jtmeis, I9I5 Pticcinia Graminis 197 



injury, and kill those at some distance from the original point of infection. 

 In a few cases it was observed that hyphae grew from the upper to the 

 lower epidermis, killing a few cells about halfway between the two and 

 killing a number of them near the lower epidermis. Such cases are 

 rather exceptional; the death of the host cells usually follows promptly 

 after the hyphse reach the cells. Whereas in the case of normal infection 

 pustules with very abundant spore production are being formed within 

 about 7 to 12 days from the time of inoculation, in such cases as those 

 described above a few host cells have been killed and the fungus has 

 reached its limit of development within the same length of time. 



The hyphae do not grow much after the death of the cells. In some 

 cases they were found to be surrounded by dead cells as early as 3^-^ days 

 after inoculation, and they themselves showed distinct signs of extreme 

 unthriftiness — viz, large vacuoles alternating with coarsely granular 

 areas. Other hyphae appeared very much as do the older portions be- 

 neath an old pustule in cases of normal infection. These hyphae had 

 grown across the substomatal space and had killed all the mesophyll 

 cells bordering on it, but had not completely killed any deeper lying 

 cells, although some of those just beyond the border cells were some- 

 what affected. Under such circumstances it is conceivable that the 

 fungus may have died from lack of nourishment, since practically all 

 the food material stored in the spore had probably been used up in the 

 growth of the germ tube along the length of about 10 epidermal cells, 

 in the formation of the substomatal vesicle, and in the growth of the 

 numerous infection threads across the substomatal space. It seems 

 quite possible that the fungus, having exhausted the supply of nutrients 

 stored in the spore, precluded the possibility of its further growth by 

 killing very quickly the first cells with which it came in contact, thus 

 shutting off its only source of food material. 



The action is not always as rapid and sharp as in those cases just 

 described. Hyphae at the end of five days from the time of inoculation 

 have sometimes killed most of the cells in their immediate vicinity, but 

 still remain alive, although they are usually not vigorous. Only the 

 tips of the branches retain protoplasm, while the remainder of the 

 hyphae are completely vacuolated, with apparently no film of proto- 

 plasm next to the walls. The tips at this time usually are also vacuo- 

 lated very distinctly and show very definite signs of approaching death. 

 It seems clear, therefore, that whatever the intimate physiological rela- 

 tions between host and parasite, the death of the host cells is the direct 

 result of the presence of the hyphae, and that for some reason the hyphae 

 themselves succumb soon after. 



The essential fact is that the fungus gains entrance in the same man- 

 ner in susceptible and resistant forms, but acts differently thereafter. 

 In susceptible forms it grows vigorously without seriously affecting the 

 host cells for some time. In resistant forms, on the other hand, a very 



