GRAIN RUSTS 245 



specialised forms of species. In addition to growing on 

 cereals, these fungi are also met with on numerous wild 

 grasses ; Puccinia graminis occurs on a hundred different 

 grasses in Sweden. It was considered until quite recently 

 that the cereals could be infected by the spores of a rust 

 produced on any wild grass. This is not the casej but as 

 a rule the various forms are almost exclusively confined to 

 one host, and are unable to infect any other cereal but 

 that one. Thus stems of oats bearing black rust can 

 propagate black rust to oats, but not to rye, wheat, or 

 barley ; stems of rye bearing crown rust can propagate 

 crown rust to rye, but not to wheat ; and so on. Plants of 

 rye and barley attacked by black rust make an exception 

 to this rule, as they are able to infect one another; in 

 some cases also those of wheat propagate black rust to 

 other cereals. 



The propagation of rust is often slight between grasses 

 capable of serving as a host to the same specialised form. 



The germinating power of uredospores and aecidiospores 

 is often small, or at best capricious. It was discovered 

 that chilling the spores by putting them on ice or in cold 

 water for several hours increased the power of germination. 



The germinating power of teleutospores depends upon 

 certain external conditions, and is restricted to a short 

 period of time, and it appears that only the crop of teleuto- 

 spores maturing during late autumn is able to germinate 

 the following spring. 



All the modes of propagation enumerated appeared 

 inadequate to account for the enormous amount of rust 

 usually present, and the following observations led Eriksson 

 to formulate a theory of the occurrence of rust to another 

 cause than that solely due to infection by spores : — 



