1 8 Ward . — Recent Researches on the Parasitism of Fungi. 
as to the validity of Eriksson’s ‘ species,’ and as to the exact value of his 
‘form-species.’ My own opinion is that no species can be accepted as 
valid until it is capable of definition in morphological terms ; and we shall 
see that the results of cross-infections have shown that bridging species (of 
host-plants) seriously affect the question of autonomy regarding the form- 
species. 
Much remains to be done, however, before we can settle these questions, 
and I would point out that here, again, a definite line of research exists in 
the inquiry after the as yet undiscovered aecidia, and the testing of form- 
species on different hosts. 
Eriksson, meanwhile, appears to insist on the rigid specialization of 
these forms, each to its own hosts ; and in one case at least the aecidium 
form discovered for P. dispersa , f. sp. bromina , bears out his contention, 
though other results shake his conclusions, as will appear in the sequel. 
The researches of the last few years have brought out the fact, soon 
suspected and looked for, that specialized parasitism is not confined to the 
Uredineae. 
Magnus (107, p. 81) in 1894 pointed out that Peronospora parasitica 
and Ustilago violacea show a, so to speak, choice of hosts which points to 
the same phenomenon; and Rostrup in 1896 (146, p. 116) called attention 
to several Ascomycetes, e.g. Dasyscypha Willkommii , D. calycina , D. abietis ; 
species of Sclerotinia , Epichloe , &c. ; Sphaerotheca pannosa ; as well as certain 
Ustilagineae, &c., as indicating by their behaviour a similar specialization. 
In 1902 (120, p. 342) Neger showed, by means of cultures, that specialized 
parasitism is probably quite common in the Erysipheae, at any rate as 
regards the conidial form ; and the phenomena have been very exhaustively 
followed out by Salmon (150), who has proved the question up to the hilt, 
and has shown that the course of events is similar in all respects to what 
occurs in the Uredineae. Much of this work was done in my laboratory, 
and I can testify to the accuracy and thoroughness of Salmon’s work. 
Giesenhagen (76, p. 319) in 1895, an< ^ Ludi (105, p. 1) in 1901, have 
also discovered similar phenomena in Exoasceae and Chytridiaceae ; and 
Beijerinck’s (26) results with the bacillus of the leguminous root-nodules point 
to the same conclusion, as do many other investigations in Bacteriology. 
Stager in 1903 (165) showed the same to be true for Claviceps ; see 
also Ed. Fischer (71, p. 53). 
On the other hand, Cystopus, Botrytis , and others appear to be 
pleophagous so far. 
Nor is specialized parasitism confined to plants. Species of Chermes , 
according to the researches of Cholodkowsky (39), and of Burdon — the 
latter working in my laboratory — are specialized more or less to certain 
trees ; and similar results obtain for the Pine-beetle, Willow Galls, and 
even the Hessian Fly and, according to recent work, the root Nematodes. 
