ORDER UREDINALES (tHE RUSTS) 



389 



oiPuccinia graminis on wheat, Waterhouse (1929), in Australia, obtained 

 two hitherto unknown forms which are probably to be interpreted as 

 crosses. Similar experiments by Miss Newton (1930) and others in Canada 

 and by Stakman (1930) and collaborators in Minnesota resulted not only 

 in interracial crosses but in crosses between P. graminis tritici and P. 

 graminis secalis and between the former and P. graminis agrostidis. In 

 the course of the various crosses several new physiological races have been 

 produced. The study of the results reveals that many of these races must 

 be heterozygous. This was further demonstrated by the production of 

 selfed strains. Thus physiologic race #17 when selfed (i.e., fertilized by 

 sperm cells of the same race but of opposite sexual phase) produced 

 aeciospores which gave rise to #17 and seven others besides, while race #53 

 gave besides itself seventeen other races. On the other hand race #9 

 proved to be homozygous. 



Fig. 129. Subclass Teliosporeae, Order Uredinales. 

 Receptive hypha with attached sperm, just below 

 mouth of stoma. (Courtesy, Andrus: /. Wash. Acad. 

 Sci., 23(12) :544-557.) 



Andrus (1931, 1933) demonstrated for the rusts Uromyces phaseoli 

 typica Arth. and U. phaseoli vignae (Barcl.) Arth, that there are certain 

 elongated filaments, which he called "trichogynes," whose tips emerge 

 from the stomata or from between epidermal cells and to which the sperm 

 cells adhere. The nucleus of the sperm enters the trichogyne and passes 

 down through it. Only then do the multinucleate cells appear in the aecial 

 primordium. Miss Allen (1932a) demonstrated the occurrence of similar 

 receptive hyphae in Puccinia triticina and other species of rusts and J. L. 

 Forsberg (in an unpublished thesis, 1932) has shown their presence in 

 Kunkelia nitens (Schw.) Arth. and Gymnoconia peckiana (Howe) Trotter, 

 but not in the monocaryon race of the former in which functional spermo- 

 gonia are not produced. Miss Rice (1933) showed that in a number of 

 other rusts similar structures are formed. According to Miss Allen (1932a) 

 the receptive cells, into which the one or more (up to ten or so) sperm 

 nuclei pass, elongate and branch and form a mycelium which penetrates 



