66 BLACK BUST 



Arrhenatherum elatius, Alopecurus pratensis, Avena pubescens, 

 and Festuca ovina ; and Barley only from Triticum and Loliwm 



perenne. According to him it seems that only Barley and 

 Wheat could infecl each other directly, although it is known 

 from oili.r sources that Wheat can also infect Rye; this 

 could, however, be done even according to Jaczewski's state- 

 ments, it' A'irojii/ron 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. Tritici -on Wheat, Barley, Hordeum murinum, Koeleria eristata, 

 Festuca gigantea, Dactylis glomerata, Agrostis alba. 



■2. f. Avenae — on Oat, Avena pratensis, A.fatua, Hordeum murinum, 

 Dactyl 'is glomerate. , Kot '■ ria eristata, Arrhenatherum elatius, Holcus 

 mollis, Ammophila armaria, Alopecurus pratensis. 



In a farther 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. 

 gra/minis by its uredospores alone for two years without any 

 loss of vigour. They found that the uredospores of f. Tritici 

 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 



