350 BROOKLYN BOTANIC GARDEN MEMOIRS 



minute structural differences. The poplar rusts of the group Melamp- 

 sora Tremulae, with their uredo and teleuto stages on Popiilus alba 

 and P. tremida and their aecidial stage on such widely separated hosts 

 as Larix decidua, Piniis silvestris, MercuriaUs 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 Compo sitae- Car ex rusts under Puccinia ex- 

 tensicola Plowr. In a similar way Tranzschel (155) combines the 

 various Centaurea-Carex rusts under the name Puccinia Centaureae- 

 Caricis. 



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 Rusts^ — 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, ii), 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: 



