198 BARTHOLOMEW: HYALOPSORA POLYPODII 
of Abies and Pinus with Hyalopsora, future workers will probably 
find the aecial host of Hyalopsora as has been done for different 
species of Uredinopsis. 
An explanation for the production of two kinds of uredinio- 
spores at the present time may be at least suggested.* Of the 
FIG. 3. Fragments of binucleate hyphae taken from the base of a sorus in which 
most of the spores were thin-walled, X about 680. Camera lucida drawing. 
groups of plants which now serve as hosts to the various rusts, the 
pteridophytes are the oldest. This gives some ground for as- 
suming that the fern rusts are the most ancient of the rusts now 
living. Following the pteridophytes came the gymnosperms, a? 
it is upon the conifers of this group that, as far as is now known, 
the aecial stage of the fern rusts appears. We may suppose, then, 
that some of the conifers became susceptible to infection by one 
stage of the rusts already parasitic upon the pteridophytes. It 
follows that one of the present spore forms of the fern rusts is @ 
* Thi 5 Pee wee age 
Ry - to the writer by Dr. J. J. Davis of this laboratory: 
