234 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
ency to usefulness on the part of the parasitic organism, and by the utmost 
tolerance on the part of the host. In many instances the nature of the 
reaction is not constant, but varies with the progress of the host-parasite 
relation. In this intergrading series of possible host-parasite relations, the 
inverse relation between host vigor and parasite virulence obtains only in 
the instances and phases where the reaction of the host to the parasite is 
one of active antagonism ; here a more vigorous host means a host of greater 
physiological capacity to combat the progress of the invader. But when 
the relation between host and parasite is of a symbiotic type, a more vigorous 
host means a host in which more food is available for the development of 
the parasite. Because, of the general class of parasitological phenomena, 
the instances mainly in the field of pathological interest (the diseases 
ordinarily so called) are an artificially selected group in which relations of 
violent antagonism between host and invading organism are most promi- 
nently in evidence, thought in the field of pathology has developed with the 
physiological antagonism of host and parasite as its basal concept; and the 
theories of immunity extant are largely concerned with the nature of the 
antagonistic reactions. 
In the group of the fungi the transition from violent and destructive 
parasitism to parasitism of the symbiotic type is accompanied by a transi- 
tion from facultative to obligate parasitism, as if the physiological corollary 
of parasitism of the latter type is extreme specialization in food preferences. 
The series in the fungi grades from violent and destructive parasites like 
Botrytis, on the one hand, to, on the other hand, so benign an infestation as 
the seed fungus of Lolium temulentum (described by Freeman, 1903) in 
which the relation is so intimate and devoid of any untoward effect on the 
host, and the life history of the cohabiting organism is so parallel with that 
of the grass, that its distinct individuality is almost open to question. 
The mutualistic nature of the relation between host elements and 
fungus in rusts of the type of the cereal rusts is commented on by Tubeuf 
(1897, p. 91) who very aptly compares the mass of chlorophyll-bearing 
leaf cells infested with the rust mycelium to a lichen structure, especially 
to those lichens whose algae obtain water and inorganic materials direct, 
rather than through the fungous hyphae. Certainly, during the greater 
part of the relation, there is here no evidence of any deleterious effects on 
the host cells. While the contribution of the affected elements to the 
growth and fruiting economy of the host plant as a whole may be diminished, 
the infected protoplasts continue essentially unimpaired in structure and 
function. The parasite does not attack the living substance of the host 
protoplast, but confines itself to establishing such a relation with the latter 
that it shares the available food resources of the cell ; and the rust haustorium 
is not an implement for mechanical disruption, but a structure more in the 
nature of the placenta of the mammalian foetus for establishing physiological 
communication with the food resources of the host. 
