502 



Rust in Wheat. 



[JULY, 



that three distinct types would be present in this generation, 

 namely : — 



(1) With dense ears, similar to the parent " Club " wheat. 



(2) With an intermediate ear-shape resembling that of the 

 hybrid. 



(3) With loosely set ears like those of Michigan Bronze. 



On taking this feature into consideration, as well as the 

 behaviour of the plants to the attacks of yellow rust, it was 

 found that each of the three types was composed of plants 

 showing either susceptibility or immunity to yellow rust. 

 In the original parents immunity was associated with dense- 

 ness of the ears and susceptibility with laxness, but in this 

 generation there were susceptible as well as immune samples 

 of " Club " wheat and also immune and susceptible examples 

 of a wheat similar to Michigan Bronze in the shape of the ear. 



Immunity and susceptibility to the attacks of yellow rust 

 then may be treated as unit characters, since they may be 

 obtained in combinations differing from those in which they 

 occur in the original parents. To any one with any experience 

 of modern plant breeding the problem was solved at this stage, 

 and it was evident that the breeder could build up disease- 

 resisting varieties " to order." Nevertheless, to test the 

 matter further, another generation was grown to determine 

 whether each of the new t}^pes was fixable in accordance with 

 '.Mendel ian expectations. At this time it was known that the 

 dense and lax types would breed true to these particular 

 characters and that those with an intermediate ear shape 

 were unfixable. From the experience already gained from a 

 study of the generation raised from the hybrid, it was foreseen 

 that the immune types also would breed true to this particular 

 feature, whilst on the average only one in three of the sus- 

 ceptible plants could be relied upon to produce only susceptible 

 plants, the remainder giving a mixed offspring. 



Individuals showing such combinations of characters as 

 immunity to yellow rust and loosely set ears, or immunity 

 and densely set ears, should therefore breed true from the 

 outset. Tests made with a considerable number of individuals 

 showed that this was the case without exception. The sowings 

 from susceptible plants in some cases bred true to this character, 

 and in others produced a number of immune plants as well, 



