154 Wisconsin Academy of Sciences, Arts, and Letters. 
compressa bore only a few patches of mycelium; conidia were 
not produced abundantly. In the other experiment, however, 
there was a very good infection of blue grass. 
As seen from these experiments the mildew on P. pratensis 
will not readily infect the other species of Poa experimented 
with. Still under certain conditions it seems able to pass over 
to some extent to P. nemoralis, and, in a lesser degree, to P. 
Irivialis and P. compressa. It is possible that there is a “sub- 
infection”, as Salmon calls it, in these cases. The question of 
the purity of the seed and the identification of the seedlings is 
very important in connection with experiments on the various 
species of Poa. 
My observations indicate that the group is an interesting one 
and further experiments may be expected to throw light on the 
real nature of physiological species and their method of origin. 
The facts thus far brought out lead us to expect that among 
the mildews on the remaining grasses which have been reported 
as hosts many will be found which have become more or less 
physiologically specialized and limited in their power of pro¬ 
ducing infection. I have farther carried out a series of experi¬ 
ments with spores of rye and blue grass mildews to determine, if 
possible, whether these forms can pass over to any other grasses 
the seed of which I could obtain. All of these grasses are re¬ 
ported as hosts of Erysiphe graminis DC. The results of these 
experiments are summarized in the following tables and further 
notes on each grass are given below. 
