Ernest S. Salmon. 
germ-tube of the conidium to penetrate the leaf-cells of the plant, 
but is due to the inability of the fungus to develop further the 
incipient haustorium which is formed, or to the incapacity of the 
fully-formed haustorium to adapt itself to the intra-cellular condi¬ 
tions. The incipient haustorium becomes arrested and gradually 
disorganized under the influences at work in the cell of the “ wrong ” 
host-plant, or if the haustorium attains to its full size it is hindered 
by these influences from carrying on its normal functions, and thus 
supplying to the fungus the food necessary for the production of 
mycelium, &c. 
We are justified in assuming that susceptibility or immunity, at 
all events in the case of these host-plants and their “ biologic 
forms,” by no means depends on the absence or presence of a chemo- 
tactic substance in the cells of the host-plant, as Massee (8) supposes, 
but on the capacity or incapacity for maintaining certain working- 
relations between the haustorium and the host-cell. Immunity in these 
cases depends on the power possessed by the host-plant of pre¬ 
venting by means of certain physiological processes the attainment 
of that balance whereby these working-relations are brought about 
and maintained. True infection does not merely depend on the 
attacking of the cell by the penetration-tube,—a process with which 
chemotaxis may be concerned—nor even on the successful pro¬ 
duction of the haustorium, but is determined by far more compli¬ 
cated factors, concerned it may well be with the production of 
special substances by the protoplasm of both the haustorium and 
that of the host-cell. We may confidently expect that with the 
discovery of the precise nature of the physiological inter-relations 
between the haustorium and the host-cell the causes of the 
immunity and susceptibility of the host-species of “biologic forms ” 
will be explained. With regard, too, to the question of the 
raising of “immune” races of plants of economic importance, it 
will probably be found that the place of the decisive conflict 
between host and parasite (the result of which gives immunity 
or susceptibility to the plant) will be intracellular rather than 
extracellular. 
The results obtained in the present experiments support the 
view advanced by Marshall Ward fifteen years ago, that in cases of 
true infection through the formation of haustoria, “the mycelium 
enters into a peculiar symbiotic connection with the cells, and for 
sometime merely taxes them, as it were, rather than injures them 
directly” (6), (7). Using “symbiosis” in this sense, we may say 
