INJURIOUS AND OTHER FUNGI. 41 



entirely neglected by botanists. As a preliminaiy step it will be 

 well to trace the development of the knot as it occurs on a single 

 species, and for this purpose the choke cherry {Prunus Virginiand) 

 may be selected." The size of the knots varies gTeatly, being 

 found on the species of Prunus under consideration all the way from 

 a few lines to several inches in length, with an average of two 

 inches in circumference. The knot does not usually entirely sur- 

 round the branch, but, growing from one side, often causes the stem 

 to bend or twist into an irregular shape. In the winter, when the 

 branches are leafless, the knots are much more noticeable, and at 

 this season they are often cracked, broken, worm-eaten and hollow. 



In the swollen portions of the branch, above and below the knot, 

 sections under the microscope show the vegetative portion of the fun- 

 gus in the form of minute threads, twisted together and extending 

 from the cambium towards the outer portion of the stem, where they 

 become separated. The fungus first reaches the cambium, either by 

 germination of spores on the surface of the branch, or by mycelium 

 proceeding from a neighboring knot. The part of the cambium free 

 from these bundles of mycelial threads, grows in the usual manner ; 

 and in an old branch shows one more layer of wood on the sound 

 side than on the diseased side. From this it is to be concluded 

 that the growing layer of tissue of the plum or cherr}^ branch, is 

 the place in which the fungus begins its destructive work. In the 

 spring the swollen portion of the branch increases in size, and the 

 mycelium soon reaches and bursts through the bark, so that bj' the 

 time the choke cherr}^ is in flower the knob has reached nearly its 

 full size, though differing from an old one in being still greenish in 

 color and solid or pulpy in consistency. 



With a hand lens one can see small hemispherical protuberances 

 which are the beginnings of the " perithicea," or pits in which the 

 sexual spores are to form. The whole surface of the protuberances 

 is covered with filaments, which are somewhat flexuous and branched. 

 At the terminal joints, or frequently a little to one side, conidial 

 spores are borne. These spores continue to be formed until near 

 the close of summer, when the filaments dry up and onl}' their 

 shrivelled remains are to be seen. The knots now assume their 

 black color, the inner pulp being either destroyed by insects or re- 

 duced to a powdery mass, with only the hard outer shell which con- 

 tains the perithicea left in place. About the middle of January the 

 spores in the sacs in the perithicea begin to ripen. 

 6 



