Top 20 Most Common Mushrooms in Tonbridge

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 centimetres (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). Its photobiont is the green algae Trebouxia impressa, a species associated with many Physcia lichens. The lichen's cortex and medulla react positively with potassium hydroxide (K), turning yellow. Cortex and medulla react positively with para-phenylenediamene (Pd) as well, also turning yellow. They produce no reaction with calcium or sodium hypochlorite (bleach – C), nor with KC (potassium hydroxide quickly followed by bleach). Among the substances produced by the lichen are atranorin and zeorin.

Physcia tenella grows initially in small rosettes, which later often flow together like a lawn. The lobules of the thallus are white gray to gray and 0.5 to 1 mm wide. The lobes end flat or in lip-shaped straightened soralen, where vegetative dissemination units (soredia) are formed. These "lip sensory" distinguish them from the otherwise very similar Physcia adscendens (which has helmet-like bulges at the lobes). Gray-white fibrils sit at the edges. Fruit bodies (apothecaries) are rarely trained.

It was first described in 1753 by Carl Linnaeus with the name Lichen saxatilis. Erik Acharius transferred it to Parmelia in 1803. The lichen has a greenish-gray to bluish-gray thallus that can turn brown in exposed locations. It typically grows on rock, although it is sometimes found on bark or wood. Parmelia saxatilis is used to make dyes with deep red-brown and rusty-orange colors. Known as "crotal" in Scotland, it was used to dye traditional cloths including Harris tweed. A somewhat similar species with a cosmopolitan distribution is Parmelia sulcata, which generally grows on trees.

Farinose cartilage lichen has a bushy appearance and is easy to identify by its long, slender branches. The reproductive structures are often scattered on the fungi. The lichen thrives in warm moist, mixed forests, forming on second-growth trees and shrubs.