154 SYSTEMATIZED PLANT-BREEDING 



conditions. These determinations were repeated for 

 several years and it was found that the varieties 

 could be grouped into a number of arbitrary classes 

 showing either no disease at all, traces, or slight, 

 moderate, or bad attacks. This classification was 

 found to hold from year to year, the varieties always 

 falling into the same groups as those in which they 

 had been included in former seasons, and this in 

 spite of the varying intensity of the attack in the 

 different years. Amongst the set was one variety, 

 found originally as a single plant in a plot of 

 Northern Duluth wheat, which year by year was 

 absolutely free from pustules of yellow rust and 

 which still after twelve years' cultivation here 

 retains its striking immunity. This variety known 

 under its notebook name of "American Club" was 

 crossed with Michigan Bronze, the most susceptible 

 variety which could be found amongst those 

 under trial. The resulting F x plants proved to be 

 exceedingly susceptible to the attacks of yellow 

 rust. In this respect they were indistinguishable 

 from Michigan Bronze, but either the attack was 

 not in reality as severe or possibly because of the 

 extra vigour of the hybrids a fairly good crop of 

 grain was secured. From this an F 2 generation 

 of over 2000 plants was raised in the following year. 

 An inspection of the plots in the early summer 

 soon made it clear that the power of resisting the 

 attacks of yellow rust was an inheritable character, 

 for standing out clearly amongst the mass of the 



