86 
biologic forms of the Erysiphe almost as complete as it is in the 
case of the rusts of the Gramineae. The widely distributed 
species Eerysiphe graminis, for instance, has been shown to be 
an aggregate of distinct biologic forms. The form on wheat 
refuses to infect normally growing barley, that on barley 
(Hordeum sativum) will not infect AH. sylvaticum and so on 
and further they have proved to be as distinct from one another 
in the ascus stage as they are in the conidial stage. 
In this particular direction we have reached closely to the 
limits of human knowledge, and looking beyond we can see 
more or less clearly that a closer study of this subject from a 
physiological point of view, will give us some clue as to the con- 
ditions obtaining both in the host plants and the parasites which 
have led to this singular separation of biologic forms within 
single species. In this connection the discovery of “ bridging 
species” by Marshall Ward is of unusual interest. The expen- 
ments were carried out with a biologic form of P. dispersa 
occurring on various species of the genus Bromus, and resulted 
in the proof that the fungus might infect a host plant A but 
be incapable of infecting a second species B, yet B might be 
infected by spores of the same fungus produced on a: third 
species C from a sowing of spores from A. The species C then 
acts, from the point of view of the fungus as a bridge between 
A. and B., enabling it indirectly to bring about infection where 
direct infection is impossible. Following this Salmon has 
proved that bridging species exist, again amongst the Bromes 
which permit of the indirect passage of a biologic form of 
Erysiphe graminis from one host to another where direct in- 
fection is impossible. “The spreading of parasitic fungi is not so 
sharply restricted then between certain host-plants as at first 
sight it appeared to be. These results point to the fact that 
some change in the constitution of the parasite itself is brought 
about solely by its temporary sojourn on a different host plant 
since then it can bring about infection where it could not before. 
How this change is effected or what it really 1s we cannot say, 
for we know nothing of the factors determining constitution i 
either host or parasite. Further than this it has been shown by 
Salmon that under certain conditions a given biologic form 
can be induced to infect hosts normally perfectly immune to Its 
attacks. This has been accomplished by lowering the vitality 
of groups of cells in the leaves of the plant by such drastic 
methods as bruising, cutting, burning, treating with anaesthetics 
and so on. After such treatment the injured portions of the 
leaves of barley become slightly susceptible to the attacks of 
Erysiphe graminis growing on wheat, in spite of the fact that 
the fungi on wheat and barley belong to sharply distinct biologi 
forms. The fungus is capable of getting such a hold on the 
