Chapter 7 

 THE BASIDIOAiYCETES 



The Basidiomycetes, a group of 20,000 to 25,000 species, in- 

 clude the fungi that bear their spores exogenously on basidia. 

 The basidia may occur singly or be variously arranged on hy- 

 menia. On the basis of the shape and structure of basidia and 

 of their arranofement, the Basidiomycetes are divided into two 

 groupings, the Heterobasidiomycetes and Homobasidiomycetes. 

 The Heterobasidiomycetes include the fungi commonly known 

 as smuts, rusts, and jelly fungi, and the Homobasidiomycetes all 

 others, including the gill, pore, leathery, and coral fungi, puff- 

 balls, stinkhorns, and bird's-nest fungi. Essentially all the Hetero- 

 basidiomycetes are gymnocarpous; that is, their basidia arise on the 

 free surface of the fructification. The Homobasidiomycetes are 

 further commonly divided into two subclasses, Hymenomycetes 

 and Gastromycetes. The Gastromycetes include those fungi 

 whose hymenia are formed within the fructification; hence they 

 are spoken of as angiocarpous. Transitional conditions of gym- 

 nocarpy and hemiangiocarpy exist among the Hymenomycetes. 

 Among pileate species the sporiferous layer is generally differ- 

 entiated on the interior of tuberous or "button-like" fundaments, 

 and the hymenium is eventually exposed by expansion and rup- 

 ture. These fungi are spoken of as hemiangiocarpous. Among 

 leathery funo-i and pore fungri crenerally, the basidia from their 

 beginning comprise a free hymenial layer. 



The basidium, the characteristic structural feature of the Basid- 

 iomycetes, is typically club-shaped and non-segmented and bears 

 exogenously a complement of four basidiospores. The shape 

 and structure of basidia, as will be pointed out, vary. Initially 

 they are the ends of hyphae composed of dicaryotic (two- 

 nucleate) cells. As each terminal cell enlarges, the two nuclei 

 enlarge also. These nuclei soon fuse to form a diploid fusion 

 nucleus, that is, one with 2x number of chromosomes, which 



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