A SERIOUS LETTUCE DISEASE. 
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by air currents, they may be carried quickly to great distances. The 
spores under suitable surroundings, germinate and send forth small 
thread-like sprouts (Fig. 7). These sprouts, with suitable nourishing 
material, grow rapidly into a vigorous mycelium, which is again ready 
to invade the living lettuce plant; to again cause the drop. 
Fig. 7.—Ascospores germinating. A. At two hours; B, at five hours; C, at seven hours. 
This history of the drop fungus may be epitomized by saying that 
the mycelium grows within the lettuce plant, causing the drop; pro¬ 
duces sclerotia when nutriment is exhausted; rests in the sclerotial 
condition until opportune conditions prevail; the sclerotia then pro¬ 
duce apothecia bearing asci in which are spores; these spores produce 
a new mycelium which again invades lettuce plants. 
The Fungus That Causes the Disease. 
Morphology. 
The mycelium in gross, as developed external to the leaf in humid 
air, appears as a coarse cottony mass most abundant around the stem 
of the plant among the leaf bases. It is also found to a great extent 
upon the lower diseased leaves that lie upon the surface of the ground. 
It is more plentiful on the under sides of these leaves or between 
leaves if two or more are lying upon each other on the ground, but 
very seldom makes any extensive showing upon the ground itself un¬ 
less under artificial conditions of excessive humidity. 
