302 THE LOWER FUNGI— PHYCOMYCETES 



are not homologous structures. The ascus is essentially sexual 

 in character. In the young condition it is binucleate. The 

 nuclei fuse, and the fusion nucleus, thus formed, undergoes 

 three mitoses resulting in eight nuclei. At the close of the last 

 mitosis the astral rays cut out the ascospores in the interior 

 of the cytoplasm, the process being one of free-oell formation 

 and thus differing wholly from that of progressive cytoplasmic 

 cleavage by means of which sporangiospores are formed. In 

 some Ascomycetes additional mitoses occur and the mature 

 ascus is polysporic. The number of ascospores, even in such 

 cases, may be definite, and is never wholly indefinite in the com- 

 plete sense in which this is true of sporangiospores. The cyto- 

 plasm not incorporated in the ascospores is termed epiplasm, 

 and functions in many species in maintaining turgor and in 

 effecting spore discharge. Epiplasm is wholly lacking in the 

 sporangium, though in a few forms a mucilaginous secretion lies 

 between the spores and gives a similar aspect. 



Though the ascus, strictly speaking, is not homologous with 

 any structure in the Phycomycetes, and, in its typical form in 

 the higher Ascomycetes, is clearly a new development, it gives 

 evidence of having been derived from the sexual apparatus 

 of the Phycomycetes rather than from the sporangium. The 

 sporangium may be conceived to have given rise to the conidium 

 in the Ascomycetes and Basidiomycetes as well as in the higher 

 Phycomycetes. The Ascomycetes include a very large number 

 of fungi, and in certain features of morphology are extremely 

 various. In the standard classification presented by Schroter 

 (1894) the group is split into the three primary subdivisions, 

 Hemiascomycetes, Ptotoascomycetes and Euascomycetes. 



The group Euascomycetes is much the largest of the three, and 

 includes all the highly developed forms. In practically all of its 

 members, asci are aggregated in definite fashion in a more or less 

 conspicuous fruit body termed the ascocarp. In one of the major 

 subdivisions of the Euascomycetes, known as the Piscomycetes, 

 the ascocarp is a typically open, cup-shaped or saucer-shaped 

 body lined with a palisade layer of asci, and receives the special 

 name apothecium. In another large group, the Pyrenomycetes, 

 it is a typically closed, spherical or flask-shaped structure 

 enclosing the asci, and is termed the perithecium. 



The Protoascomycetcs as treated by Schroter include the 

 Saccharomycetaceae (yeasts), Endomycetaceae, Exoascaceae (leaf 



