RUST-RESISTANT VARIETIES OF WHEAT 49 



used in such concentration as to permit of the normal development 

 of the host plants. 



9. A careful comparison of the sequence of infection in such 

 a susceptible form as Minnesota No. 163 with that in such an immune 

 form as Khapli reveals the fact that the fungus gains entrance in 

 the same manner in both cases. The rust mycelium is able to grow 

 luxuriantly in Minnesota No. 163 and produce spores in great abun- 

 dance. In Khapli, however, it does not thrive. The reason seems to 

 be a physiological incompatibility as evidenced by the killing of host 

 cells by the fungus and the more or less sudden death of the fungus 

 itself. Infection may occur and pustules may be developed, but it is 

 evident that the fungus is not in a congenial environment. The 

 conditions seem to be essentially similar when examination is made of 

 a cereal almost completely immune to a biologic form, such as Min- 

 nesota No. 163, inoculated with Puccinia graminis avenae. Here, 

 however, the host cells and rust hyphae are killed earlier and the leaf 

 area involved is consequently smaller. This, however, requires further 

 study. 



10. The question as to the immediate instruments of immunity 

 can probably only be answered by means of biochemical investigations. 

 In the meantime, morphological and histological characters being 

 clearly of minor importance in determining immunity, only the per- 

 formance of a supposedly resistant variety under varying conditions 

 can be depended on for a criterion of its value in this respect. 



ACKNOWLEDGMENTS 



The writer takes pleasure in making acknowledgment to E. C. 

 Johnson for suggestions and material, and especially to Dr. E. M. 

 Freeman, under whom the work was done, for many suggestions and 

 much criticism during the progress of the work. 



