LIFE HISTORIES OF RUSTS. 15 



may really not be identical, since they are not interchangeable from 

 one host to the other. For instance, the leaf rusts of wheat and rye 

 can not be distinguished from each other under the best microscope 

 lenses of the present day ; yet the leaf rust of rye can not ordinarily 

 be transferred by inoculation from rye to wheat and probably is not 

 so transferred in nature. In other words, one finds here two fungi 

 exactly similar in morphological characteristics, but physiologically 

 different. These have been variously designated, 1 probably the most 

 convenient and expressive term being "biologic forms." It is seen 

 at a glance that the biologic forms may complicate very greatly the 

 rust life history. They offer great difficulties to the investigator of 

 rusts and, at the same time, are the basis for a most promising field 

 of work of much importance, viz, the study of rust-resistant varieties. 

 A further complication arises from the- facts obtained through 

 experiments in various countries, which have shown that what is 

 apparently the same species (the host being morphologically the 

 same) may consist of a large number of strains or varieties which 

 may behave differently in different geographic areas. The stem 

 rusts of wheat and barley, for instance, are very similar, interchanging 

 hosts easily and being capable of transfer to various grasses in this 

 country. (See pp. 17-21; also Carleton, 30, pp. 54-56.) The 

 researches of Eriksson (41, p. 70; 40, p. 294; 42, p. 500; 46, p. 198) 

 show that in Sweden the stem rust of wheat goes with difficulty to 

 barley and rye, while the stem rusts of barley and rye interchange 

 hosts very easily. The chief factors in the complexity of the life 

 history of cereal rusts may be summarized thus: (1) Alternation of 

 hosts for different spore forms; that is, between the barberries and 

 grasses. (2) The restrictions of different biologic forms of a single 

 species of rust to various definite groups of host plants; as, for 

 instance, Puccinia graminis avenae on oats; also found on Dactylis, 

 etc., but not on wheat. (3) The variation of the biologic forms in 

 different geographic areas. 



The biologic forms of cereal rusts have been somewhat fully worked 

 out by Eriksson and Henning, Klebahn, and others for various 

 localities in Europe. The reader is referred to these authors for 

 more complete details (39 and 63). 



The forms in this country have received some attention, though 

 scarcely as much as those in Europe. Practically the only work 

 done in this line has been that of Hitchcock and Carleton (58) and 

 of Carleton (30 and 31). The results of the latter agree in the main 

 with the results recorded in this paper, but differ considerably 



1 Some of the terms used are Species sorores, Schroter (94, p. 69); biologische Spezies, Klebahn (62, pp. 

 232, 258); biologiske arter, Rostrup (88, p. 40); physiological species, Hitchcock and Carleton (58, p. 4); 

 formae speciales, Eriksson (40, p. 292); Gewohnheitsrassen, Magnus (69, p. 82); and biologiske rassen, Ros- 

 trup (89, p. 116). 

 216 



