Soredia are asexual “reproductive bodies” found in lichens. They are knots of hyphae containing a few algal cells. They are small granule-like or bud-like outgrowths on the upper surface of the thallus. Each soridium is formed of one to a few algal cells surrounded by closely arranged fungal hyphae produced by branching of a hypha from the algal region.
Isidia are small, finger-like or conical outgrowths developed on the upper surface of the thallus and are meant for increasing the surface area. Isidia develop into new plants when they detach. Isidia have the same fungal and algal components as those of the thallus. The isidia vary in form in different species of lichens. They maybe rod-shaped (Parmelia), cigar-shaped (Usnea), corolloid or coral-like (Peltigera) etc.
The more organized soredia occurring on localized area are called soralia. These are pustule-like and are seen as small white areas.