CH.X] THE SAC FUNGI 449 



chogyne by a sperm cell. While this arrangement occurs 

 in some of the group, it is suppressed in others, in all degrees 

 down to the formation of the asci without any external trace 

 of fertilization or sexual structures. In the Basidiomycetes 

 the distinctive structure is a BASIDIUM, or enlarged end of 

 a hypha, on which are developed naked spores, typically 

 four in number. These basidia arc usually collected in large 

 parallel groups which form without any external evidence of 

 fertilization or sexual structures, though probably they are 

 homologous with the asci and were originally formed like 

 them. 



Although external structures and processes connected with 

 sexual reproduction are absent from many Ascomycetes and 

 most Basidiomycetes, there do occur within their cells certain 

 fusions of nuclei, and other processes, strongly suggesting a 

 form of sexuality ; and it may yet prove that practically all 

 of the Fungi do possess a sexual reproduction, physiologically 

 like that of other plants, though morphologically very 

 different. 



The homology of the methods of sexual reproduction in 

 some of the Ascomycetes with those in Red Algae suggests 

 that these Fungi also should exhibit an alternation of genera- 

 tions. This in fact is found to be true, and the ordinary 

 mycelium in the Ascomycetes and Basidiomycetes represents 

 the sexual generation, while the ascocarp, and its equivalents 

 in the various Fungi, represent the asexual generation. 



Some 19,000 species of Ascomycetes are known. They 

 are divided into several orders, the relationships of which are 

 somewhat doubtful, though in practice we may conveniently 

 treat them as follows : 



ORDER 1. DISCOMYCETES: THE CUP FUNGI. 



ORDER 2. PYRENOMYCETES: THE BLACK FUNGI. 



ORDERS. ERYSIPHE&: THE POWDERY MILDEWS. 



ORDER 4- PLECTASCALES : THE BLUE AND GREEN MOLDS. 



ORDERS. EXOASCI: THE NAKED-ASCUS FUNGI. 



ORDER 6. SACCHAROMYCETES : THE YEASTS. 



ORDER?. LICHEN ES: THE LICHENS. 



2G 



