CHAPTER IX 

 ZYGOMYCETES 



The Zygomycetes form an isogamous series, parallel to the Oomycetes, 

 which has become adapted to terrestrial habitats. The thallus in the 

 simple forms is coenocytic like that of the Oomycetes, in the higher forms, 

 secondarily divided into cells. In several families, under certain condi- 

 tions, the hyphae may fragment into oidia, hyphal bodies, etc., which 

 develop further as sprout mycelia. Under unfavorable conditions, 

 small portions of hyphae are thickened to gemmae. 



The fructifications are aerial. The haploid portion produces sporan- 

 gia, conidia and gametangia; the diploid, zygospores. The sporangia and 

 conidia are each characteristic for a special developmental series. As in 

 the higher Oomycetes, zoospore formation is absent. Instead the spo- 

 rangia form non-motile, endogenous sporangiospores, while the conidia 

 germinate by a single germ tube. While the conidia remain equivalent 

 to each other, the sporangia, even within a single family, undergo funda- 

 mental changes and in various ways are reduced to conidia. 



In both sporangia and gametangia, the individual daughter cells are 

 no longer motile and the gametangia fuse, like those of the Oomycetes, 

 in the coenocytic condition. The sexual act is essentially isogamous, in 

 spite of heterothallism. The higher Zygomycetes tend more and more 

 toward heterogamy. Within the gametangia in some Zygomycetes (as 

 in the higher Oomycetes) there appears a tendency to privilege a few 

 gamete nuclei; only in typical cases the division between functional and 

 supernumerary nuclei occurs before the demarcation of the gametangia, 

 so that the supernumerary nuclei may still migrate, while in the Oomy- 

 cetes they are differentiated after the delimitation of the gametangia and 

 then degenerate within them. 



The products of the sexual act, the zygospores, are essentially different 

 from the oospores of the Oomycetes in that their wall is itself formed by 

 the gametangial wall while the walls of the oospores are new structures. 

 In the higher Zygomycetes, especially in the Endogonaceae, between 

 plasmogamy and fertilization (as in the Peronosporaceae), a short 

 dicaryophase is inserted. In the Zygomycetes the development pro- 

 ceeds further and removes caryogamy both in time and place; for the 

 first time in the fungi, caryogamy no longer occurs in the place designed 

 for it (in the gametangia), but is retarded and shifted to an outgrowth 

 of the female gametangium. 



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