Genus
Porpidia consists of
crustose lichens, meaning the
thallus (the main body of the lichen) is closely attached to the surface it grows on. The thallus can vary in appearance, from thick and scaly to almost invisible, and may be solid or cracked into small pieces called . Its colour typically ranges from grey to white, though some species may show orange hues. A , a thin, often black or orange border between neighbouring lichens, may be present but is not always visible. The lichen's inner structure, or
medulla, may or may not react with
iodine (I+ or I–), and some species produce powdery
soredia for reproduction, while others do not. One species of
Porpidia even has
isidia—small, finger-like structures that aid in reproduction. The lichen's
green algae partner is usually from the genera
Trebouxia or
Asterochloris. The
apothecia (fruiting bodies) are common and can appear either scattered or in clusters. These round, dark brown to black are often relatively large, ranging from 0.5 to 3 mm in diameter, and may be coated with a frosty layer (a ). The apothecia have a distinct outer margin, which may fade as the lichen ages, and are seated directly on the surface of the thallus without a (a rim formed by the thallus itself). The rim surrounding the apothecia is usually dark blue-black or brown, and made of tightly packed fungal filaments. Under the surface, the reproductive layer (the
hymenium) reacts with iodine (I+) to turn blue, a useful trait for identification. The
asci, the spore-producing sacs, each hold eight spores and have a distinctive shape called the "
Porpidia-type". The are typically
ellipsoid and measure 15–22 μm long, with a thick outer layer. In some species, other reproductive structures, known as
conidiomata, are present and often have a rough surface. ==Species==