Primary thallus: squamulose, persistent; squamules: 1-6 mm long, 2-5 mm wide, sometimes with granular soredia; podetia: 2-30 mm tall, 1-4 mm wide, grayish to whitish green, unbranched to sparingly branched, subulate or blunt, not cup-forming; surface: sorediate, with disappearing cortex on upper portions, but basally thickly corticate (also below apothecia); soredia: coarse at base, sometimes intergrading with isidioid structures; Apothecia: scarce, 0.5-3 mm wide, red; ascospores: oblong, 8-13 x 3-4 micro meter; Pycnidia: common, on primary squamules or tips of podetia, ovoid to cylindrical, with red gelatin; conidia: 3-8 x 0.5-1 micro meter; Spot tests: K+ bright yellow, persistent, or K-, C+ & KC+ canary yellow, or C- & KC-, P+ orange, or P-, UV-; Secondary metabolites: thallus with two major chemotypes: (1) thamnolic, barbatic (inconstant), and didymic (accessory) acids; (2) barbatic acid, didymic acid (accessory); many minor accessory metabolites also present (Ahti 2000); apothecial discs with rhodocladonic acid as a red pigment.; Habitat and ecology: on dead wood, tree bases, or sometimes over soil or rocks; World distribution: all continents (Antarctica uncertain); mainly temperate to boreal; Sonoran distribution: Arizona, southern California, Baja CalIfornia, and Sinaloa.; Notes: Cladonia macilenta is often divided into two species, called C. macilenta and C. bacillaris (the nomenclature is in need of clarification) based on the presence or absence of thamnolic acid (revealed by K and P reactions). They are here regarded as chemotypes, following Christensen (1987) and Ahti (2000), although some authors claim that they have subtle morphological differences. The barbatic acid chemotype seems to be the only one in inland areas of the Sonoran region, while the thamnolic chemotype is more frequent in the coastal lowlands.