402 BROOKLYN BOTANIC GARDEN MEMOIRS 



influence in a striking way the capacity of a parasite for infecting 

 hosts. 



The extent of the stability of speciaHzed races must vary greatly 

 in different cases. Some of the forms above considered are doubtless 

 as fixed and constant as many parasites which show structural differ- 

 ences. Mention may be made of the races of Melampsora Tremidae 

 as illustrating a good case of physiological species. In studying the 

 races of Erysiphe graminis one also gets a strong impression of their 

 constancy and definiteness and they seem as real as though separable 

 by structural features. 



In other cases, however, this is not true. One is not impressed 

 with the definiteness of races in Puccinia Helianthi, P. Hieracii and 

 others. These are not characterized by well-defined host limitations. 

 Perhaps one is justified in distinguishing different types of specializa- 

 tion as physiological species, races, strains, etc. 



But little data are available for comparing the specialization of 

 the same fungus in widely separated localities. The specialization of 

 Puccinia graminis is apparently the same in Sweden and Russia but 

 it has taken a quite different course in North America. Treboux 

 (159, 160) finds a different condition in Puccinia coronata in southern 

 Russia from what Eriksson (37, 42) finds in Sweden and Miihlethaler 

 (102) finds in Switzerland. Carleton's (26) results in the United 

 States also diverge widely from those of Eriksson. Arthur (11, 13) 

 does not find evidence for specialization in Puccinia Ribesii-Caricis 

 in this country, while Klebahn (87, 91) reports several fairly well- 

 defined races of this rust in Europe. On the other hand, there seems 

 to be no essential difference in the specialization of Erysiphe graminis 

 in Europe and North America. The same sharp host limitation seems 

 to occur in both countries. Further data are necessary before we are 

 able to determine the relation between the specialization of parasites 

 in different regions. 



In a few cases the races are also characterized by minor structural 

 differences. Freeman and Johnson (57) and later Stakman and 

 Piemeisel (149) have noted variations in the size and shape of the 

 uredospores which distinguish the races of Puccinia, graminis. Kle- 

 bahn (87, 91) has noted similar differences in the spores of the various 

 races of Puccinia Absinthi, Melampsora Larici-epitea and others. 



Fischer (46, 47), as well as others, has suggested that physiological 

 specialization is a starting point for the origination of forms distinct 

 on structural grounds. The suggestion is plausible, for it is possible 

 to arrange a series of forms ranging from races differing in host rela- 

 tions through all stages to others showing constant structural differ- 

 ences. 



