304 THE BASfDIOMYCETES 



indefinitely in a yeast-like manner. Then the original sporidia or 

 the sprout cells may copulate directly or through copulation 

 tubes, with the result that the cells become binucleate. These 

 binucleate cells may sprout, each bud containing a conjugately 

 formed pair of nuclei. 



Kniep (1919), Bauch (1925), and Stakman and Christiansen 

 (1927) have shown that sporidia and sprout conidia arising from 

 the same promycelial cell do not copulate with each other. In- 

 stead the 4 promycelial nuclei are sexually differentiated in pairs; 

 hence their descendants are also sexually differentiated in pairs. 

 The fusion of 2 strains of opposite sex is known to be prerequisite 

 to the formation of chlamydospores, at least among certain 

 species, such as Ustilago violacea, U. levis, and U. zeae, and may 

 reasonably be presumed to be necessary for all species. More- 

 over, Stakman and Christiansen (1927) have shown that smut galls 

 are not formed on corn when this plant is inoculated with 1 sex- 

 ual strain of U. zeae. 



In some species of smuts the binucleate condition does not 

 arise by copulation of sporidia or of their progency. Rawitscher 

 (1912) observed that the hvphae of U. zeae within the host con- 

 sist of uninucleate cells and that plasmogamy takes place within 

 the sorus. In Cinctractia fiiojitagnei and Ustilago bromivora 

 [Bauch (1925)] copulation involves pairs of neighboring cells of 

 the promycelium, and either the wall of contiguous cells is re- 

 sorbed or else copulation tubes are formed between the cell pairs. 

 In Ustilago mida, U. hordei, and U. tritici the promycelium may 

 form sprout mycelia, and conjugation may involve hyphal cells 

 of either a single promycelium or of different promycelia [Ra- 

 witscher (1922)]. 



Although the 4-celled promycelium is normal among Ustilagi- 

 naceae, it is by no means constant. In Ustilago pamci-jnimenta- 

 cei the promycelium is 2-celled; in U, longissijna and Thecophora 

 deforntaiis a true promycelium is lacking, and in its stead a much- 

 branched hypha- is formed that bears 3-celled sprout conidia. 

 Fusion then involves either different sprouts or a pair of sexually 

 different cells of the same sprout. 



The studies by Hanna (1929) show that sexual behavior in 

 Ustilago zeae is controlled by 2 allelomorphic pairs of genes lo- 

 cated on separate pairs of chromosomes. If both pairs of chromo- 



