56 
E. S. Salmon. 
DC., E. Cichoracearum DC., E. Galeopsidis DC., and Sphaerotheca 
Humuli (DC.) Burr. 1 
This phenomenon of specialized adaptive parasitism is not 
peculiar to the Erysiphaceae. It was originally discovered by 
Eriksson 2 in the Uredineae, and its existence has lately been proved 
in other groups of parasitic fungi. 3 
Some recent work on the specialization of parasitism in the 
Erysiphaceae, which I have carried out at the Cambridge University 
Botanical Laboratory, has brought to light certain facts which 
show the existence of complicated inter-relations between “ biologic 
forms” and their host-plants. These facts show (1) the high 
degree of specialization reached in the adaptive parasitism of 
E. Graminis to the various host-species within a single genus; 
(2) the existence of “ biologic forms ” of host-plants; and (3) 
the existence of certain host-species which serve as “ bridging 
species.” 
In the series of inoculation-experiments conidia of the forms of 
E. Graminis occurring on species of Bromus were used, and 1650 
inoculations were made on leaves of various species of the genus. 
The results of the experiments show that a considerable number 
of “ biologic forms ” exist within the genus Bromus. Thus the fungus 
occurring on B. inter ruptus, B. “ hordeaceus,*’ B. commutatus, B. 
racemosus, B. velutinus, B. arvensis, B. tectorum, and B. cirduennensis 
proved in a series of comparative infection experiments, to be in 
each case a “ biologic form ” possessing distinctive infection- 
powers. 
These facts show the high degree of specialization which the fun¬ 
gus has reached in fitting itself to the various species of the genus 
Bromus. They show also that each species of Bromus has distinctive 
physiological (or constitutional) characters existing concomitantly 
with the specific morphological characters. We are able by using the 
index of the reaction to the attacks of the “biologic forms” of the 
fungus to show the presence of specific physiological (or constitutional) 
1 See Neger, F. W., in Flora XC., 221-272 (1902); Marchal, E. 
in Comptes Reudus, CXXXV, 210-212 (1902) niul CXXXVI., 
12S0-1281 (1903); Salmon, E. S., in Reihefte z. Botau. 
Centralbl., XIV., 261-315, Taf. XVIII. (1903),and in Journ. of 
Rot., XU., 159165. 204-212 (1903). 
2 See Eriksson, J in Rot. Gaz. XXV. 26-38 (1898), and in Anu.Sci. 
Nat. VIII.ser.,XIV., 1-124(1901), andXV., 1-155(1902); also 
Fischer, E.; Die biologisclien Arten der parasit. Pilze 
(I9<>3k 
3 Thus in the Hypocreaceae (Claviceps), see Steiger, R., in Bot. 
Zeit. LXI., 111-158 (1903). 
