Species of
Agonimia grow as a crust that clings tightly to bark or rock. In most taxa this crust is built from tiny, grain-like clumps of algae and fungus (), but some develop minute scale-like lobes () or even coral-like branches; one outlier forms delicate, leaf-like up to 5 mm across. Where squamules are present their outer cells unite into a thin, skin-like layer (a ) that may be tinged brown and often bears one or more small warts (). In an extraordinary species these papillae stretch and fuse into stiff bristles. The thallus lacks the dead, protective film common in many lichens, except for a single species in which the outer cells collapse to leave a refractive crust. Squamules are usually fragile and crumble into powdery or irregular fragments that serve as asexual
propagules; some species also shed —minute buds that break away—or produce structures resembling
isidia or
soredia. A distinct (a hyphal fringe at the margin) is absent or inconspicuous. The
photosynthetic partner is a
green algal cell of the type, measuring roughly 4–13 × 3.5–7.5
μm. Fruiting bodies are flask-shaped perithecia that appear black, or grey-brown if overlain by a thin unpigmented film, and sit between the squamules or grains. Each perithecium is nearly spherical to barrel-shaped; the usual dark cap () is missing, so its wall () grades seamlessly outward. That wall is thick and stratified: a pigmented outer zone of rounded cells, a middle layer of similar but colourless tissue, and an inner layer of compressed, clear cells. The brown pigments turn grey-brown, reddish-brown, or greenish in
potassium hydroxide solution. Within, the hymenial gel
stains weakly with
iodine—red in strong solution, blue in dilute—a property termed hemiamyloid. Only short ostiolar threads ( and ) line the neck; the longer interascal filaments found in many lichens are absent. The
asci open by splitting their walls (), contain either two or eight spores, and show no iodine reaction. The
ascospores are colourless, divided by multiple cross-walls (
septa) into a brickwork pattern (), and may brown slightly when over-mature.
Asexual reproductive bodies (pycnidial conidiomata) are rare; when present they release rod-shaped, colourless
conidia.
Thin-layer chromatography has so far failed to detect any
secondary metabolites. ==Species==