50 Immimiti/ in Cereals 



One (if the t'uiidanicntal pheiioincna in the parasitism of fungi on 

 plants, playing a decisive 7-ule in the selection of immune sorts, is the 

 specialization of parasites. The majority of pai'asitic fungi are, by their 

 nature, sharply limited in the choice of their hosts, and are attached to 

 definite genera and species of plants. In many cases they are limited 

 to one plant genus. In others, especially in the group of rusts and 

 mildews, there are cases in which the fungus is limited to a few, or even 

 to one host-species only. 



In rusts, mildews and a few other families of fungi, the differentiation 

 went so far that the same morphological species are not seldom divided 

 into many independent physiological races, so-called " biologic forms," 

 which are attached to different host-jjlants. In some instances the 

 various stages of fructification of the same fungus may be differently 

 specialized'. 



The study of the causes of the imniunity and susceptibility of plants 

 hitherto has not made much headway ; the phenotnena of parasitism 

 being too comj)lex, and the drawing up of general statements a matter 

 for the future. But still it has been shown, by the works of Eriksson, 

 as well as by Marshall and his school (17), that immunity depends, 

 not on the anatomical peculiarities of plants, but on the properties of 

 their protoplasmic cell-contents. Salmon and Miss Gibson have also 

 establisheil that positive chemotactic attraction of the germ tubes of 

 fungi by the juice of the host-cells, is not sufficient to produce the 

 normal growth of fungi on the plant. In fact it is now clear to us 

 that immunity depends on very complicated physiological inter-relations 

 between the protoplasm of host-cells and fungus, and that the external 

 differences in regard to immunity to fungal diseases, which are per- 

 ceptible in various races of plants, are an indication of internal hereditary 

 differences in the constitution of their plasma. 



Starting from the nature of immunity, as we undcrst;ind it at the 

 present day, and from the fact of the specialization of parasites— 

 the connection between the phenomena of immunity and genetics 

 becomes evident. It is obvious that fungi, and, in particular, narrowly 

 specialized ones, may be used in some cases as a physiological test (or, 



' A very interestiiiR case of this was recently reported by J. Norton for the autoecious 

 rust of asparagus, Puccinia asparagii D.C. lu regard to tlie " uredo '' or summer stage, 

 some immune sorts of asparagus have been found, but these resistant plants are all 

 susceptible to the same fungus in the "aeoidio" stage. {Methods used in breeding 

 Asparnpus for Rust Resistance, by .J. B. Norton. Bureau of Plant Industry, Washington. 

 Bui. No. 263, 1913, pp. 23—24.) 



