A SERIOUS LETTUCE DISEASE. 
97 
who published on parasitic fungi from 1813 to 1837. The plant body 
of this fungus consists of delicate branching, mold-like threads called 
the mycelium, which may, with the microscope or if abundant with 
the naked eye, be seen in or on the affected parts of the lettuce plant. 
ISTo diseased part is free from them and they are, on the other hand, 
never present without being accompanied by a condition of disease in 
the adjacent parts of the lettuce plant. It has been definitely proved 
beyond all peradventure that these fungous threads are the actual 
cause of the disease and that nothing else Can cause this disease. 
This fungous mycelium, coming in contact with a lettuce leaf, exudes 
a poison which kills the near-by cells of the lettuce plant. The cell 
walls are then dissolved and the mycelium makes its way through or 
between them. It dissolves also the contents of the cells and absorbs 
the resulting nutrient solutions to further its own development. The 
mycelium thus grows larger, kills more cells, consumes them and con¬ 
tinues To advance rapidly through the affected leaf until the whole 
leaf is a soft, slimy, rotten mass. The invasion continues into the 
main stem, then upward to the central bud and “heart” of the head, 
out into other leaves, downward through the root until every portion 
of the host plant has been killed, and the nutritious parts consumed. 
Environmental conditions may, to some extent, change the course 
of the disease. The fungus grows best in abundant moisture. Some¬ 
times this leads to a more rapid decay of the inner protected dry 
portion of the head, and a plant which to casual inspection appears 
healthy may prove upon close examination to be, at heart, a slimy, 
rotten mass. Again through one-sided infection the decay may pro¬ 
gress much more rapidly upon one side than upon another, resulting 
in complete death of one side before the other shows any symptom of 
disease. 
As a rule the mycelium is not, visible to the naked eye on leaves 
until the nutriment within the leaves has been nearly or quite, exhausted 
by the fungus. When this time is reached the mycelium begins its 
external appearance as the loose cottony growth referred to above. 
The most profuse development of this aerial mycelium occurs in the 
region of more humid atmosphere, such for example as on the under 
sides of leaves lying upon the ground, between leaves, or at the bases 
of leaves, in fact anywhere that the air is so situated as to cause it to 
remain especially humid. 
Soon after the appearance of the aerial mycelium in profuse quan¬ 
tity, it may be noted that in each region where the mycelium is dense 
there appear one or more centers of aggregation, composed of very 
densely intertangled and interwoven mycelial threads. These denser 
masses enlarge, increase in density and finally become solid masses of 
tightly compacted mycelium. These bodies are called sclerotia. The 
sclerotia are at first colorless, then pale salmon color, and finally black 
on their exterior and flesh colored within. When first formed they 
are buried in and covered by mycelium, and are only to be seen by 
tearing this mycelial covering away. As time passes this mycelium is 
lost, the remains of the lettuce plant disappear, and the only visible 
