220 MENDELISM 



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 

 Puccinia 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 plant without exception was 

 badly rusted, but fortunately a considerable number 

 of ripe grains was obtained, and these were sown 

 to produce the second generation. When the plants 

 of this generation had grown up it was observed 

 that among a majority of badly-rusted plants certain 

 individuals stood out fresh and green, being entirely 



