Isidium

thumb|A herbarium specimen of the lichen [[Leptogium cyanescens, magnified 40X, with lobule-shaped isidia]]

An isidium (plural: isidia) is a tiny, wart- or finger-like outgrowth on the thallus surface of certain lichen species. It is one of two principal types of vegetative reproductive structures in lichens, the other being soredia. Each isidium contains both fungal and algal partners and is wrapped in a thin protective layer (the ), distinguishing it from soredia, which lack this covering. While both function in vegetative reproduction, the heavier, corticate structure of isidia means they tend to establish in microhabitats close to the parent thallus, often favouring stable, humid niches where mechanical protection improves survival. Unlike spores, which are microscopic and easily carried over long distances by wind, isidia are larger, multicellular fragments that rely on external forces such as wind, rain, or animal contact, but typically disperse over much shorter ranges. Isidia are morphologically diverse, ranging from spherical and cylindrical to club-shaped or scale-like, typically measuring 0.01–0.03 mm in diameter and 0.5–3.0 mm in height, and may be smooth, knobby, shiny, , or hollow.