22 



STRUCTURE AND CLASSIFICATION OF FUNGI 



of mycelium send out bud-like processes. Sometimes one of these 

 is larger than the other, i.e., there may be a differentiation into an 

 antheridium and oogonium. These unite, their nuclei fuse, and spores 

 are formed immediately as in some of the yeasts. Various stages are 

 shown in Fig. 13. 



In most of the Ascomycetes, the Euascomycetes, however, the proc- 

 ess is not so simple. After fertilization of the oogonium by the 

 antheridium, the nuclei do not fuse at once and the resulting cell 

 does not at once develop ascospores, but instead gives rise to new 

 filaments of mycelium, the ascogenous hyphae, with paired nuclei 

 which divide conjugately. In many of the Euascomycetes the 

 oogonium is fertilized by an antheridium from the same thallus. 

 They are homothallic and heterogamous. The nuclei do not unite 

 at once but divide separately and occur in pairs, one nucleus of each 

 pair derived from the oogonium, one from the antheridium. These 

 ascogenous hypha is often at first without crosswalls but eventually 

 septa are formed across at least the end cells. These form their 

 sexual spores and in some species proliferate in a manner which has 

 many analogies with the conjugate nuclear division and basidiospore 

 formation of the Basidiomycetes. 



a 



O 



O 



HJH 



Fig. 14. Crozier and ascospore production in Ascomycetes. Diagrammatic. 

 Explanation in text. Drawing by Hazel Jean Henrici. 



