YELLOW RUST 67 
view we may adduce the fact that Eriksson could infect Berberis 
vulgaris with teleutospores obtained from many grasses (Wheat, 
Oat, Barley, Rye, Arrhenatherum elatius, Agropyron repens, 
A. caninum, Dactylis glomerata, Agrostis stolonifera, Elymus 
arenarius, Poa compressa, P. pratensis, Aira caespitosa, Bromus 
secalinus, and many others, non-British) while Bolley was able 
to infect a large number of the grasses with spores taken from 
a single Barberry hedge. 
The economic importance of the matter lies in the fact that, 
if the specialisation is as strict as Eriksson maintains, the corn- 
crops cannot often (in the absence of Berberis) be infected by 
rust on other cereals or on the wild grasses in the neighbouring 
hedges. This excessive strictness, however, no one else is 
prepared to admit: in any case there are obviously plenty of 
“bridging” species which would enable the rust to get at the 
corn at the second step, if not at the first. The reason why we 
cannot, so far, attribute any very great accuracy to the state- 
ments regarding specialisation is that the conditions required 
for infection are demonstrably very complex and at present ill- 
understood, so that a negative result, even when repeatedly 
occurring, often proves nothing whatever. This is manifest 
from Eriksson’s own complaints about the “ capriciousness ” of 
the germination of the spores, and from the frequent recurrence 
of such remarks as this—“ Uredospores from Aira caespitosa 
would not always infect Aira caespitosa.” There is another 
very important conclusion that can be drawn from this survey, 
viz. that the life-histories of hetercecious rusts must always be 
worked out separately for each country in the world. 
A second example of specialisation is given by the Yellow 
or Golden Rust (P. glumarum): it is divided by Eriksson into 
five biological races : 
f. Triticc—on Wheat alone 
f. Hordei—on Barley alone 
f. Secalis—on Rye (perhaps also on Wheat) 
f. Llymi—on Elymus arenarius alone 
f. Agropyri—on Agropyron repens, 
but of the last Eriksson remarks that he could not infect this 
host with its own uredospores. With the exception mentioned, 
5—2 
SP wb 
