RUST-RESISTANT VARIETIES OF WHEAT 29 



could not always be correlated with resistance. They suggest that re- 

 sistance is of a complex chemico-physiological nature and is inherent 

 and fairly constant within the plant. Ward (1902) decided in connec- 

 tion with Puccinia dispersa on the bromes that resistance was indepen- 

 dent of any recognizable morphological character and suggested that 

 the problem was much the same as that dealing with the factors gov- 

 erning fertility and sterility of stigmas to pollen. Biffen (1907, p. 128) 

 concludes that resistance to Puccinia glumarum is independent of any 

 discernible morphological character. He reasserts this principle in a 

 later work (1912, pp. 421-429). Bolley (1908), although not positive, 

 inclines to the view that disease-resistance is physiological rather than 

 morphological in its nature. Cook and Taubenhaus (1912) show that 

 various vegetable acids are toxic to parasitic fungi and that the amount 

 of some of these acids present depends on the stage of ripening of the 

 fruit. Jones, Giddings, and Lutman (1912, p. 83) conclude that the 

 resistance of potato tubers and leaves to Phytophthora infestans is due 

 to somtthing either largely or wholly within the tissues. The consen- 

 sus of opinion among the more recent investigators seems to be that 

 there is a very delicate balance maintained between host and parasite. 

 This balance is dependent, to a certain extent, on the environmental 

 conditions under which host plants are grown. This, however, as well 

 as other phases of the problem, will be discussed more fully under the 

 various sections into which the question naturally subdivides itself. 



FORMS WHICH ARE RESISTANT 



Attention has frequently been called by various observers to the 

 fact that freedom from disease does not necessarily indicate resistance. 

 Varieties which mature early may escape the disease, and various other 

 factors, mainly ecological, may influence the degree of resistance. 

 Careful experiments, however, have shown that some varieties of 

 Triticum durum are really resistant. The resistance of durum wheats 

 varies with locality according to Bolley's observations (1906, p. 662). 

 Consequently to determine absolute resistance the plants should be 

 grown under controlled conditions. Further, they should be subjected 

 to conditions favorable for infection. 



FIELD OBSERVATIONS 



Field observations were made on various forms which were grown 

 in a rust plat. An epidemic was induced by spraying frequently with 

 water containing a large number of spores of Puccinia graminis tritici. 

 Frequent observations, were made on the amount of rust. Final notes 

 were taken at ripening time. The percentage indicates in each case 

 the estimated percentage of resistance, assuming absolutely immune 

 forms to have a resistance of 100 per cent. 



