296 



BOTANY: PRINCIPLES AND PROBLEMS 



single cell, possessing a definite nucleus, cytoplasm, and sap-cavity 

 and producing, by the process of "budding", a loose, irregular 

 chain of cells. When conditions become unfavorable, the con- 

 tents of some of the cells divides into four spores, forming a 

 structure which is thought to represent a modified ascus. The 

 yeasts are saprophytes on sugary substances, thriving in the 

 absence of free oxygen, and are the chief agents in alcoholic 

 fermentation, a process which has been described in a preceding 

 chapter. Their use in the raising of bread is familiar to everyone. 



Fig. 173. — Spore production in a basidiomycete. ^, cross section of a few of 

 the gills of a mushroom. B, a portion of the surface of one of the gills, much 

 enlarged. The stout cells are basidia. Each of them bears four basidiospores on 

 slender stalks or sterigmata. The lowest basidiuna has shed its spores. 



Basidiomycetes or Basidia Fungi. — Like the Ascomycetes, 

 this is a very large and varied group containing over 20,000 

 species. Its specialized reproductive structure, the basidium 

 (Fig. 173), is the swollen terminal cell of a hypha and typically 

 bears four basidiospores, each supported on a delicate stalk 

 or sterigma (plural sterigmata). The basidia are arranged in a 

 more or less definite hymenium. Aside from this difference in the 

 method of spore production, the group as a whole is distinguished 

 from the Ascomycetes by the almost complete absence of sexual 

 processes and by the larger and more conspicuous fruiting bodies. 

 The Basidiomycetes are regarded as the highest of the fungi 

 and are believed to have come from Ascomycetes, the basidium 

 representing a much modified ascus. 



A few orders, notably the Smuts (Ustilaginales) and the Rusts 

 (Uredinales) differ from the typical Basidiomycetes (or Auto- 

 basidioniycetes) in having basidia, or structures thought to repre- 



