Hymenium

The hymenium is the fertile, spore-bearing tissue layer of many fungal fruiting bodies. It occurs in both Ascomycota and Basidiomycota, where it is made up chiefly of asci or basidia and often also contains sterile cells such as paraphyses or cystidia. The hymenium usually forms a palisade-like layer of cells extending outward from the supporting tissue on which spores are produced. In basidiomycete mushrooms and related fungi, the hymenium lines gills, pores, teeth or smooth surfaces; in ascomycetes it may cover the exposed surface of a cup-like fruiting body (apothecium) or line the inside of a flask-shaped one (perithecium). The arrangement of the hymenium was historically central to fungal classification, particularly in Fries's early nineteenth-century separation of fungi with exposed hymenia from those with enclosed spore-bearing tissues, although modern molecular phylogenetics has shown that many broad morphology-based groups evolved repeatedly rather than from a single common ancestor. Microscopic hymenial characters such as the form of cystidia, the structure of paraphyses, and the type of ascus or basidium remain widely used in identification and taxonomy. In lichenised fungi, especially apotheciate lichens, the hymenium is commonly described together with adjacent tissues such as the hypothecium and epithecium.