INTRODUCTION 17 



with minute fruit bodies. It is the largest of the three major 

 subdivisions of the fungi, and contains about twenty orders. 



In the Basidiomycetes the thalhis exists as a well developed 

 mycelium. The individual hyphae often lie close together and 

 run more or less in parallel forming heavy strands or cords termed 

 rhizomorphs. The fruit body usually results from differentiation 

 at the tip of a rhizomorph. The hyphae are septate and are 

 often provided at the septa with peculiar structures termed 

 clamp connections. The clamp is of the nature of a curved 

 lateral branch connecting the two cells otherwise separated by 

 the septum, and is probably homologous with the ascus hook 

 developed in crozier formation in the Ascomycetes. The 

 Basidiomycetes are distinguished by the basidium, an organ 

 similar to the ascus and probably homologous with it, but differ- 

 ing in that the spores are formed exogenously (outside) rather 

 than endogenously (inside). The sexual nuclei fuse in the 

 basidium and reduction occurs there. The resulting nuclei 

 pass out of the basidium through minute sterigmata into vesicular 

 enlargements at their tips, which then mature into spores 

 (basidiospores) and are ejected into the air. Usually four 

 spores are formed. The basidia usually arise in or on special- 

 ized fruit bodies and commonly stand side by side forming a 

 paUsade layer termed the hymenium. Asexual reproduction 

 by means of conidia of various types commonly occurs. The 

 group may be divided roughly into smuts, rusts, Hymenomycetes 

 and Gastromycetes. It contains the mushrooms, bracket fungi, 

 coral fungi, gelatinous fungi, stink-horns, puff-balls, bird's nest 

 fungi and other prominent forms. It is a large group but has 

 been more thoroughly studied than have the Ascomycetes. 

 Sexual organs, present in many Phycomycetes and numerous 

 Ascomycetes, are apparently absent in the Basidiomycetes. The 

 group is characterized by a distinct alternation of generations 

 in which a mycehum composed of uninucleate cells alternates 

 with mycehum in which the cells are binucleate. The same 

 type of cycle somewhat less sharply marked occurs in the 

 Ascomycetes. 



For a more detailed treatment of the Ascomycetes and Basidio- 

 mycetes, especially with regard to the problems of comparative 

 morphology and cytology, the student is referred to the recently 

 published books of Gwynne-Vaughan (1922) and Gaumann 

 and Dodge (1928). The taxonomic hterature is extensive and 



