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.
Definition The hymenium is the tissue layer in which sexual spores are produced in many ascomycete and basidiomycete fruiting bodies. In a typical hymenium, the spore-producing cells are closely packed into a more or less palisade-like layer, often with interspersed sterile elements. The word derives from New Latin hymenium, from Ancient Greek ὑμένιον (hymenion), a diminutive of ὑμήν (hymēn, 'membrane'). Its earliest recorded use in English dates to 1830. The hymenium is distinct from the hymenophore, which is the larger supporting structure—such as a gill, tube, tooth or smooth surface—on which the hymenium sits. The hymenium is only the thin fertile cellular coating on that structure, not the structure itself. Not all spore-bearing fruiting bodies have a strongly organised hymenium. In gasteroid basidiomycetes, the fertile tissue develops internally as a gleba rather than remaining exposed, and in some closed ascomata the asci are scattered rather than arranged in a distinct hymenial palisade.
Structure and associated tissues
In Basidiomycota, the hymenium consists mainly of basidia, usually club-shaped cells that bear basidiospores externally on short projections called sterigmata. Many basidiomycete hymenia also contain basidioles (immature basidia) and sterile elements collectively known as cystidia; cystidia on the gill face are called pleurocystidia, and those on the gill edge cheilocystidia. The term "hymeniform" is also used for any palisade of club-shaped cells arranged like a hymenium, even when the cells are sterile. For example, the cap surface (pileipellis) of some agarics may be described as hymeniform because its cells resemble basidia in shape and arrangement, though they produce no spores. In Ascomycota, the fertile cells are asci, within which ascospores are formed internally, commonly eight to an ascus. The asci are often interspersed with paraphyses, sterile filaments that arise from the subhymenium and extend upward between the asci. In some groups, especially the Dothideomycetes, the interascal elements are instead pseudoparaphyses, which grow downward from above the locule rather than upward from below, or paraphysoids, which are attached at both ends. The sterile elements within an ascoma are collectively termed the hamathecium in specialist literature. Beneath the hymenium lies the subhymenium, a layer of supportive hyphae from which the hymenial cells arise (often called the hypothecium in lichenological usage). Below that is the sterile tissue of the supporting structure: in gills, tubes and teeth this is the trama; in many lichen apothecia the tissue enclosing the hymenium laterally is the exciple or excipulum. The organisation of the subhymenium and trama is itself taxonomically informative in many groups. The uppermost zone of many lichen hymenia is termed the epithecium, the region of paraphysis tips above the asci, which is often pigmented. In lichenological usage, epihymenium is sometimes treated as a synonym of epithecium, but some authors use it more narrowly for a thin layer of interwoven hyphae on the hymenium surface rather than the zone of paraphysis apices, so terminology varies between publications.
Occurrence and morphology
In agarics, the hymenium covers the vertical faces of the gills; in boletes and polypores it lines the inside of tubes that open as pores; in tooth fungi it coats spines or teeth; and in crust fungi it lies on a smooth or wrinkled surface directly applied to the substrate. In apothecial ascomycetes, such as cup fungi, the hymenium forms the exposed upper surface of the fruiting body. In perithecial ascomycetes it lines the inner wall of a flask-shaped cavity, and the spores are discharged through an ostiole. Closed ascomata may have a reduced or poorly differentiated hymenium, and in ascolocular forms the asci develop in locules within a stroma rather than in a conventional apothecial or perithecial layer. The same basic concept also applies in lichens, most of which are lichenised ascomycetes. Their apothecia or perithecia contain hymenia built from asci and sterile filaments, but the surrounding tissues and terminology are often described in more detail than in general mycology.
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