S. F. Armstrong 59 



protoplasm to overcome the resistance of the cells of the host by means 

 of enzymes or toxins, and reciprocally, on that of the protoplasm of the 

 cells of the host to form an ti- bodies which destroy such enzymes or 

 toxins "(19). In another place Ward (18) was careful to point out that, 

 " The failure to find any structural or mechanical explanation of the 

 phenomenon (in the sense here imphed) does not necessarily involve the 

 assumption that there is no mechanism in the hving plant which is 

 answerable for the obstruction, or aid, to infection exhibited by the 

 species. It only points to the conclusion that the mechanism is of that 

 more refined and subtle nature which determines such fundamental 

 properties as specific relationship, variation, heredity, and other bio- 

 logical phenomena." 



These quotations from Ward's papers have been made because, in 

 the first place, they represent the final con-elusions of an investigator 

 whose work in this direction has never been surpassed; and further, if 

 these conclusions are correct it is reasonable to expect that such features 

 as immunity, etc., should be subject to the general laws of heredity. 



Ward (20, p. 37) also inoculated the fohage of Rivet wheat — which is 

 only shghtly susceptible to Yellow Rust — with the uredospores of that 

 fungus; at the same time he inoculated a very susceptible variety (Red 

 King), and hybrid plants obtained from a cross between these varieties. 

 Microscopical examination of serial sections showed that inoculation 

 and infection very readily occurred in the very susceptible parent and 

 the hybrids. In the case of the resistant variety (Rivet) it was found 

 that the uredospores germinated, and sent their germ tubes into the 

 stomata as frequently, or nearly so, as they did into the more susceptible 

 wheats. Up to the fourth or fifth day after entry the course of events 

 was also similar, but invariably after this period a process of complete 

 degeneration was observed in the hyphae, and the parasite failed to 

 establish itself. The host cells in the immediate neighbourhood of such 

 hyphae were generally collapsed and their contents disintegrated. From 

 a comparison with experimentally starved hyphae, Ward concluded that 

 the hyphae in this resistant wheat were undergoing death-changes either 

 as the result of starvation or poisoning. 



These observations have been confirmed and extended by other 

 workers. Miss Gibson (7) showed that the mere inoculation of the wrong 

 host plants by the uredospores is quite a common occurrence among the 

 Uredineae. She found that in such cases the germ tube enters the stoma 

 as in a normal infection, but that after a period of about four days or 

 less the resulting hypha becomes exhausted and dies. Her work emphasized 



