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 f 

 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 

 Puccinia glumarum. Among them Mr. BifTen 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 grams could ever be 

 obtained from them. 



Bifien crossed these two types together. In the 

 first generation every plant without exception was 



