350 BROOKLYN BOTANIC GARDEN MEMOIRS 
minute structural differences. The poplar rusts of the group Melamp- 
sora Tremulae, with their uredo and teleuto stages on Populus alba 
and P. tremula and their aecidial stage on such widely separated hosts 
as Larix decidua, Pinus silvestris, Mercurialis perennis and Chelidonium 
major, certainly differ from each other in a fundamental way. The 
difference between these is doubtless as significant as some of the 
minute structural differences which distinguish other species. 
It is, however, certainly important to recognize the fact that these 
rusts, mildews, etc., referred to as specialized races, physiological 
species, etc., can be distinguished only by cultural tests. This can 
most easily be done by grouping them together on the basis of struc- 
tural similarity. Klebahn (92) has done this recently in his scheme 
for illustrating the relationships of the willow and poplar rusts, and 
also in the case of the Ribes-Carex rusts. Arthur (10) adopts the same 
plan in combining the Compositae-Carex rusts under Puccinia ex- 
tensicola Plowr. In a similar way Tranzschel (155) combines the 
various Centaurea-Carex rusts under the name Puccinia Centaureae- 
Caricts. 
At the present time, a large number of the parasitic fungi have 
been investigated from the standpoint of specialization to particular 
hosts and the phenomenon has been found to be of wide occurrence. 
The present paper is an attempt to bring together the results of 
numerous investigations bearing on this point. 
THE RustTs—UREDINEAE 
Puccinia graminis Pers. Extensive studies have been made in 
both Europe and the United States on the specialization of the black 
stem rust of the cereals and other grasses. Eriksson (34, 37, 38, 41) 
in Sweden, Jaczewski (68) in Russia, Carleton (25, 26), Freeman and 
Johnson (57), Arthur (2, 5, 6, 7, 8, 10, 11), and Stakman and co- 
workers (143-149) in the United States, have reported the results of 
their cultural experiments with this rust. Experiments have been 
reported in which inoculation tests with both uredospores from the 
various grass hosts and aecidiospores from the barberry have been 
used. The general results of these experiments may best be sum- 
marized as follows: 
+e 
