Rhizocarpon haidense spreads as a thin, crust-like growth (the thallus) that blankets its rocky home in a patchwork of tiny, creamy to pale-brown polygons. These angular patches (areoles), are only 0.3–0.4 mm wide and rise just enough to look slightly domed. Unlike many crustose lichens, they lack an outer skin (cortex), so the surface merges directly into the inner tissue (medulla). Under a hand lens the medulla is packed with microscopic crystals; a drop of potassium hydroxide solution (the common K-spot test) dissolves the crystals and turns the solution bright yellow. The lichen's photosynthetic partner (photobiont) is a green alga with spherical cells 7–12 μm across.[2]
The sexual fruiting bodies (apothecia) are plentiful and conspicuous. Each is a black, button-like disc 0.4–0.7 mm in diameter that lies level with, or slightly above, the thallus surface. A thin, paler rim surrounds the disc and persists with age. Inside, the cup wall (exciple) is made of colourless, radially arranged cells that, like the medulla, are laden with K-soluble crystals. The clear spore-bearing layer (hymenium) stands 90–100 μm tall and is threaded by slender, scarcely branched filaments (paraphyses). Beneath it sits a dark-brown support tissue (hypothecium). The spore sacs (asci) are of the Rhizocarpon type, slightly club-shaped (50–60 × 15–18 μm) and each holds eight transparent, two-celled ascospores that average 17 × 8 μm; the septum only slightly pinches the spore's waist, and both ends are evenly rounded. No asexual reproductive structures have been observed.[2]
Standard chemical spot tests on the thallus give K+ (yellow), PD+ (orange) and C− reactions, indicating the presence of the lichen substances stictic and constictic acids, sometimes accompanied by a trace of norstictic acid and several unidentified compounds revealed by thin-layer chromatography.[2]
