Members of the Rhizocarpaceae are mostly rock-dwelling
crustose lichens—forming a thin, paint-like layer that adheres tightly to the
substrate—although some species grow as small, scale-like or as
rosettes whose lobed margins radiate from a central point. Their upper surface (the ) is usually present and may be smooth, cracked, or warted and display an array of hues from grey and green to yellow-green when
rhizocarpic acid is concentrated near the surface. A few species begin life on the
thallus of another lichen (
lichenicolous) and may never form a thallus of their own, producing only reproductive bodies on the
host. Vegetative
propagules such as
soralia (powdery eruptions of algal–fungal tissue) or (tiny outgrowths that detach as whole-thallus clones) occur sporadically. The
photosynthetic partner is always a
green alga with rounded cells (a photobiont).
Sexual reproduction takes place in black
apothecia that sit flush with, or project slightly above, the thallus. These may be round, angular, or on rare occasions slit-like; they lack a pale outer rim, a condition termed . The fleshy rim () is built from radiating or intertwined fungal threads (
hyphae) and may contain crystals visible through polarising light. Inside, the colourless
hymenium turns blue with iodine (I+ blue), revealing its
amyloid nature. Slender, branching weave through the hymenium, often swelling at their tips and sometimes capped by a dark pigment. The asci are —meaning two wall layers separate during spore release—and carry one to eight
ascospores. When mature, the spores are one-celled to (divided by several internal walls), colourless to dark olive-brown, and wrapped in a gelatinous envelope ().
Asexual reproduction is rare and involves
pycnidia, minute flask-shaped structures sunken in the thallus that release cylindrical
conidia. Alongside
rhizocarpic acid, the
medulla may host various
depsides,
depsidones, and
fatty acids that aid species identification through simple chemical
spot tests. ==Ecology==