Asci (s. ascus) are the spore-containing structures of a fungus found in the hymenium.  In lichens, the mycobiont  produces spores through meiosis, which are housed in an ascus.  A single ascus represents a single meiotic event followed by a mitotic event.  Therefore each spore within an ascus results from the same mother cell after it undergoes meiosis, making ascospores useful in the study of fungal genetics.

Asci are visible in the cross section of an apothecium from Peltigera leucophlebia. Paraphyses (sterile filaments) are also visible. (Nomarski/DIC micrograph A. Fischer 3/2011)