CHAPTER XI 



ASCOMYCETES 



The Ascomycetes, the largest class of the fungi, containing 

 approximately half of all the described species, have perhaps 

 one common characteristic, the ascus, or spore sac, generally 

 with a definite number of spores (ordinarily eight). The ascus 

 is of many types and may be produced in a variety of ways, 

 sometimes apparently in a mariner analogous to simple spo- 

 rangia ; again, it may be formed upon the surface of, or within, 

 more or less complex fruit bodies. The fruit bodies, in turn, may 

 be within or upon a modified mycelial tissue, termed a stroma. In 

 some cases the fruit body and asci are developed after cell and 

 nuclear fusion in special organs, phenomena indicating sexuality. 

 The size, form, and consistency of the fruit bodies are extremely 

 diverse, examples of these diversities being well borne in mind 

 by a comparison of the large edible morel with the minute fruit 

 bodies (perithecia) of the lilac mildew. 



Nonsexual, or conidial, spore forms of manifold variety are 

 known, and a single species may possess several of these forms. 

 In general, the mycelium is considerable, exposed or imbedded 

 in the substratum, septate, and relatively thick- walled. Some 

 orders contain only a few and others many parasitic species. 



As usually considered, the Ascomycetes include about ten orders 

 and more than sixty families. For convenience, two subdivisions, 

 with rather artificial limitations, may be recognized in this class 

 among those with definite fruit bodies, namely, (i) the Dis- 

 comycetes, in which the asci are produced in a body finally open- 

 ing more or less as a cup-shaped or saucer-shaped apothecium ; 

 and (2) the Pyrenomycetes, in which the asci are developed 

 within a perithecium, or an enveloping structure, which may be 

 entirely closed, or open by a relatively small mouth, the ostiolum. 



