SECTION OF VEGETABLE PATHOLOGY. 349 
sharply defined. Sometimes the spots coalesce or run together, and 
in this manner the entire apple is soon affected. Toward the center 
of the diseased spot there 1s usually a very dark, frequently almost 
black, discoloration. 
Upon close examination it is seen that the darker portions are 
studded with minute black points, which are slightly raised above 
the surrounding tissue, imparting to their surfaces a somewhat rough- 
ened appearance; occasionally these points are arranged in circles or 
grouped in little clusters. 
On cutting through a diseased apple while the spots are small it 
is seen that the decaying tissue extends to quite a distance into the 
fruit, and as the growth of the fungus continues the entire apple soon 
becomes a soft, yellowish-brown mass. 
(c) BOTANICAL CHARACTERS OF THE FUNGUS. 
The body of the parasite, or the mycelium, consists of slender 
threads, which push their way through the tissue of the fruit and 
destroy the parts with which they come in contact. These threads 
vary in size and color. At first they are nearly transparent ; later, 
however, they become darker, and when old they are usually brown- 
ish-black and somewhat thickened. 
Frequently the thicker tubes send out lateral branches, which are 
at first colorless. These are divided by more or less frequent trans- 
verse partitions or septa, and soon assume the usual brownish color. 
The walls of the tubes are constricted at the septa, sothat when the 
latter are roughly handled they frequently break apart at these 
points. In the process of growth the mycelial threads become 
thickly matted and interlaced at certain points, and as these mats 
become more compact they gradually take a more or less globose 
shape. Transverse sections through a portion of a diseased spot 
will reveal these little rounded masses in all stages of development. 
Their formation begins at a point some distance beneath the epi- 
dermis, and as they continue to enlarge they finally rupture the lat- 
ter and appear on the surface in the form of the minute black specks 
already described. A thin transverse section through one of the 
mature black pustules shows their structure. With the aid of the 
microscope it is seen that they consist, for the most part, of compact, 
dark-colored, many-septate threads, which are arranged with their 
ends pointing towards or frequently protruding through the ruptured 
cuticle. Before the cuticle is ruptured this arrangement is not so 
apparent as the mass at this time has a rounded form. When the 
cuticle is torn, however, the hitherto globose mass spreads out and 
becomes somewhat fan-shaped above (see Fig 2). 
Upon the tips of the closely-compacted threads spores, or repro- 
ductive bodies, are borne (Fig. 3). These are colorless, or nearly so, 
and somewhat variable in shape; usually, however, they are more or 
less cylindrical, rounded at each end, and, occasionally, somewhat 
curved (Fig. 3, B). 
Kach thread develops a single spore, but the former are so thickly 
compacted that an immense number of the latter are developed in 
each pustule. When sown in water the spores germinate within 
ten hours by sending out one or more rather thickish germ tubes 
(Fig. 5). When first sown there are usually no signs of nuclei in 
the spores. Just before germination, however, nuclei (or vacuoles) 
appear, and the contents of the spore become granular, 
