Cetradonia

Cetradonia is a lichen genus in the family Cladoniaceae. A monotypic genus, Cetradonia contains the single species Cetradonia linearis (formerly known as Cladonia linearis and as Gymnoderma lineare). The genus was circumscribed in 2002 by Jiang-Chun Wei and Teuvo Ahti. The genus was once placed in the family Cetradoniaceae (created in 2002) until that family was subsumed into the Cladoniaceae in 2006.

Cetradonia linearis, commonly known as the rock gnome lichen, is a squamulose lichen found in the higher elevations of the southern Appalachian Mountains. Populations are only known to exist in Georgia, North Carolina, South Carolina and Tennessee. The lichen occurs only in frequent fog, or in deep river gorges. Because of its specialized habitat requirements and heavy collection for scientific purposes, the lichen has been listed as an endangered species since January 18, 1995. It is only one of two lichens on the endangered species list, the other being the Florida perforate cladonia.

Description
Genus Cetradonia features a thallus that is enduring and well-formed, typically aggregating into clumps. These thalli are characterized by their almost cylindrical base stalks that evolve into linear, flattened, and strap-like. These lobes are somewhat upright, ranging from unbranched to minimally branched, and have a texture. They measure between 5 and 25 mm in length and 0.5 to 1 mm in width, often connecting at the base through horizontal structures known as rhizomorphs. The upper surface of the thallus is densely covered by a cortex, with colors from greyish green to pale yellowish brown or olive-green, enclosing a cartilaginous core. In contrast, the lower surface has a thinner cortex, lacks, though it may show transverse wrinkling towards the lower end and has furrows and pits near the tips, transitioning in color from white or cream at the tip to black towards the base.

Podetia, which are small stalked structures, emerge predominantly from the upper section of the thallus's lower surface, often near the tip, and have a dirty white hue. These structures can range from 0.3 to 2 mm in height, are covered by a, solid in form, and may or may not contain algae. The reproductive structures, known as ascomata, host clusters of hymenial that can expand up to 2 mm across, with individual discs varying from 0.1 to 1 mm in diameter. These discs are flat to spherical in shape and dark brown to black in color, occasionally stacking atop one another in a manner similar to Cladia aggregata. The asci, or spore-bearing structures, are slender, with stalks that can extend up to 2 mm; conidia (asexual spores) have not been observed to occur in this species.

In terms of chemical composition, Cetradonia contains atranorin and lichesterinic acids.