294 A TEXTBOOK OF THEORETICAL BOTANY 



Relationships 



The discovery of these Fungi naturally stimulated the view that the Fungi 

 as a whole were derived from the. Algae, for it was maintained, quite reason- 

 ably, that both the development of the female reproductive organ and also 

 the presence of non-motile spermatia were features remarkably similar to 

 those seen in many of the Rhodophyceae of the present day. Moreover, the 

 fact that similar female organs and also spermatia, though apparently not 

 functional, occur in certain Lichens, emphasized that the Laboulbeniales 

 were not alone in possessing these Algal-like characters. 



Despite these similarities, however, most mycologists now prefer to 

 consider that the Laboulbeniales have in reality been derived from simpler 

 Fungi. 



BASIDIOMYCETES 



The Basidiomycetes are Fungi consisting of a septate filamentous 

 mycelium which ramifies either through the host tissues, in the case of 

 parasitic species, or in the soil in many saprophytic ones, and produces 

 highly complex fruiting bodies. The club-shaped cells which bear the 

 spores are called basidia, and these may be formed either over the whole 

 surface of the fruiting body or localized on certain special structures. From 

 the basidium four spores, basidiospores, are externally developed. These, 

 when they fall to the ground, form fresh mycelia. The mycelium which 

 grows from a spore is composed of uninucleated cells. When two mycelia 

 of different strains meet, a cell fusion takes place and a secondary mycelium 

 is formed consisting of binucleate cells. In many species fruiting bodies are 

 formed only on secondary mycelia, and all the cells of the fruiting body are 

 binucleate. Sexual union consists in the fusion of the two nuclei in the 

 basidium. This fusion is immediately followed by meiosis, so that the 

 basidiospores are monoploid and uninucleate. 



Asexual reproduction, where present, is by means of conidiospores 

 borne on the mycelium. 



Many of these Fungi are of economic importance in causing plant diseases. 

 Many form mycorrhiza with the roots of trees, a factor which influences 

 the ecological distribution of both partners in the association (see 

 Volume IV). 



The Basidiomycetes are divided into a number of orders, of which we 

 shall consider examples of the following : — 



1. Urediniales {e.g., Ptdccinia). 



2. Ustilaginales {e.g., Ustilago). 



3. Aphyllophorales {e.g., Polyponis). 



4. Agaricales {e.g., Psalliota). 



5. Gast^romycetales {e.g., Cyathus, Phallus). 



