28 A STUDY IN CEREAL RUSTS 



on semi-immune forms, may have some influence in breaking down 

 biologic forms. 



4. There appears to be a physiological and even a slight mor- 

 phological change in the rust fungus when grown continuously on a 

 semi-immune host. The physiological change manifests itself as an 

 adaptation to the new host, which, however, is very gradual. 



i 5. There are indications that biologic forms of cereal rusts, at 

 least Puccinia graminis tritici, do not lose their specialization tenden- 

 cies when grown on barberry. 



6. The degree of incompatibility of host and parasite varies 

 greatly. In semi-compatible forms, fairly large leaf areas are some- 

 times killed, indicating a killing of host cells by fungus and consequent 

 death of the mycelium itself. In this respect they resemble very closely 

 some of the rust resistant forms of wheat. The biologic forms of rusts, 

 therefore, with susceptible and immune varieties of host plants, throw 

 light on the question of the nature of resistance to Puccinia graminis. 



PART II. RUST-RESISTANT VARIETIES OF WHEAT 



HISTORICAL 



It has long been a matter of common observation that some wheats 

 are more resistant than others to the attacks of Puccinia graminis and 

 other rusts. Among the earlier observers Henslow (1841, p. 3), La 

 Cour (1863, p. 326) and Little (1883, p. 634) note that some wheats 

 are less injured by rust than are others. Bolley (1889, p. 16) observes 

 that those varieties least susceptible to rust are "hardy, stiff-strawed 

 wheats, having smooth, fibrous leaves." Anderson (1890, p. 84) says 

 that hard, flinty wheats are more rust-resistant than others. He thinks 

 it may be due to the large amount of silica in the hard wheats. Cobb 

 (1892) advanced his "mechanical theory" to explain resistance. It 

 was due, according to his idea, to morphological characters of the 

 host, namely, thick cuticle, waxy coating, and small stomata. Hitch- 

 cock and Carleton (1893, p. 12) also correlated resistance with morpho- 

 logical characters, asserting that resistant forms had stiff, upright 

 leaves with a thick epidermis. Eriksson (1895, p. 199) and many 

 others since have shown, however, that a wheat resistant to one spe- 

 cies of rust is not necessarily resistant to another species, thus indicat- 

 ing a rather delicate relationship as the basis of resistance. 



Eriksson and Henning (1896, pp. 332-365) were unable to sub- 

 stantiate Cobb's mechanical theory, since morphological characters 



