Blue-gray Rosette Lichen

Physcia caesia is a foliose lichen that forms small rosettes, typically measuring 2–3 cm (0.79–1.18 in) across at maturity, though it can reach diameters of up to 7 cm (2.8 in). The thallus is pale gray shading to darker gray in the center, and adheres closely to the substrate on which it grows. Its convex lobes are linear, typically measuring 0.5–1.0 mm (0.02–0.04 in) in width, though occasionally as wide as 2 mm (0.08 in). Forms in some areas, including Greenland, can be considerably darker, with a thallus ranging from gray to dark gray; they can also have broader or narrower lobes. The lichen's surface is white-spotted, though this is not always obvious. It has blue-gray soredia, which are piled in round mounds (such aggregations are known as soralia) across the thallus's upper surface. The lower surface is white to brown with short, dark rhizines. Physcia caesia only rarely has apothecia. Where present, these are black discs measuring up to 2 millimetres (0.08 in) across, with prominent thalline margins (which means the thallus extends up around the edges of the apothecia) and a grayish pruinescence. Each ascus contains 8 spores, which are thick-walled and brown, measuring 18–25 μm (0.00071–0.00098 in) x 6–10 μm (0.00024–0.00039 in). …

Physcia caesia is widespread across much of the world, found in Arctic, boreal and temperate vegetation zones. Widely distributed across Europe, it is one of the most common species of Physcia in Greenland, and is abundant throughout Britain. Though found across much of North America, it is absent from central and southeastern United States, parts of the Great Basin, and northern Alaska. In South America, it is found in Argentina, Chile, and Peru; common and widespread in temperate areas, it occurs only rarely and only at elevations above 3,700 m (12,100 ft) in tropical regions. It occurs patchily across Asia—in India, Bhutan, Nepal and Japan—as well as in East Africa. It is described as "cosmopolitan" in New Zealand, has been reported from a number of sites in southeastern Australia and Tasmania, and also occurs in Antarctica. It is common on calcareous substrates, growing on limestone (including tufa) and concrete. It also grows on basaltic and siliceous rock, as well as on bone. It is particularly common on rocks where birds perch and defecate. It is uncommon on bark; however, in the Colorado Rocky Mountains, it is one of the predominant lichens on juniper trees, growing primarily within 0.1 m (3.9 in) of the ground on the northern and eastern side of trunks. …

Physcia caesia tolerates a variety of locations, occurring on vertical, angled and horizontal surfaces, as well as rocky overhangs. It is found on both dry stone and stone moistened by seepage, and can tolerate being irregularly submerged underwater. It survives in hot desert conditions, primarily on the north side of slopes. It also thrives in cold conditions, and is able to photosynthesize at temperatures as low as −14 °C (7 °F). Like many lichens that grow on rocky substrates, Physcia caesia is able to accumulate a number of necessary nutrients—including nitrogen, phosphorus, potassium and iron—from the substrate upon which it grows, as well as from rainwater and atmospheric dust. Its hyphae can extend into substrate as much as 17 mm (0.67 in). It is a nitrophyte and can tolerate higher levels of atmospheric ammonia than can many other species of lichen, because it can survive on substrates with a higher pH. It has been recorded growing on a lead surface (probably a weathered oxide). Physcia caesia is known to accumulate high concentrations of heavy metals, including chromium, zinc, copper and iron. …