Isidia

Morphology

Isidia appear in various shapes, including spherical (globose), cylindrical, scale-like (squamulose), coral-like (coralloid), club-shaped (claviform), disc-shaped (disciform), cup-shaped (cupuliform), and wart-like (verruciform). Although they are always small (around 0.01–0.03 mm wide and up to 3 mm tall), their surface may be smooth or knobby, shiny or matt. Most isidia are solid, but some lichens feature hollow, inflated isidia. In some lichens, soralia—defined areas on the thallus where soredia are produced—and isidia can form in overlapping locations. Soralia may grow at the tips of isidia, and isidia can sometimes develop within a soralium, where the loose network becomes compact and forms a secondary cortex.