soredium (noun, pl. soredia) – a microscopic group of photobiont cells aggregated by loosely interwoven hyphae, always lacking a cortex or pseudocortex; erupting from cracks or pores in the thallus surface; appearing finely powdery to coarsely granular, and functioning as somatic (= vegetative) propagules; most often produced within localized masses (soralia) or covering large diffuse areas of the thallus surface (i. e., from an erose surface, not forming soralia); usually with a greenish tinge, but sometimes variously pigmented. Related terms: isidiose soredium, isidioid soredium, soralia. This is:part of upper thallus surface. Kind of:upper surface structure.