Description
Amygdalaria forms a firmly attached crust on siliceous rock. Seen close up, the surface is broken into angular, sometimes rather coarse blocks ( areoles ) that may carry a faint pinkish tint. Scattered in the tiny gaps between these blocks are wart-like cephalodia—specialised pockets that house a second, blue-green alga ( Stigonema ) and allow the lichen to fix atmospheric nitrogen. The main, green partner is a Trebouxia -type alga, and the internal white layer (medulla) gives no colour reaction with iodine (I–). [4]
The fruiting bodies (apothecia) develop deep inside the areoles; externally one sees only a black to dark-brown disc that is flat or slightly cup-shaped and may sit on a tiny central bump ( umbonate ). These discs have no rim of thallus tissue (a thalline margin ), and the inner cup wall is often poorly formed, consisting of packed, radially arranged hyphae. Under the microscope the spore layer (hymenium) is unusually tall and stains blue in iodine, while the supporting tissue beneath ( hypothecium ) varies from nearly colourless to dark, carbon-black; when the latter is well developed it forms a small cup and flashes reddish in potassium hydroxide solution. Instead of the usual simple paraphyses, the hymenium contains a web of slender, branched threads whose bead-like upper cells interconnect. Asci are elongate clubs of the Porpidia type, each with a thin amyloid coat and a thickened cap ( tholus ) pierced by a pore that stains deep blue in iodine. They hold eight large, smooth, single-celled ascospores wrapped in a tight gelatinous envelope. Minute, sunken pycnidia occasionally produce rod-shaped conidia for asexual spread. Chemical tests commonly show gyrophoric acid, sometimes accompanied by lecanoric acid, and occasionally other orcinol-derived lichen products. [4]
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