37 6 THE WHEAT PLANT 



severely infected that the upper parts of the young leaves are covered 

 almost entirely with the bright yellow sori of the fungus, and the surface 

 of the soil around the plants is bright yellow with the fallen uredospores. 



Although the extent of the attack on the different wheats varies from 

 year to year, the classification into resistant and susceptible forms is easy, 

 and infection or freedom from attack are found to be constant hereditary 

 characters. 



It was Biffen who first investigated the inheritance of resistance and 

 susceptibility to the attacks of P. glumarum, and discovered that these 

 characters form a Mendelian allelomorphic pair. On crossing resistant 

 forms of T. turgidum (Rivet), T. compactum (American Club), and T. 

 vulgare (Hungarian Red) with very susceptible forms of T. vulgare 

 (Michigan Bronze, Preston, and Hungarian White) susceptibility was 

 found to be a dominant character, the hybrid F x generation being as 

 easily attacked as the susceptible parents. 



In F 2 immune and susceptible segregates were obtained in the pro- 

 portion of 3 susceptible to i resistant, which bred true in the F 3 genera- 

 tion, immunity proving a recessive character. 



While the immune or resistant forms were not completely free from 

 attacks of the rust, they could be distinguished without much difficulty 

 from the susceptible plants. 



Among the infected segregates there appeared to be different degrees 

 of susceptibility, and the plants in the F 3 bred more or less true to the 

 variable character. Later Nilsson-Ehle also examined the inheritance of 

 the same species of rust in hybrids between more or less susceptible and 

 resistant forms of T. vulgare. He confirmed Biffen's results, but observed 

 that well-marked dominance of susceptibility was rarely found, the F x 

 being intermediate. 



In the F 2 generation segregation was evident, but the establishment 

 of definite numerical ratios was not possible, the segregates being finely 

 graded between those which might be classed as resistant and the most 

 susceptible plants. Moreover, in some of the crosses transgressive segre- 

 gation occurred in F 2 , plants more susceptible and more resistant than 

 either of the grandparents being found in this and subsequent generations. 



Biffen also observed plants more susceptible than either of the grand- 

 parents in the F 2 generation of the hybrid Rivet x Red Fife. 



Nilsson-Ehle attributes these results to the existence of multiple 

 factors for the susceptible and resistant characters (cf. inheritance of 

 grain colour, pp. 373-375). 



The researches of Pole-Evans in South Africa regarding the inherit- 

 ance of susceptibility and resistance of wheats to the attacks of Black 

 Stem-rust (Puccinia graminis, Pers.) show that susceptibility is the 

 dominant, resistance the recessive character as in P. glumarum. In the 



