20 Ward. — Recent Researches on the Parasitism of Fungi. 
Having selected this Fungus for the work, the next thing was to become 
so familiar with its characters that it could always be distinguished from 
other morphologically distinct species. 
But on extending my collection of Bromes — I have now obtained and 
grown every species that could be got from all parts of the world — it soon 
became evident that nothing short of a revision of that genus would suffice 
to reduce the chaos inflicted on them by well-meaning authorities and 
amateurs, and I had carefully to work out each species and variety as 
occasion demanded. Only those who have acquaintance with the forms of 
Bromus erectus , B. secalinus and its allies, and such species as the South 
American B. unioloides and the North American B. ciliatus , will be able 
to appreciate the labour here involved ; but it was clearly useless to 
attempt any strict delimitation of the experiments without attempting 
this. 
One of the first general results was the demonstration that uredo- 
spores taken from B. mollis , of the group Serrafalcus, will not infect 
B.sterilis or B. erectus, species of the groups Festucoides and Stenobromus, 
or do so very rarely ; whereas they readily infect B. secalmus and its allies 
of the same group. Again, spores from B. sterilis (Stenobromus) refused 
to infect B. erectus (Festucoides) and B. mollis (Serrafalcus), but readily 
infect others of its own group. 
In short, the uredo on the species of Brome belonging to any one 
group are remarkably closely specialized to species of that group ; and this 
was borne out by Freeman, who repeated and extended these experi- 
ments. 
It was interesting to notice that we both found a few exceptions to the 
rule, of which more later on. 
Immunity and Susceptibility. 
Obviously the results above referred to on specialization of parasitism 
raise in a new form the old question of immunity and susceptibility, and 
the problem resolves itself into this : — Why is it that, of two closely allied 
species, or varieties, both equally exposed to infection, one will prove 
immune and the other susceptible ? 
The suggestion that the immunity depended on structure, and that 
immune varieties would be found to have thicker cell-walls, fewer or smaller 
stomata, more or longer hairs, waxy cuticles, &c., had been made from 
several sides, but I believe it was Cobb, in Australia, who first put this 
question to the test in 1890-3 ( 41 ), and he thought that a thicker cuticle, 
more stomata, and absence of waxy bloom, made for immunity. 
Eriksson was unable to confirm this ( 62 , pp. 355-63). 
