isidium (noun, pl. isidia) – a small (mostly 0.5-1 mm) outgrowth of the thallus with a cortex, containing both mycobiont and photobiont (organized as in the thallus), serving as an asexual (somatic) dispersal unit (propagule); often granular, warty, pin- or finger-shaped, club-shaped, less often compressed and spathulate, squamose (scaly), or even shield-like ("peltate"); either simple or subdivided, sometimes elaborately branched and coralloid. The term isidium is often used rather broadly, to include all corticated propagules, even ones only externally similar, such as dactyls or pseudoisidia (see isidiomorphs and papillae). Related terms: columnar isidium, false isidium, fibril, sorediate isidium, isidiomorph, spine, spinule, modified thallus part, thallus outgrowth. This is:part of upper thallus surface. Kind of:upper surface structure.