isidium

Isidium (plural isidia) is a type of vegetative reproductive structure found on the thalli of many lichens. It appears as a small outgrowth on the surface of the lichen, often at the margins or on projections, and can be cylindrical, stalked, or flattened in form. An isidium typically contains fungal hyphae with embedded photobiont cells and is usually covered by a protective cortex on its exterior.

Isidia function as asexual propagules. When mature, they detach from the parent thallus and disperse, bringing both fungal and algal partners to new locations. This allows the lichen to colonize suitable substrates without relying on sexual reproduction. The production of isidia can be constant or triggered by environmental conditions such as light, moisture, or physical disturbance. Because their presence and shape are often distinctive, isidia are useful features in the identification and characterization of lichen species.

In contrast to isidia, soredia are another form of vegetative propagule that consists of loose aggregations of algal cells wrapped in fungal hyphae, usually lacking a surface cortex. Isidia and soredia therefore represent different strategies for lichen reproduction and dispersal. The term isidium denotes the singular form; isidia is the plural.