248 Journal of Agricultural Research voi. xv, no. 4 



(11) Many attempts were made to increase the virulence of biologic 

 forms on resistant hosts by successive transfers to these hosts. P. gra- 

 minis triciti, P. graminis avenae, P. graminis phleipratensis , and P. gra- 

 minis agrostis were used. The results indicated that rust forms do not 

 gradually adapt themselves to resistant or semicongenial hosts. 



(12) Biologic forms seem to be roughly analogous to pure lines. Plus 

 and minus fiucutations may occur, but there is always a tendency to re- 

 turn to normal. 



(13) It is possible but not demonstrated that some biologic forms may 

 be mixtures from which "pure lines " can be isolated. P. graminis avenae 

 is a possibility. 



(14) The facts given in this paper do not support the conclusions of 

 previous workers that the pathogenicity of biologic forms is easily 

 changed by host influence. 



(15) From the practical standpoint the constancy of biologic forms is 

 of great importance. Breeding for rust resistance can proceed with con- 

 siderable assurance that the same rust will not adapt itself quickly to new 

 varieties. 



(16) Biologic forms may have arisen either by mutations or by gradual 

 process of evolution. These processes may be operative yet, but the 

 writers have not been able to detect any mutation nor to induce percepti- 

 ble evolutionary changes experimentally. The possible role of hybridiza- 

 tion will be investigated. 



LITERATURE CITED 

 (i) Arthur, J. C. 



1910. CTJLTURES OF UREDINEAE IN 1909. In Mycologia, v. 2, no. 5, p. 213-240. 



(2) BiFFEN, R. H. 



I912. STUDIES IN THE INHERITANCE OF DISEASE RESISTANCE. II. In Jour. 



Agr. Sci., V. 4, pt. 4, p. 421-429. 



(3) BoLLEY, H. L., and Pritchard, F. J. 



1906. RUST problems: facts, observations and theories; POSSIBLE MEANS 



OPCONTROL. N. Dak. Agr. Exp. Sta. Bui. 68, p. 607-672, 30 fig. 



(4) DiETEL, p. W. 



1887. BEITRAGE ZUR MORPHOLOGlE UND BIOLOGIE DER LTREDINEEN. In Bot. 



Centbl., Bd.32,No. 2, p. 54-56; No. 3, p. 84-91; No. 4, p. 118-121; No. 

 5, p. 152-156; No. 6, p. 182-186; No. 7, p. 217-220; No. 8, p. 246-250, 

 pi. I. 



(5) Eriksson, Jakob. 



1S94. UBER die specialisirung des parasitismus bei den getreiderostpil- 

 ZEN. ZwBer. Deut. Bot. Gesell.,Bd. 12, Heft9, p. 292-331. Litteratur- 

 Verzeichniss, p. 330-331. 



(6) 



1902 . UEBER DIE SPEZIALIZIERUNG DES GETREIDESCHWARZROSTES IN SCHWEDEN 



UND IN andEREN landERN. In Centbl. Bakt. [etc.], Abt. 2, Bd. 9, 

 No. 16, p. 590-607; No. 17/18, p. 654-658. 



(7) Evans, I. B. Pole. 



191 1. SOUTH AFRICAN CEREAL RUSTS, WITH OBSERVATIONS ON THE PROBLEM OP 



BREEDING RUST-RESISTANT WHEATS. In Jour. Agr. Sci., V. 4, pt. I, 

 p. 95-104. Bibliography, p. 104. 



