BIOLOGIC SPECIALIZATION IN THE GENUS SEPTORIA 
29 
spores in a moist chamber. It is well to make a record of the source of the 
material collected, and the conditions under which it was found. {Cf. graph 
I, C, D, G, and H.) 
Host limitations. The experimental results thus far obtained indicate 
that the species of Septoria do not have a broad host range. Each can 
infect vigorously one or a few closely allied plants, and can infect to a less 
degree a number of hosts that stand in rather immediate relation to the 
vigorously infected ones. In some cases this host range does not extend 
beyond the limits of a genus, and in other cases includes but two or three 
related genera. Although some of the fungi studied have been found to 
have approximately the host range previously reported for them, in many 
instances the host ranges established by the experiments have been much 
more narrow than the reports on hosts would lead one to conclude, a few 
forms being limited to the species upon which they were collected. Notable 
illustrations of this are the forms of Septoria on wheat. Convolvulus sepium, 
and possibly upon Ruhus occidentalis and Helianthus sp. Further infection 
experiments would doubtless reveal more cases of identity both morphologi- 
cal and biological among forms now classed as separate species, while at 
the same time some of the present species would be shown to consist of 
more than one morphological form, or species. 
The value of disease characters. The variable nature of disease charac- 
ters, as manifested by the host, has been well demonstrated. These varia- 
tions are dependent upon the species, the age, and the part of the host as 
well as upon environmental conditions. On this account these characters 
lose much of their value in taxonomy, but inasmuch as the host ranges of 
the species of Septoria are not broad, and the number of forms parasitizing 
a single host is very fe\v, such characters may be of some use in distinguish- 
ing the parasites on individual hosts. For these same reasons the host 
itself will continue to be a valuable key in the determination of the fungus. 
Biologic specialization. The experiments herein described have not 
been broad enough to include forms from all the hosts reported for any 
species, especially with such fungi as Septoria ruhi upon numerous members 
of Rubus, 5. graminum or S. tritici upon several different genera of Gramin- 
eae, and S. polygonorum upon various species of Polygonum. Great 
difficulty will inevitably be met in bringing together even a major portion 
of the respective forms for comparative study, such as would be necessary 
to establish firmly the existence of biologic specialization, and to ascertain 
the number of biologic forms. Still, such data as the present investigations 
furnish indicate clearly that biologic specialization exists in many species 
of Septoria. This is shown by the fact that in many instances the Septoria 
from one host either fails to infect, or infects to only a slight degree, certain 
hosts upon which morphologically similar forms of Septoria are known. 
In illustration of this, the species of Septoria from wheat, from Ruhus occi- 
dentalis, and from certain species of Helianthus and Polygonum furnish 
examples. 
