t 
BIOLOGIC SPECIALIZATION IN THE GENUS SEPTORIA 9 
Although a vigorous parasite upon the few species of Polygonum, the 
form of Septoria polygonorum dealt with here is not at all cosmopolitan in 
its parasitism. A special attempt was made to infect a plant of P. con- 
volvulus kept constantly covered with a paraffined paper bag to insure a 
humid atmosphere and provide partial shading, but with negative results. 
One hundred leaves of P. hydropiper also gave negative results. The fungus 
was induced to attack detached leaves of P. convolvulus kept in a moist 
chamber in the moist laboratory, spots and pycnidia developing in seven 
days ; but it would not attack Rumex crispus or Fagopyrum esculentum in a 
similar way. Thus Septoria polygonorum appears as a species with fairly 
fixed infection-powers, possibly embracing forms differently specialized in 
addition to the one experimented with here. 
The disease spots are not the same on the various species of Polygonum. 
The distinctions are illustrated in Plate I, figures 1-4. The specimens were 
selected as representing the usual character of the spots. Specimens of 
the same species might have been selected showing scarcely distinguishable 
spots. Upon P. pennsylvanicum the spots attain the greatest size, often 
become irregular in outline, have but a very narrow, dark brown border, 
and have pycnidia uniformly distributed throughout their area (fig. i). 
The spots upon P. persicaria are intermediate in size, and are chiefly char- 
acterized by a rather wide, dark reddish-brown border. The spots seldom 
lose their orbicular shape, and the pycnidia are grouped nearer the center 
(fig. 2). Upon P. lapathifolium the spots range smallest in size, and are 
yellowish-brown in the fresh leaves, with a border of the same color, but in 
the dry specimens the spots become reddish-brown (fig. 3). In size and 
shape the spots on P. orientale resemble those on P. pennsylvanicum, but 
are yellowish-brown in the fresh leaf. The reddish-brown border appeared 
when the leaf was put into a moist chamber for a day. The variability in 
the biological or host characters shown here illustrates the untrustworthy 
nature of these characters when used to distinguish species. 
Septoria lactucicola Ell. & Mart. 
Septoria lactucicola has been reported upon Lactuca floridana, L. scariola, 
and L. canadensis, but in most instances on the last-named plant. The 
results in diagram 2 show that this Septoria is somewhat adaptive in its 
parasitism, being able to attack three genera and five species. Were the 
experiments conducted on a broader scale, the host range would probably 
be enlarged. There is a gradation in susceptibility passing from L. canaden- 
sis through to Sonchus oleraceus. Although the fungus attacked L. sativa 
and L. scariola rather readily, with 16 spots on 30 leaves and 10 spots on 35 
leaves respectively, the infection was slight compared with 23 spots on 7 
leaves of the original host. Moreover, there were but few fruiting bodies 
on these new hosts. The most of the 17 spots on Prenanthes sp. were very 
