304 DISEASES OF CULTIVATED PLANTS 



are specialised, or biological forms capable of infecting one, 

 or at most a few allied species. Thus the black rust of oats 

 can infect oats, but not rye, wheat, or barley ; crown rust on 

 rye cannot infect wheat, etc. Eriksson has proved that the 

 germination of uredospores and aecidiospores is often small, 

 or at best capricious. The germinating power of teleuto- 

 spores depends upon certain external conditions, and is 

 restricted to a short period of time, and only the crop of 

 teleutospores maturing during late autumn is able to ger- 

 minate the following spring. The propagation by spores 

 alone is considered by Eriksson as inadequate to account for 

 the enormous amount of rust appearing simultaneously over 

 very extensive areas. This and other reasons has led the 

 author to the conclusion that, in addition to the ordinary 

 method of infection by spores, there exists in the embryo of 

 the seed, in some form or other, the germs of disease, which 

 under favourable conditions grows up with the young plant, 

 and results in a rusted crop. This idea is generally known 

 as the mycoplasm theory, and is formulated by Eriksson as 

 follows : 



' The fungus lives for a long time a 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 favourable, it develops 

 into a visible state, assuming the form of a mycelium.' 



This theory has caused much controversy, and was 

 investigated in detail by Professor Marshall Ward, who 

 considered it to be unfounded, and in this country it is 

 generally considered that Ward's investigations proved 

 Eriksson to be wrong. There is one point in Ward's 

 argument which perhaps weakens his supposed refutation 

 of the theory. Eriksson distinctly admits that a certain 

 amount of infection takes place, where the mycelium sends 

 haustoria into the cells of the host. On the other hand, 

 when the mycoplasm present in the cells materialises, 

 Eriksson considers that it does so under the form of minute 

 bodies already in the cells, which send out tubes through 

 the cell-wall and form an intercellular mycelium. Now Ward 

 infected his plants with spores, and of course found haustoria 

 in the cells, connected with intercellular mycelium ; these 

 haustoria Ward considered to be identical with the bodies 

 supposed by Eriksson to represent the earliest materialisation 

 of his mycoplasm. This may be correct or it may not, but 





