Heterodermia typically forms a foliose
thallus—occasionally shading towards a somewhat
fruticose habit—that is continuous, lobate and irregular, or arranged in loose
rosettes across. Neighbouring thalli can merge, producing broad radiating mats or tangled clumps. The lobes may lie separate or touch; they range from closely adnate and appressed to partly ascending and loosely attached, and are linear to linear-cuneate or spoon-shaped (
spathulate). Branching is mostly dichotomous but can be irregular, and the margins are often fringed with cilia that are simple or densely branched. The upper surface is whitish-, grey- or yellow-grey, flat to convex—sometimes concave near the edge—and may be dull or glossy. It can carry
isidia,
soredia, or , but never
pseudocyphellae. The cortex consists of longitudinally aligned hyphae (a , superficially like plant
collenchyma). A is absent. The
photobiont forms a continuous band above a well-defined white
medulla that may be tinged yellow, orange or brown. The lower surface, which may lack a cortex or bear a prosoplectenchymatous one, is white to whitish grey and often darkens to purple-grey, grey-black, or partly yellow, orange or brown.
Rhizines are white to black, simple to densely branched—sometimes long enough to project beyond the lobe margins—and only rarely absent. The
ascomata (fruiting bodies) of
Heterodermia are
apothecial and (ringed by thallus-like tissue). They sit on the thallus surface () and are rounded, either or borne on a short stalk. The exposed hymenial surface, or , ranges from pale to dark brown or black; it can be concave or nearly flat and may appear frosted () or smooth. A exciple rims the disc—prominent or —and remains distinct throughout the apothecium's life. In section, the epihymenium is pale brown to brown-black, while the underlying
hymenium is colourless. The is usually colourless, only rarely tinged pale yellow.
Paraphyses branch toward the top, where their terminal cells broaden and turn brown. The
asci are cylindrical to somewhat club-shaped—
Lecanora-type—with eight
ascospores. Their apex is
amyloid and thick-walled, enclosing a clear axial body. The spores develop one or more (small
vacuoles in the wall). They turn grey-brown to dark brown, are
ellipsoidal to oblong or fusiform, and have a single
septum that often causes a slight constriction. Walls are very thick; internal apical thickenings only appear after the septum forms. A is thin or absent, and the surface remains smooth.
Conidiomata lie immersed in the thallus at first, later becoming emergent. Their cells form short, branched chains and produce
conidia enteroblastically (inner-wall budding). The resulting conidia are
bacilliform to short-cylindrical. The genus
Physcia most closely resembles
Heterodermia. Unlike
Heterodermia, however, it has a differently structured upper cortex consisting of (which gives it a uniform, unoriented appearance), and its ascospores are different. ==Species==