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tholus (noun,ascus term, pl. tholi; synonym: apical dome) – the more or less thickened inner part of the ascus apex, often developing into a distinct cap-like structure and frequently functioning as an apical apparatus, i. e., ejecting spores during ascus dehiscence (active discharge). The tholus is typically differentiated into different ascus wall layers that frequently show distinct amyloid reactions with Lugol's or Meltzer's iodine. According to these reactions different ascus types can be distinguished. For example, asci may have a distinctly amyloid axial mass, sitting above a non-amyloid apical cushion; others have amyloid tholus flanks that surround a non-amyloid central cone; etc. The tholus may be penetrated by an axial canal or extend along the sides of an ocular chamber like a saddle (= bourrelet). Some asci almost entirely lack a distinct tholus (e. g., prototunicate, hypodermataceous, etc.). This is:part of ascus.

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