26 INTRODUCTION [CH. 



Freeman and Johnson, in 1911, described barley as a bridging host for 

 biological forms of Puccinia Graminis on other cereals and, in the same 

 year, Pole Evans found that the heterozygote between strains immune and 

 susceptible to infection by black rust was even more susceptible than the 

 susceptible parent and was capable of acting as a bridging species from 

 which the immune parent could be infected. 



On the other hand an increasing number of investigators, working with 

 isolated strains under rigidly controlled conditions, have failed to confirm the 

 existence of bridging species and, according to Stakman and his assistants, 

 there is no evidence that biological species are changed by their sojourn on 

 a particular host. Such a change would imply a certain physiological 

 plasticity on the part of the fungus, since it would be capable of becoming 

 acclimatized, on the bridging host, to conditions similar to those awaiting it 

 on the form resistant to direct attack. 



In the case of Puccinia Graminis the problem is further complicated by 

 the fact that a considerable number of biological forms are parasitic on the 

 varieties of wheat, so that the same variety may be susceptible in one 

 locality and immune in another according to the distribution of the parasite. 



SPECIALIZATION OF PARASITISM ETC.: BIBLIOGRAPHY 



1863 Dfc BARV, A. Recherches sur le developpement de quelques champignons parasites. 



Ann. Sci. Nat. seV. 4, xx, p. 5. 

 1894 ERIKSSON, J. Ueber die Specialisirung des Parasitismus bei den Getreiderostpilzen. 



Her. der dent. Bot. Ges. xii, p. 292. 

 1898 ERIKSSON, J. A General Review of the Principal Results of Swedish Research into 



Grain Rust. Bot. Gaz. xxv, p. 26. 

 1902 DIKDICKE, H. Ueber den Zusammenhang zwischen Pleospora- und Helmintho- 



jr/or/ww-Arten. Centralbl. f. Bakt. ix, Abt. ii, p. 317. 

 1902 WARD, H. MARSHALL. On the Relations between Host and Parasite in the Bromes 



and their Brown Rust, Puccinia dispersa (Erikss.). Ann. Bot. xvi, p. 233. 



1902 WARD, H. MARSHALL. Experiments on the Effect of Mineral Starvation on the 

 Parasitism of the Uredine Fungus, Puccinia dispersa, on Species of Bromus. Proc 

 Roy. Soc. Ixxi, p. 138. 



1902-3 MARCHAL, E. De la Specialisation du parasitisme chez VErysiphe Graminis. 

 Comptes Rendus, cxxxv, p. 210, and cxxxvi, p. 1280. 



1903 WARD, H. MARSHALL. Further Observations on the Brown Rust of the Bromes 

 Puccinia dispersa (Erikss.) and its adaptive Parasitism. Ann. Myc i p 1*2 



1903 SALMON, ES. On the Specialization of Parasitism in the Erysiphace'ae. Bei'heftc 2, 

 Bot. Central, xiv, p. 261. 



903 * ^'^ f ASC Sp reS in the Erysiphaceae. Journ. Bot. 



1904 MASSEE, G. On the Origin of Parasitism in Fungi. Phil Trans B 



1904 



' and its 



