142 Wisconsin Academy of Sciences, Arts, and Letters. 
Various terms have been applied to these physiologically dif¬ 
ferent varieties, as “special forms,” “biologic forms,” “adapted 
races,” “biologic species,” “physiological species.” Some re¬ 
gard such specialized forms as incipient morphological species. 
It is possible that the specialized form of mildew adapted to 
growth upon wheat, but not on other grasses, is in course of 
becoming a species of Erysiphe which may eventually show 
morphological differences in addition to the physiological ones 
it already manifests. 
Salmon believes that the physiological species are restricted to 
certain host plants because of the presence of certain enzymes 
or toxins, or perhaps both, in the cells of the parasite and anti¬ 
toxins, or similar substances, in the cells of the host plant. 
Under normal conditions certain substances peculiar to each spe¬ 
cies of host plant are able to prevent the attacks of any mildew 
except the specialized form adapted to withstand these sub¬ 
stances. 'When the host plant is injured these substances are 
not formed by the cells in sufficient quantities to prevent the 
attack of other specialized forms of the fungus and accordingly 
infection results. 
It is difficult to see how the spores from an injured leaf, which 
has become infected by the special form from another host plant, 
are able to infect uninjured leaves of the same plant. The cells 
of the healthy plant, of course, form these substances which nor¬ 
mally inhibit the development of this special form of fungus. 
In one generation, however, the fungus becomes able to resist 
these substances and produce infection. The fungus must be 
extremely variable and must readily adapt itself to the sub¬ 
stances formed by the cells of the plant infected. It is evident 
also that the differences betvceen the various physiological spe¬ 
cies are very slight. They also probably differ among them¬ 
selves as to the degree of their specialization. 
Just to what extent injured plants may serve to bridge these 
specialized forms over from their own host plants to others must 
be determined much more fully. W 7 e shall probably find that 
the various specialized forms differ remarkably in this respect. 
The farther apart the host plants are in their characters the 
