64 CORN-RUST 
spores carefully placed between the lines with a camel-hair 
brush. 
A few instances of specialisation will now be given, in 
addition to the less complicated cases which are treated of 
under the several species in the systematic part. It will be seen 
that greater economic importance attaches to this specialisation 
than might at 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 
P. graminis is distinguished as Black Rust, on account of the 
conspicuous black striae 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 
who 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 versd. 
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 
Erysiphacee. 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 
