CHAPTER VI 

 BASIDIOMYCETES 



THE Basidiomycetes include over 13,000 species possessing a well-developed 

 mycelium, which, among the higher forms, builds up an elaborate fruit-body 

 such as may be observed in the toadstools, bracket fungi and puff balls. 



They are characterized by the fact that their principal spores, the basidio- 

 spores, are borne externally on the mother-cell or basidium. The young 

 basidium contains two nuclei ; these fuse, and the fusion nucleus divides twice, 

 providing the nuclei of the four spores ; each spore is formed at the end of 

 a stalk or sterigma through which the nucleus passes to enter the developing 

 spore. The two divisions in the basidium constitute a meiotic phase. In the 

 Autobasidiomycetes the basidium is without septa, and the spores, except 

 where some fail to develop, are regularly four in number for each basidium. 

 In the Protobasidiomycetes the basidium is divided into four cells, each of 

 which gives rise to a single spore ; the walls are transverse in the Uredinales 

 and Auriculariales, longitudinal or oblique in the Tremellales. In the Hemi- 

 basidiomycetes (Ustilaginales) septa may or may not be present in the 

 basidium, but the fusion nucleus divides more than twice, and more than 

 four spores are produced. 



In Puccinia, Phragmidium and other Uredinales, and in Sirobasidinvt 

 and its allies, the basidia are developed in chains, in other cases they are 

 borne singly. In the Ustilaginales and in the majority of the Uredinales the 

 nucleus and cytoplasm of the basidium are at first enclosed in a thick wall 

 formingthebrand-sporeorteleutospore cell, which becomes detached, form- 

 ing an additional means for the distribution of the plant; later the contents 

 are extruded as a thin-walled promycelium on which the basidiospores are 

 produced. In other Basidiomycetes the basidia are thin- walled throughout 

 their development and produce spores while still attached to the mycelium. 



The basidiospore is unicellular, round or oval in shape, usually with 

 a smooth, rather thin wall. Echinulate or warted spores occur in a few 

 species, and in many families, especially among gill-bearing fungi, dark or 

 bright-coloured spores are common. 



In a considerable number of genera accessory spores are also produced. 



Except for the production of their characteristic spores externally on 

 basidia, the Ustilaginales and Uredinales differ in almost every particular 

 from the majority of the Basidiomycetes; they are obligate parasites with 

 a delicate mycelium ramifying in the tissues of the host and they lack the 

 elaborate stroma characteristic of the Autobasidiomycetes. It has therefore 

 seemed advisable to deal with them as distinct groups, separating them for 

 purposes of description from the Basidiomycetes proper. 



