PRACTICAL APPLICATIONS 221 



free from infection. On examination it was found that 

 every plant could be placed in one or other of two 

 categories either it was badly rusted or it was 

 entirely free from rust ; and the numbers of the two 

 kinds of plants were as follows : 1,609 infected, 523 

 immune. 



It is clear, then, that immunity and susceptibility 

 to the attacks of yellow rust behave as a simple pair of 

 Mendelian characters, immunity being recessive. And 

 it is, therefore, possible to obtain by crossing, in three 

 generations, a pure rust-free strain containing any other 

 desired quality which is similarly capable of definite 

 inheritance. 



