PRACTICAL APPLICATIONS 237 
We may next turn to an even more important 
achievement. In many countries the annual loss of 
crop due to the attacks of yellow rust, Puccinia 
glumarum, amounts on a moderate estimate to a con- 
siderable number of millions of pounds sterling. 
Certain strains of wheat exist, indeed, which are more 
or less completely immune to the ravages of this fungus, 
but these are usually wanting in other qualities which 
are indispensable to the farmer. If it should be found 
that immunity to rust is a simple Mendelian allelo- 
morph, it would be possible to combine this quality 
with any other useful character which obeyed the same 
law of inheritance—as several useful characters have 
already been shown to do. At one time it must have 
been thought that a similar method of inheritance of 
the character rust-immunity was too excellent a boon 
to be reasonably hoped for. 
Among a great number of strains of wheat grown 
on the Cambridge experimental farm, several types 
showed marked differences in the degree of their 
immunity from, or susceptibility to, the attacks of 
Puccina glumarum. Among them Mr. Biffen found 
one which was apparently quite immune, and, though 
grown in the midst of numbers of rusted plants, itself 
never showed a trace of infection. Of another type, 
known as Michigan bronze, no single individual ever 
escaped the rust, and so badly were the plants of this 
strain diseased that very few ripe grains could ever be 
obtained from them. 
Biffen crossed these two types together. In the 
first generation every nlant without exception was 
