CHAPTEE VIII 



ASCOMYCETES 



The Ascomycetes are those fungi in which meiosis occurs in characteristic 

 sporangia with endogenous spore formation. These sporangia are called asci 

 and their spores, ascospores. Their thallus is generally well developed; its 

 hyphae (in contrast to those of the Phycomycetes) are regularly divided by 

 septa into uni-, bi- or, rarely, multinucleate cells. Under certain environ- 

 mental conditions, they may continue growth by sprouting ; in the yeasts and 

 a few other forms, only the sprout mycelium is known. 



The imperfect forms reach the culmination of development in this group, 

 especially among the pathogens of the higher plants. Besides oidia, hypno- 

 spores, etc., the most varied types of conidia are found, often produced in 

 highly specialized organs which at times approach the perfect (sexual) forms 

 in complexity. In certain families, several imperfect forms may be produced 

 successively or even simultaneously in the same species, a condition usually 

 referred to as polymorphism. In case the perfect stage is unknown, these im- 

 perfect stages are given a name and classified among the Fungi Imperfecti. 



The sexual organs of the primitive groups with which we are concerned 

 more or less resemble those of the Phycomycetes, especially those of the Muco- 

 rales. In the most primitive family we have gametes differentiated and set 

 free to copulate in pairs. These produce a diploid ascogenous hypha. These 

 conditions approximate those in the Oomycetes, although the ascogenous 

 hypha and ascus seem to be a new development. Also in the Ascoideaceae we 

 have a proliferation of the gametangium or ascus which is suggestive of the 

 Oomycetes. Aside from these very primitive forms there are simple isogamous 

 or heterogamous copulation branches very much as we found in the Mucorales. 



In the higher groups, there is an extensive functional and morphologic 

 differentiation, the male being differentiated as an antheridium and the female 

 as an ascogonimn. A unicellular antheridium approaches a unicellular ascogo- 

 nium and is surrounded by the filamentous end of the ascogonium, known as 

 the trichogyne. In the Plectascales, the only group of interest to medical men, 

 there is not a great differentiation of trichogyne from the ascogonium. 



In most groups plasmogamy has lost its obligatory character and becomes 

 facultative. Morphologically this functional disturbance first affects only the 

 antheridia ; these disappear and amphimictic fertilization is replaced by many 

 deuterogamous processes. (For details, see Gaumann & Dodge 1928.) Grad- 

 ually this functional degeneration extends to the female organs which also 

 disappear in many groups. Eventually no sexual organ is formed and plas- 

 mogamy becomes pseudogamous. 



In conjunction with this degeneration, there is a shifting in the signifi- 

 cance of the sexual organs for the formation of fructifications. In the lower 



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