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1898 | RESEARCH INTO GRAIN RUST 37 
But in what form are these internal germs of disease living? Is 
it easy to follow and identify them with the microscope? Not 
at all. They can only be detected just before the breaking out 
of the young pustules. The microscope examination induces 
me to suppose that 
10. The fungus lives for a long time a latent symbiotic life as a 
mycoplasma in the cells of the embryo and of the resulting plant, and 
that only a short time before the eruption of the pustules, when outer 
conditions are favorable, it develops into a visible state, assuming the 
form of a mycelium. 
To speak comprehensively, the investigation above outlined 
gives the following general conclusions : 
A. The outbreak of grain rust is due (a) in the first place to 
germs of disease in the host plant itself, which in certain cases ave 
inherited from the parent plant through the seed, and in which they 
lead a latent symbiotic life as a mycoplasma and continue to do so 
afterwards for a long-time in the resulting plant, and (b) in the 
second place to external infection from the vicinity. 
B. The intensity of grain rust is due (a) in the first place to 
the degree with which the dominant outer circumstances (weather, 
soil, manuring, and so forth) are able to convert the inner germs of 
disease from the latent stage of a mycoplasma into a visible stage 
of mycelium, and.(b) in the second place to the accession of infect- 
tve material from without. 
So far have we now gone in our knowledge of the nature of 
grain rust. Many things that before seemed incomprehensible 
have now a natural explanation, and our point of view has been 
very much changed. Especially have the experiments so far 
carried out provided a new method for explaining the varying 
Susceptibility of different varieties of cereals, and have thus 
given a new point of departure for continued efforts for the 
mastery of the disease in the open field. We are warranted in 
suggesting that the predisposition of the Hosford wheat to yel- 
low rust may be explained by assuming that between this variety 
of wheat and the yellow rust an extremely vital mycoplasma- 
symbiosis is to be found, while on the contrary the Squarehead 
