papilla (noun, pl. papillae) – a small (visible only with a magnifying lens), domelike to hemispherical, conical, pimple-like or nipple-like (sometimes wart-like) bump (process or protuberance); either (1) on the lower thallus surface of foliose lichens (e. g., some Nephroma spp.), or (2) on the surface of fruticose lichens (e. g., in Usnea, where they are globose or short-cylindrical, concolorous with the surface or paler at the tip, but distinguished from tubercles by a smaller size and an intact cortex, thus not apically opening with a pore and not becoming sorediate), or (3) on the upper surface of foliose lichens (e. g., in Melanelia, where papillae are usually tipped by a pseudocyphellium when young and usually develop into an isidium). Related terms: dimple. This is:part of thallus surface. Kind of:upper surface structure.