Parasitism in the Erysiphaceae. 
55 
RECENT RESEARCHES ON THE SPECIALIZATION OF 
PARASITISM IN THE ERYSIPHACEAE d 
By Ernest S. Salmon, F.L.S. 
[With Figs. 63-65.] 
I N the case of certain parasitic fungi the process of becoming 
adapted to different species of host-plants has resulted in the 
evolution of “biologic forms.” An instance taken from the 
Erysiphaceae will show the nature of this specialized parasitism. 
The species Erysiphe Graminis DC. occurs on wheat, barley, 
oats, rye, and a number of wild grasses belonging to various genera 
( Bromus , Poa, Dactylis, Festuca, &c.). 
We will confine ourselves for the present to the forms of the 
fungus on wheat and on barley. In all morphological characters— 
even when examined under the microscope—the form on wheat is 
indistinguishable from the form on barley, both in the conidial 
(Oidium) stage and in the perithecial stage. But inoculation- 
experiments show at once that these two forms possess distinctive 
and sharply defined physiological (or biological) characters. 
If the asexual conidia or the sexually produced ascospores of 
the form of the fungus on wheat are sown on wheat and barley, 
infection at once results on wheat, but never on barley; and simi¬ 
larly with the form of the fungus on barley when sown on barley 
and wheat. Further, if the fungus on wheat or barley is sown on 
species belonging to any other genus in the circle of host-plants of 
E. Graminis, no infection results. 
The forms on wheat and barley are therefore “biologic forms,” 
since they are morphologically identical, but differ physiologically 
or biologically in possessing distinctive and sharply defined infection- 
powers. 
Inoculation-experiments have shown also that the forms of 
E. Graminis in Secale, Avena, Poa, Agropyrum, Bromus and Dactylis 
constitute likewise “biologic forms,” and that the same phenomenon 
is found in other species of the Erysiphaceae, e.g. in E. Polygoni, 
1 We are able to include this article, containing a general sketch 
of his more recent investigations, through the courtesy of 
the author; Mr. Salmon's detailed paper will appear in a 
forthcoming number of the Beihefte zum Botanischen 
Centralblatt. Ed. — N.P. 
