BLACK KNOT 219 



354. A good illustration of the plants of this order is 

 the Black Knot (Plowrightia morbosa), which attacks the 

 plum and cherry. In the spring the parasitic filaments, 

 which the previous year penetrated the young bark, 

 multiply greatly, and finally break through the bark, 

 and form a dense tissue. The knot-like mass grows 

 rapidly, and when full-sized is usually from 2 or 3 to 10 or 

 15 centimeters long, and from 1 to 3 centimeters in 

 thickness; it is solid and but slightly yielding, and is 

 composed of filaments intermingled with an abnormal 

 development of the bark-tissues of the host-plant. 



355. The knot at this time is dark-colored, and has a 

 velvety appearance, which is due to the 



fact that its surface is covered with 

 myriads of short, jointed, vertical fila- 

 ments, each of which bears one or more 

 conidia. The conidia, which fall off 

 readily, are produced until the latter part 

 of summer, when the filaments which 

 bear them shrivel up and disappear. 



356. During the autumn asci are produced, but re- 

 quire the greater part of winter to come to perfection. 

 The asci grow in the cavities of minute papillae (peri- 

 thecia), and are intermingled with slender filaments 

 (paraphyses) . Each ascus contains eight spores, which 

 eventually escape through an apical pore. These spores 

 germinate by sending out a small filament, or sometimes 

 two. 



357. No sexual organs have as yet been observed. 

 Possibly they exist in the dense tissues of the knot, and 

 fertilization may occur in the spring or early summer, 

 but they may have disappeared through the excessive 

 parasitism of these plants. 



358. The parasitic filaments of each year's knot gener- 



