Ernest S. Salmon. 
2 17 
ON THE STAGES OF DEVELOPMENT REACHED BY 
CERTAIN BIOLOGIC FORMS OF ERYSIPIiE IN CASES 
OF NON-INFECTION. 1 
[Plate V.] 
By Ernest S. Salmon. 
mHE morphological species Erysiphe Graminis DC. has been 
J_ proved to consist of a number of “ biologic forms,” each of 
which is sharply restricted either to a single host-species, or to a 
few closely-allied species. 2 Thus the biologic form on wheat cannot 
infect barley, rye, oat, &c.; that on barley cannot infect wheat, rye, 
oat, &c. ; and so forth. In some cases this specialization of para¬ 
sitism has proceeded to such an extent that we find distinct and 
sharply-marked biologic forms on very closely related host-plants; 
thus, the two biologic forms on Bromus commutatus and B.racemosus 
are unable to cause the reciprocal infection of their host-plants, 
notwithstanding the fact that the two plants are so close morpho¬ 
logically that the majority of systematists consider that B. rcicemosus 
is nothing more than a variety of B. commutatus. 
Now in these cases when the conidia of a certain biologic form 
are sown on the host-plant of another biologic form of the same 
morphological species, although no true infection, involving the pro¬ 
duction of a mycelium and conidiophores, results, yet the conidia 
germinate and reach certain further stages of development. The 
object of the experiments described below was to investigate these 
stages of development, and to ascertain at what stage and in what 
manner the action of the “ wrong ” host-plant stopped the further 
growth of the germinating conidium. The inoculations made were 
as follows:—(1) Conidia from wheat were sown on barley ; (2) Coni¬ 
dia from Bromus mollis and B. racemosus were sown on B. commu¬ 
tatus ; (3) Conidia from oat were sown on wheat. These cases were 
selected because it has been proved by a very large number of 
experiments that no true infection resulted when these inoculations 
were made [see (1), (2), (3) and (4)]. We will consider separately 
the results obtained in each case. 
(1) Conidia were taken from wheat and sown at a marked place 
1 From the Jodrell Laboratory, Royal Botanic Garden, Kew. 
2 A general account of the phenomena met with in the specializa¬ 
tion of parasitism in the Erysiphaccae, and a definition of 
“biologic forms” have been given by the writer in this 
journal (Vol. III., p. 55.) 
