76 Transactions British Mycological Society. 
These results appear to be conclusive and show that suscepti- 
bility and immunity to Yellow Rust can be considered to be 
genetic factors operating in a Mendelian way. 
Resistance to Yellow Rust has thus been shown by Biffen 
and Armstrong to be a Mendelian recessive character. Re- 
sistance to Mildew (Evysiphe graminis) as regards wheat has 
been shown on the other hand by Armstrong to be a dominant 
character. Thus the variety Wilhelmina, which is very sus- 
ceptible to mildew, on being crossed by the variety Persian 
which is immune to mildew, gave hybrids which were also com- 
pletely immune to this disease. The F, generation of this cross 
grown during 1920 consisted of some goo plants about 100 of 
which were attacked by mildew. The rest remained free from 
attack, but owing to the general weakness of many of the 
plants, the thinness of the crop, and other conditions unfavour- 
able to mildew attack, Armstrong states that no accurate 
statistics are available. Further researches therefore are neces- 
sary to show how far resistance to mildew can be correlated 
with the mode of inheritance of resistance to Yellow Rust. 
Little work has yet been done concerning the mode of in- 
heritance of resistance to other plant diseases, but there is a 
wide field of research available here and one full of promise. 
Indications are to hand that resistance and susceptibility to 
wart disease of potatoes may be a pair of allelomorphic char- 
acters, the practical bearing of which is obvious. 
The breeding of disease-resistant varieties of cultivated plants 
is clearly one of the most effective means of combating plant 
diseases of economic importance, and this method applied to 
cereals has already given results of considerable value. 
In this connection, however, there are some considerations 
which must be taken into account, that tend to limit the 
universal application of the methods of the plant breeder to 
cure all the ills to which plants are subject. Immunity is a term 
covering many shades of meaning, and sometimes the so-called 
immunity to disease is merely the expression of the accidental 
escape of the host from coming into contact with the germs 
of the parasite that cause infection. Where immunity is really 
of the nature of active disease-resistance, the basis of immunity 
is probably very diverse in different diseases. Thus immunity 
to a malady such as wart disease of potatoes which is caused 
by an organism which can only live parasitically, i.e. is an 
obligate parasite, probably depends upon factors different 
from those conferring immunity to a wound parasite, where 
the fungus begins growing as a saprophyte prior to invading 
tissues consisting perhaps essentially of dead cells. There is as 
yet no information available as to the inheritance of disease- 
