2 98 Ward. — On Relations between Host and Parasite 
latter regain their vigour, and thus the infection becomes 
abortive. 
If the Uredospores germinate on a leaf of some other 
species of Br omits than the one which bore them,— e.g., if 
spores from B. mollis germinate on leaves of B. sterilis — the 
infection may be rendered abortive, owing to the non-adapta- 
tion of the parasite to the host, in other ways. Thus the 
infecting tube may attack the stomata and underlying cells too 
vigorously, as it were. In this case the cells first attacked 
die, and the dead tissues are so unsuitable as pabulum for 
the incipient mycelium, that the latter dies off from starvation, 
a corroded patch showing where the injury is localized. This 
appears to be a common event when the tips of the young 
leaves of B. sterilis , B. madritensis , or B. maximus are invaded 
by vigorous spores derived from B. mollis. 
In such cases the non-adaptation of the parasite to its new 
host is shown, not by any active resistance of the latter, but 
by the too vigorous onslaught of the former producing such 
disastrous results that the mycelium cannot advance through 
the ruined tissues its ill-timed ravages have brought about. 
In sections of such, the short stumpy mycelium, with fatty 
contents suggesting a starved condition, may be observed 
among the dead and dying cells of the leaf-tissue, evidently 
unable to grow. 
Yet another case of non-adaptation of the parasite appears 
to occur when the Uredospores from one species attack a 
species unsuitable to them, e.g., when spores from B. mollis 
germinate on the leaves of B. inermis or of B. erectus , &c. 
In these cases it seems that the infecting tubes reach and 
penetrate the stomata, but are unable to form a mycelium in 
the tissues, evidently because the living cells are really resistant. 
How the latter prevent the hyphae from putting haustoria 
into their cell-cavities is not yet clear, but it looks as if they 
exerted some deleterious influence on the delicate tips and 
thus brought their efforts to abortion. 
It will be obvious that the foregoing explanation of the 
phenomena differs entirely from Eriksson’s My coplasm hypo- 
