BIOLOGIC SPECIALIZATION IN THE GENUS SEPTORIA 27 
and indefinite outline. The spots on A. laevis were very small and brown, 
indicating that the fungus was less well adapted to this host than to A. 
ericoides. As the spores for inoculation were from pure culture, and adja- 
cent leaves not inoculated showed no symptoms of disease, it appears 
certain that the spots in question were due to the Septoria applied. 
General Discussion 
Age incidence. In the field Septoria spots are more frequently observed 
upon the old or fading leaves of plants, but this is often due to the fact that 
the young leaves have recently expanded, and have not had time to become 
infected. The immature foliage is sometimes the more susceptible, but 
this varies with hosts. From the results of the infection experiments the 
leaves of Lepidium virginicum appear to be equally susceptible at all ages. 
Old leaves of Lactuca scariola in which the edges are cracked and drying are 
infected without difficulty, but the spots remain small. Partially grown 
leaves of this host are more easily infected, and the spots become larger. 
The older leaves of Convolvulus arvensis and C. sepium are the most heavily 
attacked, while inoculation of the immature leaves seldom produces in- 
fection. 
The influence of age upon susceptibility is very well shown in Malva 
rotundifolia and Althaea rosea. Disease first develops on the oldest leaves; 
in those of intermediate age the incubation period has greater length, while 
those partially grown are resistant. A plant of Althaea rosea with seven 
leaves was thoroughly wetted with a suspension of spores of Septoria mal- 
vicola, and kept under a bell jar four days. In ten days the five mature 
leaves were spotted, the oldest having the heaviest infection. It was 
nearly three weeks before the sixth leaf showed disease, and the seventh or 
youngest leaf inoculated continued healthy. In two trials with the mallow 
there were similar results, which are in accord with what one sees in the 
field, for there only old shaded leaves are badly attacked. 
Susceptibility of different leaf surfaces. Equal areas of the upper and 
the lower surfaces of separate lots of leaves of a number of plants were inocu- 
lated, and other factors made as comparable as possible. In Lactuca scariola 
the upper surface gave the heavier infection. In Polygonum persicaria 
39 spots, 1-5 mm. in diameter, resulted on the top side as compared w^ith 
six spots, 1-2 mm. in diameter, on the lower. The leaf surface of Erigeron 
annuus in 150 separate areas, 75 above and 75 below, was inoculated with 
loop-drops of a suspension of spores of Septoria erigerontis, and 14 infections 
from above and six from below were obtained. In contrast to these experi- 
ments, the inoculation of the lower surfaces of leaves of Solanum carolinense 
with Septoria ly coper sici gave abundant infection, while leaves inoculated 
above remained free of disease. No explanation of the above-described 
results can now be given, but the number of stomata, the character of the 
cuticle, the ease with which the suspension makes contact with the surface, 
and the light exposure may be factors. 
