BLACK RUST 65 



restrict themselves to a particular species of grass ; in course of 

 years they became more and more closely adapted to this host, 

 and in so doing grew less and less able to infect other species. 

 But possibly they have seldom completely lost this power, 

 as is shown by the existence of " bridging " species of which 

 Ward produced the best evidence in P. bromina ; these will be 

 referred to later. 



It is assumed that all the forms of P. graminis will infect 

 the Barberry ; the restrictions are confined to the alternate 

 host. As a result of his experiments, Eriksson divided the 

 species into the following " special forms," which are here called 

 " biological " races : — 



1. f. Secalis — on Rye 



2. f. A venae — on Oat 



3. f. Tritiei— on Wheat 



4. f. Airae — on Air a 



5. f. Agrostidis — on Agrostis 



6. f. Poae — on Poa. 



Race 1 grows not only on Rye, but also on Hordeum vulgar e, 

 H. murinum, Agropyron repens, A. caninum, Elymus arenarius, 

 Bromus secalinus etc. (In all these enumerations non-British 

 species will be omitted.) 



Race 2, on Oat, and on Arrhenatherum elatius, Dactylis 

 glomerata, Alopecurus pratensis, Milium effusum, Bromus 

 arvensis, B. madritensis, Festuca Myurus, F. sciuroides, F. ovina 

 (tenuifolia). 



Race 3, on Wheat, but also though more rarely on Barley, 

 Oat, and Rye. 



Race 4, on Aira caespitosa. 



Race 5, on Agrostis canina, A. stolonifera, A. vulgaris. 



Race 6, on Poa compressa, P. caesia, P. pratensis. 



A seventh Race, f. Hordei, is sometimes added, though 

 Eriksson included it under his f. Tritiei or f. Secalis. 



Jaczewski (1910) from numerous inoculation-experiments 

 arrived at somewhat different results : he found that he 

 could infect Rye only from Agropyron repens, A. caninum, 

 Bromus secalinus, and Dactylis glomerata; Wheat only from A. 

 repens, Festuca gigantea, and Lolium perenne ; Oat only from 



G. u. 5 



