BIOLOGIC SPECIALIZATION IN THE GENUS SEPTORIA 
3 
be presented. It may be well to mention that biologic specialization is 
also spoken of as "adaptive parasitism," and biologic forms are referred to 
as "biologic species," "biologic races," "physiological races," and "adapted 
races." 
Eriksson (8) called attention definitely to the nature of biologic speciali- 
zation through the results of his cross-inoculation experiments with Puccinia 
graminis in Sweden. He found that, although this rust upon the cereals 
and grasses represented the same morphological species, the form upon 
one host-species was not always identical with the form upon another; 
since, for example, oat rust would transfer to oats, but not as a rule to other 
species of Gramineae. He showed that this fungus embraces at least six 
distinct forms distinguished by their dissimilar powers of infection with 
respect to the species of the grass family. In a subsequent paper (9) Eriksson 
showed that the trend of specialization may be different in isolated localities. 
This was illustrated by the fact that the form on rye, Puccinia graminis 
secalis, has a relatively vigorous development in Sweden, but a relatively 
weak one in North America. In the case of Puccinia gramifiis tritici on 
wheat, the more vigorous development is in the latter country. 
Ward (46), experimenting with Puccinia dispersa upon Bromus, dis- 
covered "bridging species." The nature of a "bridging species" is de- 
scribed by this statement of the author: "Although it is generally true that 
the adapted races of Puccinia dispersa are restricted to groups of closely 
allied species, there do occur species which serve as intermediaries in the 
passage of the fungus from one section of the genus to another." 
Stakman and Piemeisel (43) claim that the biologic forms of P. graminis 
can be distinguished from one another morphologically as well as 
biologically, by the size, shape, and color of the urediniospores. Re- 
garding susceptibility they state: "All gradations in susceptibility occur 
from complete immunity to complete susceptibility to the various biologic 
forms. The following reactions may be made to inoculation: no visible 
effect, appearance of small flecks, production of very small uredinia without 
flecks, production of very small uredinia in small or large flecks, production 
of large uredinia surrounded by small dead areas or by apparently healthy 
tissue." 
Arthur (i), Hitchcock and Carle ton (13), Carle ton (2, 3), and Stakman 
(42) have also carried out valuable investigations upon biologic specializa- 
tion in rusts. 
Neger (21) and Marchal (17) were early workers upon adaptive para- 
sitism in the powdery mildews. Salmon, however, has done the most ex- 
tensive work in this field, and has published a long series of papers (30-39). 
He proved that several biologic forms of Oidium parasitize the species of 
Bromus, that an individual species may be the meeting place of several 
biologic forms, and that "bridging species" exist. An additional discovery 
is best stated in his own words (35, p. 57) : "The inter-relations of the biologic 
