78 Transactions British Mycological Society. 
the cross between this and “ Wilhelmina” seemed to have prac- 
tically no effect as regards rendering them more susceptible to 
attacks of Yellow Rust. It seems that in these plants the 
genetic constitution was strong enough to be uninfluenced by 
a heavy application of nitrogenous manure. 
Another consideration to be borne in mind by the plant 
breeder is that parasitic organisms are themselves plastic and 
liable to temporary or permanent change leading perhaps to 
the evolution of races which are more virulent than previous 
forms. The education of relatively harmless forms of bacteria 
to pronounced virulence is a well-known phenomenon, and the 
same may prove to be true of certain fungi, but there is yet 
little evidence of this. Indeed, evidence obtained by Stakman (3) 
in America points to the various races of Puccima graminis 
being very stable forms with little power of adaptability. He 
has also found that there are several races of Puccima graminis 
on wheat inhabiting different parts of the United States, so 
that a wheat resistant in one place is readily attacked at another 
because it is exposed to infection by another strain of the 
fungus. Facts of this kind necessarily complicate the problem 
of breeding disease-resistant varieties. 
Notwithstanding these difficulties, which one may perhaps be 
allowed as a plant pathologist to place before the plant breeder, 
the latter will probably always be able to keep ahead of im- 
portant changes in the resistance-power of the host, whether 
these be due to alterations in its constitution brought about by 
environmental factors or to increased virulence on the part of 
the parasite. 
It is only in the case of Yellow Rust that we have anything 
like adequate knowledge of the inheritance of disease-resistance, 
and even here, we are very much in the dark as to what is the 
essential factor conferring resistance, although, from the re- 
searches of Marshall Ward and others, it is certainly of a subtle, 
protoplasmic nature. It is suggestive that in some immune 
wheats the fungus appears to be unable to attack the host with 
sufficient vigour to establish itself, while in others, the attack 
appears to be conducted with such violence as to defeat its 
own ends, the infecting hyphae being surrounded by so many 
dead host cells that the mycelium cannot penetrate to the living 
tissues in which alone the quasi-symbiotic life necessary for the 
rust fungus can be established. 
BIBLIOGRAPHY. 
(1) Birren, R. H.—Studies in the inheritance of disease-resistance. Journ. 
Agric. Sci., 1907. 
(2) Howarp, A. and Howarp, G.—Wheat in India, 1909. 
(3) Stakman, E. C. and others.—Plasticity of biologic forms of Puccinia 
graminis. Journ. Agric. Research, 1918. 
