
251 
The leaf first 
turns light then 
deeper brown, 
and gives the 
impression to a 
casual observer 
of simply becom- 
ing prematurely 
aged. When the 
disease is severe 
the lower leaves 
die faster than 
new onesare pro- 
duced, and the 
heads of lettuce 
are consequent- 
ly rendered unfit 
for use, or the 
plantsare entire- 
ly killed. The 
presence of the 
fungus does not 
appearto change 
Fig. 3.—Vertical section of leaf of lettuce passing through a R 
perithecium of Septoria Lactucz Pass. - Magnified 250 diame- the taste of the 
ters. Original. lettuce in any 
way; the injury comes from rendering the leaves less attractive, 
and from the partial or total killing of the plant. 
If lettuce in this condition is examined more closely, both surfaces 
of the leaf will be found to be covered with minute brown or black- 
ish specks, as fine as pin points, their great abundance giving the 
rusty color. These specks are the fruiting condition of the fungus 
which is the cause of the disease. The vegetative threads of the 
fungus are not visible, being concealed in the tissues of the lettuce; 
it is, therefore, an internal or endophytic species. Like all such in- 
ternal growers, the parasite sends its threads through the tissues of 
the plant, absorbs and appropriates its nutrient juices, and in due 
time begins the production of spores for its immediate and rapid ex- 
tension to other host plants. In this case a globular spore-case— 
perithecium—is produced just beneath the surface of the lettuce 
leaf, and in thisare formed great numbers of single-celled, elongated 
spores. The perithecium soon opens by a small pore at the top and 
the ripe spores are forced out from the center by the pressure from 
the constantly forming spores near the periphery within (fig. 3). 
The number of spores poured out from a single perithecium is enor- 
mous, and when the great number of perithecia which cover each 
leaf is considered, the provision for complete and rapid extension of 
the parasite is evidently adequate to explain its utmost prevalence. 
The fungus is Septoria Lactuce Pass. This name applies to the 
stage described above, in which as a parasite it is doing the greatest 
injury. The complete, sexual condition is not known, and what be- 
comes of the fungus when the lettuce leaves die is yet to be discover- 
ed, for there isa long hiatus between the death and disintegration of 

34 ¥ + lis Seis boos 
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