CHAPTER III 



PLECTOMYCETES 



The group Plectomycetes is constituted to include those relatively simple 

 forms which possess neither the cup-shaped apothecium of the Discomycetes, 

 nor the flask-shaped perithecium opening by a definite ostiole which 

 characterizes the Pyrenomycetes. In the majority of the remaining Asco- 

 mycetes a rounded ascocarp is produced, but it opens either by the decay 

 of its walls, or by an irregular split or tear. The asci may arise from the 

 floor of this fructification, and stand parallel one to another, or they may be 

 irregularly disposed, the fertile hyphae forming a tangled weft. In other 

 families the asci are naked; they stand parallel in the Exoascaceae, but in 

 these parasitic forms their position is probably determined by the fact that 

 they grow up between the epidermal cells or under the cuticle of the host, 

 and may be without phylogenetic significance. In the Endomycetaceae 

 they are irregularly disposed on the mycelium, and in the Saccharomyceta- 

 ceae a mycelium is not developed. 



In the majority of species the asci are club-shaped, pyriform or oval, 

 they arise indifferently from the terminal or intercalary cells of the fertile 

 hyphae, and the regular bending over of the tip of the ascogenous filament 

 characteristic of the Discomycetes and Pyrenomycetes is not found among 

 them^. The ascospores are usually continuous and hyaline; in the large 

 majority of cases the gametophytic mycelium gives rise to conidia. 



In the Exoascales the asci arise on a mycelium of binucleate cells and 

 the origin of the binucleate arrangement is unknown ; but in the other main 

 groups of Plectomycetes the form of the sexual organs has been recorded, 

 and a number of species show functional sexuality. 



In the Endomycetaceae and Saccharomycetaceae fusion takes place 

 between similar or nearly similar cells, a single fusion nucleus is formed in 

 the zygote and there gives rise to the nuclei of the ascospores. 



In the Erysiphaceae the mycelial cells and gametangia are uninucleate 

 the antheridium and oogonium differ in size and after fertilization the zygote 

 (oogonium) undergoes septation and one of its cells either becomes the 

 single ascus or branches and gives rise to several asci. 



In the Gymnoascaceae and Aspergillaceae the cells of the mycelium 

 and sexual branches are multinucleate, the oogonium is furnished with a 



1 The group corresponds, therefore, to Dangeard's Rectascees with the. addition of his Game- 

 tangiees and Choristogamet^es {Le Botaniste, 1907, p. 28). 



