A SERIOUS LETTUCE DISEASE. 
105 
of corn. The large and the small are however in nature and in normal 
cultures, produced indiscriminately together, and in no case was there 
a production of either large or small ones exclusively. 
When first gathered from the field their average weight was deter¬ 
mined as .06 grams each. After dying in the laboratory for a year they 
averaged .02 grams as determined upon a weighed hundred. They are 
somewhat rough on the surface and have a rather tough exterior pro¬ 
tecting layer composed of two or three layers of parenchyma-like 
cells with thick, hard, black walls, Fig. 17. Internally the sclerotium 
consists of a compacted mass of mycelial threads, which do not vary 
perceptibly from the ordinary mycelium, except that they have much 
thicker walls and narrower lumen, Fig. 17, and their diameter is less 
than that of the vegetative hyphse. 
In cultures in the laboratory it was possible to follow the develop¬ 
ment of the sclerotia accurately. At first there occurs a massing of 
the mycelium, causing a white, somewhat raised bunch of mycelial 
threads, floccose in appearance. These masses as they grow, exude 
numerous 'small drops of a colorless watery liquid, after which they 
change to cream color, which gradually passes to a dirty yellow, the 
developing sclerotial membrane losing their floccose character. Their 
color changes to a greenish black, afterward to almost pure black as 
they mature. 
c b a 
Fig. 18.— Sclerotium with disks, a, Photographed January 10; b, January 12; c, January 14. 
Note the recurved edges of the largest disk. 
To secure germination of the sclerotia, they were placed in moist 
sand in flower pots which were kept standing in a pan of water thus 
keeping the sand wet by capillarity. After about two months the 
sclerotia began to send out minute yellowish-brown hud-like protuber¬ 
ances. These rapidly elongated to form a filament which as soon as it 
reached above the soil and into the light, began to expand, at first to 
an urn-shape and then gradually flattening into a broad, flat disk, 
Fig. 18. In some cases the disk became recurved with age, Fig. 17c, 
having the appearance of a very minute yellowish brown umbrella, 
