[newton] wheat stem RUST 197 



resistant to any one (and therefore presumably to all) of these six 

 strains must potentially efïect a tremendous reduction in the annual 

 losses from wheat rust. 



9. Genetic material bearing the necessary factors for rust resist- 

 ance is readily accessible to the plant breeder. Kanred, for example, 

 is immune to all of the 6 biologic forms predominating in the principal 

 wheat-growing areas. 



PART II 



The Development of the Parasite within the Tissues of 

 Resistant and Susceptible Hosts 



Historical Introduction 



Marshall Ward (63, 66) was the first investigator to carefully 

 work out, and accurately interpret, the intimate relationship between 

 the host and the rust parasite. The most important conclusion 

 arising out of his early work was that resistance has nothing to do 

 with anatomy, but depends entirely on the physiological reactions of 

 protoplasm of the fungus and of the cells of the host. Later (66) in 

 investigations which refuted the "mycoplasm hypothesis" of Eriksson, 

 he worked out for the first time the complete histology of the uredinial 

 cycle of a rust fungus (P. dispersa). 



Since his time considerable work has been done on the effects 

 of different rusts upon both congenial and uncongenial hosts. Miss 

 Gibson (26) inoculated a large number of unrelated plants with the 

 spores of Uredo chrysamthemi, as well as of other rusts, and found in 

 all these cases that the germ tube entered the stoma in the same way 

 as it did in a normal infection on the proper host of this fungus. 

 However, the after course of events was quite different. No haustoria 

 were formed, the hyphae appearing to die as soon as they came in 

 contact with a cell. Consequently no pustules were formed. The 

 failure of the fungus to produce haustoria was suggested to be due 

 to some poisonous or repellent substance emitted by the cells. The 

 power to form haustoria was, therefore, taken as an index of infection 

 capacity; because if the fungus cannot use the host-plant as food it 

 must shortly die of starvation. In the case of resistant varieties of 

 Chrysanthemum the germ tube entered and developed a mycelium 

 with haustoria, just as in the infection of a susceptible variety, but 

 the mycelium was unable to spread further, owing to the host tissue 

 in the neighbourhood having been killed. The author concluded that 



