- 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 rie 108, 
- owrightia. 
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 papille (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- 
