416 CLASS BASIDIOMYCETEAE 



logical phenomena of these strains within the host. Similar results were 

 obtained by Eddins (1929) in his studies of the same species. (Fig. 138B.) 

 Aside from the distribution of the two sexual phases in the promycelial 

 cells of Ustilago levis Dickinson studied the distribution of the factors for 

 color of the mycelium and for form of the colony when grown in culture. 

 These studies were made by isolating and culturing the sporidia, noting 

 their position on the promycelium. The smut used was a cross of strains 

 producing yellow and white mycelium and corrugated or depressed colo- 

 nies. Out of 22 such isolations the sexual strains A and B occurred in the 

 following order from the apex toward the base of the promycelium: 

 A ABB (3 times), BBAA (5 times), ABAB (6 times), BAB A (twice), 

 ABBA (3 times), and BAAB (3 times). The distribution of the other two 

 sets of allelomorphic factors was apparently entirely independent of the 

 distribution of the sexual strains and of each other. 



Hybridization has been brought about in cultures and also appears to 

 occur in nature. E. and J. Hirschhorn (1935) have shown that in Argentina 

 there may be present in the same smut gall on maize the three species 

 Ustilago zeae (Beckm.) Unger, U. fischeri Pass., and Sorosporium reilianum 

 (Kiihn) McAlpine, and that there occur crosses between all three, so that 

 all types of intermediate forms may occur together. Only very rarely are 

 U. zeae and S. reilianum found alone in the pure state in Argentina. 

 Fischer and Holton (1941) report the result of crosses between U. avenae 

 (Pers.) Jens, on oats (Avena saliva L.) and U. perennans Rostr. on Ar- 

 rhenatherum elatius (L.) Mert. & Koch. The race of the former that was 

 used had naked sori with indurated spore masses while the latter had 

 covered sori (i.e., the glumes were not entirely destroyed by the fungus) 

 and powdery spores. The Fi grew on A. fatua L. but not on Arrhenath- 

 erum. The F2 generations grew on A. saliva but still not on the tall oat- 

 grass. It was demonstrated that the covered character is recessive to 

 naked and the indurate recessive to powdery. Other hj^brids between 

 species of Ustilaginaceae have been reported by Dickinson (1927-1928), 

 Hanna and Popp (1931), Kniep (1926), and others. 



The occurrence of geographic races has been studied intensively in 

 Ustilago longissima (Sow.) Tul., by Bauch (1930, 1931). He showed that 

 multiple allelomorphy occurs in both the A and B factors. 



That the Ustilaginaceae are degenerate forms seems to be indicated by 

 the fact that their types of sexual reproduction are not at all standardized. 

 In the majority of cases studied the teUospore upon germination produces 

 a four-celled promycelium, from each cell of which one or more sporidia 

 are produced, but this is not universal. In some species germ tubes take 

 the place of the sporidia. In Ustilago striiformis (West.) Niessl. several 

 races are recognized on different hosts (Davis, 1935, Fischer, 1940). 

 Fischer has shown that the teliospores of forma Hordei Fisch. germinate 



