64 CORN-BUST 



spores carefully placed between the lines with a camel-hair 

 brush. 



A tew instances of specialisation will now be given, in 

 addition fco the less complicated cases which are treated of 

 under the several species in t he systemal ic part. It will be seen 

 thai greater economic importance attaches to this specialisation 

 than mighl a1 first be imagined. The first example taken will 

 be that of Puccinia graminis, which is found upon various 

 grasses, especially upon cultivated cereals. In the early days of 

 this study almost any rust upon corn was called P. graminis: 

 afterwards it was found that there are several kinds, which can 

 be easily separated by their form or colour, and the real 

 /'. ij rn minis is distinguished as Black Rust, on account of the 

 conspicuous black stria' which its teleuto-sori form upon the 

 culms in autumn. Its uredospores also can easily be dis- 

 tinguished from the uredospores of the other species which live 

 upon the corn. But even after restricting the application of 

 the name by these morphological distinctions, the species is 

 still recorded on more than 180 kinds of grass, although of 

 course some few of these records may be erroneous. 



When discussion took place in the past upon the mode by 

 which epidemics of Corn-rust were caused, apart from the 

 Barberry, year after year, it was considered sufficient to point 

 to this wide prevalence of the species, and to assert that it lived 

 through the winter upon the wild grasses and passed from them 

 to the corn when the time arrived. Eriksson is the experimenter 

 wlio has done most to refute this idea; by making artificial 

 inoculations he has proved that in certain cases the rust which 

 is found on wild grasses will not infect the wheat and vice versa. 

 In spite of this biological difference, however, in most cases no 

 morphological distinctions can be detected, or, if so, they are 

 very slight and somewhat variable. Nevertheless the difference 

 exists, though in varying degrees of definiteness ; exactly the 

 same kind of specialisation has been proved to exist in the 

 Erysiphacese. The natural explanation is that the species, 

 P. graminis, was originally parasitic on numerous grasses, quite 

 indifferently; but as time went on, certain reasons, perhaps 

 geographical or ecological, caused some sets of individuals to 



