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1898 } RESEARCH INTO GRAIN RUST 29 
Between certain of these forms, that is to say, those which 
constitute a species (for instance the three forms of black rust) 
we have not succeeded in discovering, even with the aid of a 
microscope, any distinguishing difference in the outer appear- 
ance, such as the size, color, and distribution of the pustules, 
the shape and size of the spores, etc. However, there is a dif- 
ference between them with regard to their inner nature that is 
of no little practical interest. The difference appears in that every 
form is almost exclusively confined to its particular cereal, and 
that consequently it is able to infect no 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. 
If we consider the thirty-seven kinds of grasses mentioned 
in the table, we observe that the forms of rust upon them 
distinguished at the present time reach the number of thirty, 
and that these forms are brought together under seven 
species. 
The old P. graminis has been divided into two species, that 
is to say, P. gxaminis (black rust) with ecidium, and P. Phlei- 
pratensis (timothy rust) without acidium ; P. rubigo-vera, its form 
simplex included, has been divided into three species, P. gluma- 
rum (yellow rust), P. dispersa (brown rust), and P. simplex 
(dwarf rust); and finally P. coronata into two species, P. corontfera 
(crown rust) with zcidium on Rhamnus cathartica, and P. coronata 
With ecidium on AR. Frangula. Two forms, the last two in the 
table, are for the present arranged separately because we have 
e 
*In this column are yet to be arranged the forms of black rust on the following 
nine grasses: Aira flexuosa, Alopecurus nigricans, Elymus glaucifolius, Panicum milt- 
aceum, Phieum Boehmeri, P. Michelit, Poa chaixii, P. pratensis,and Triticum unicum, 
with regard to their capacity for passing to a barberry. So far it has not been possible, 
on account of lack of «cidiospores, to range each of these forms in its proper place 
in the species. 
