Mushroom. Light micrograph (LM) of a section through a mushroom, Agaricus sp. (formerly Psalliota sp.), showing its stalk (orange) and gills (long white projections). The margins of the gills (black) constitute the hymenial layer, or hymenium, which produces the spore-bearing structures known as basidia. Many spores (basidiospores) lie at the surface of this layer or between the gills (grey).