

 quently the knot turns at a sharp angle, or it may be inclined to 

 the form of a spiral. Large branches which cannot be affected in 

 this manner generally cause the formation of knots that are more 

 or less oval in outline, the mo.st protruding portion being near the 

 center. When the knots are formed in the forks of two branches, 

 as so frequently occurs, they extend along both branches beyond 

 the point of union, and also downwards. Such knots vary 

 greatly in form. 



