446 
Journal of Agricultural Research voi. xxiii, No. 6 
RESISTANCE PHENOMENA IN GENERAL 
RELATIONSHIPS AND RESPONSES OP HOST AND PARASITE 
It has long been known that plants and animals vary greatly in their 
susceptibility to disease, and that the causal organisms are more or less 
restricted in their choice of host. This variation may be attributable to 
the inability of the causal organism to set up a pathogenic relationship 
with the host or to its inability to function after having established such 
relationship. Failure to infect may be due to an incompatible optimum 
of temperature, moisture, light, or nutrient materials. These factors may 
also provide the physiological basis for failure to function after infection 
has taken place. In addition, the outer coat may offer a mechanical 
obstruction. Thus, the bark of trees, the seed coats of grain, and the 
skins of animals offer a powerful protection against invading organisms 
which gain a ready access if injury occurs. However, a great majority of 
the pathogens that have been studied experimentally seem to have less 
difficulty in gaining entrance to closely related resistant forms than in 
continuing that existence after having gained entrance. Thus in Marryat’s 
experiments (32) infection by yellow or stripe rust [Puccinia glumarum 
(Schm.) Erikss. and Heim.] occurred with equal readiness through the 
stomata of resistant and susceptible wheats. The susceptible Michigan 
Bronze wheat seemed to nurture the infection tubes and hyphae, the 
cells accepted the haustoria without shrinking, and normal spores were 
soon produced. Hyphae never seemed to flourish in the resistant Einkom, 
but appeared stunted and watery and seldom produced haustoria. They 
soon died, but in dying they killed the host cells with which they came 
in contact—appearing to be mutually toxic. The American Club, inter¬ 
mediate in resistance, sometimes killed the fungus without reducing the 
vigor of the host cells, but in other places the hyphae were found flourish¬ 
ing while the host cells seemed to be dying. When spores did develop 
there was not enough force to break the epidermis. Apparently resist¬ 
ance in this case was produced by antitoxins furnished by the host and 
toxins furnished by the parasite. Ward (do), also working on the 
histology of resistance, showed that size of stomata, hairs, or comparative 
leaf surface had no influence whatever on the susceptibility of different 
bromes to brown rust, Puccinia dispersa Erikss. and Henn. After 
treating a mass of data statistically he drew the following conclusions: 
Resistance —is not to) be referred to observable anatomical or structural peculiarities 
but to internal, i. e., intraprotoplasmic, properties beyond the reach of the microscope, 
and similar in their nature to those which bring about the essential differences be¬ 
tween species and varieties themselves. 
This was in 1902. Since then a great mass of experimental work in 
plant pathology, bacteriology, and medicine has added weight to his 
conclusions. For example, Stakman (45), who has done much work on 
the phenomena of resistance in cereals and grasses to the stemrust, 
Puccinia graminis Pers., concludes from his histological study of hyphal 
invasion (to quote his summary) : 
1. When plants practically immune to Puccinia graminis are inoculated, the fungus 
gains entrance in a perfectly normal manner. 
2. After entrance the fungus rapidly kills a limited number of the plants cells. 
3. The fungus, after having killed the host cells in its immediate vicinity, seems 
unable to develop further. 
4. The relations between plant and parasite in partially resistant and almost wholly 
immune plants are different in degree only. 
