66 BLACK RUST 
Arrhenatherum elatius, Alopecurus pratensis, Avena pubescens, 
and Festuca ovina; and Barley only from Zriticwm and Lolium 
perenne. According to him it seems that only Barley and 
Wheat could infect each other directly, although it is known 
from other sources that Wheat can also infect Rye; this 
could, however, be done even according to Jaczewski’s state- 
ments, if Agropyron repens were employed as a “ bridging” 
species. 
Carleton, in North America, experimented with much the 
same forms, but reached a still different result. According to 
him (1899) there are only two biological races : 
1. f. Lritici—on Wheat, Barley, Hordeum murinum, Koeleria cristata, 
Festuca gigantea, Dactylis glomerata, Agrostis alba. 
2. f. Avenae—on Oat, Avena pratensis, A. fatua, Hordeum murinum, 
Dactylis ylomerata, Koeleria cristata, Arrhenatherum elatius, Holcus 
mollis, Ammophila arenaria, Alopecurus pratensis. 
In a further publication (1904) he adds to form (1) that 
Holcus lanatus should probably be included, and furthermore 
that ‘there is a form of P. graminis on Agrostis alba vulgaris 
which could not be transferred to Wheat or Oat. 
Freeman and Johnson (1911) in the U.S. cultivated P. 
gramims by its uredospores alone for two years without any 
loss of vigour. They found that the uredospores of f. Tritica 
would infect Barley easily, rarely Rye, and never Oat, but by 
using Barley as a “bridging” species, they could infect, with 
the uredospores produced on that, Rye easily, and Oat in a less 
degree. The uredospores of f. Hordei would infect Wheat and 
Barley easily, and in a less degree Oat and Rye. The uredo- 
spores of f. Secalis would infect Barley, and by using that as a 
“bridging” species would infect Oat at the second step, but in 
a less degree. The most specialised form was f. Avenae; besides 
the Oat its uredospores would infect only Barley, and not always 
that. 
From this it is evident that either (1) the specialisation of 
these races is less sharp than Eriksson would have us believe, 
or (2) the specialisation is taking place along two distinct lines 
in the United States and in Europe respectively. Probably 
both of these statements are true, but in support of the former 
