314 Ward. — On Relations between Host and Parasite 
B. secalinus and B. velutinus with larger stomata, they refuse 
to infect B. inermis with equally large stomata, and only 
once (out of 74 trials) infected B. maximus with far larger 
ones. 
Or, again, if the small size of the stomata were the expla- 
nation of the immunity of B. unioloides and B. Schraderi , 
how are we to reconcile this with the only partial immunity 
of B. brizaeformis , where they are much smaller? 
Similarly, supposing it were suggested that B. macrostackys 
is as readily infected as B. secalinus by spores derived from 
the latter, because the stomata are approximately identical in 
size and numbers ; we have still to explain how it is that 
B. inermis , with equally large and numerous stomata, is 
immune, as is also B. maximus with much larger but fewer 
stomata. 
Or again, spores from B. sterilis readily infect B. madri- 
tensis , which has larger but fewer stomata than the former 
species. How is it that B. maximus is almost immune to 
these spores, although its stomata are far larger? Is it 
because they are also fewer? If so, it seems queer that B. 
tectorum should also have proved immune, because its stomata 
are more numerous than those of B. madritensis , and hardly 
smaller than those of B. sterilis. And the results are similar 
with the other factors. It is useless to reproduce all the 
curves of the various factors, and it may suffice that I have 
tried all sorts of combinations and can discover no relations 
between them and the curves of infection. We are hence 
driven to conclude that the factors which govern predisposi- 
tion on the one hand and immunity on the other, are similar 
to those which govern fertility and sterility of stigmas to 
pollen, and I have elsewhere 1 shown that parallels between 
the behaviour of pollen (which is after all a kind of spore) 
towards the stigma of the receptive plant in cross-breeding, 
and of these Uredospores towards their host-plants, multiply 
as we examine them. The importance of all this is, I think, 
that it justifies the hopes of those who believe that the con- 
1 Proc. Cambridge Phil. Soc., 1 . c., pp. 326-328. 
