| TAXON: | Physcia caesia (Hoffm.) Fürnr. (1839) |
| RECENT SYNONYMS: | – |
| FAMILY: | Physciaceae |
| GROWTH FORM: | Foliose, heteromerous |
| SUBSTRATES: | Calcareous or eutrophicated rocks and hard artificial substrates, including concrete, cement and the like; rarely siliceous rock; occasionally eutrophicated wood and bark, including tree-bases in ‘canine zone’ |
| PHOTOBIONT: | Trebouxioid alga |
| REPRODUCTION: | Soredia; apothecia, pycnidia rare |
| ASCUS: | If present, cylindric-clavate; apex Lecanora-type; 8 spores |
| SPORES: | Ascospores if present ellipsoidal, brown, 1-septate, thick-walled, 18-25 × 6-10 µm; conidia if present bacilliform |
| NOTABLE FEATURES: | Thallus pale grey, often with bluish tinge, usually orbicular; lobes closely adpressed to substrate, radiating; cortex flecked with white pseudocyphellae; soralia mainly laminal but also marginal and apical, often hemispherical but sometimes excavate; soredia grey to blue-grey |
| CHEMICAL TESTS: | Cortex and medulla K+ yellow, K/UV(dry)+ yellow (atranorin) |
| HABITAT: | Well-lit situations wherever suitable substrate available, including urban areas |
| DISTRIBUTION: | Widespread and common in Britain |
| CONSERVATION STATUS: | Least Concern |
| LICHENICOLOUS FUNGI: | Arthonia epiphyscia, Erythricium aurantiacum, Intralichen aff. lichenicola, Lichenoconium xanthoriae, Marchandiomyces corallinus, Paranectria oropensis, Polycoccum pulvinatum, Pronectria sp. (often with Acremonium-like anamorph), Sphaerellothecium pumilum, Zwackhiomyces physciicola |
| IDENTIFICATION DIFFICULTY: | Green 1: Field identification possible |
| CONFUSION SPECIES: | Phaeophyscia orbicularis, Physcia dubia |
FIELD NOTES
Physcia caesia is one of ten species of Physica known to occur in Britain, and one of the three most common and widespread. With a preference for calcareous substrates including concrete and cement, and a liking for nutrient enrichment, it’s one of the more successful lichens in the modern British environment. It’s also one of the more successful lichens on the planet, ranging from the Arctic to the Antarctic, and found on every continent.
You’re almost certain to encounter P. caesia near you. It’s fairly easy to recognise, but if you’re not familiar with it you might confuse it with other similar lichens, particularly Phaeophyscia orbicularis or Physcia dubia. Fortunately, you can distinguish between these species using some conspicuous or easily tested features.
Like P. caesia, Phaeophyscia orbicularis is a very common lichen, and it’s found on an even broader range of substrates, including bark. Unlike P. caesia, though, it tends to have a greenish rather than bluish tint, and it often sports rhizines that protrude from the ends of its lobes, recalling the bristles of a used old brush. It also lacks atranorin, and it doesn’t test K+ yellow.
P. caesia and its cousin P. dubia are more similar to one another. P. dubia tends to prefer siliceous rock or more acid substrate than P. caesia. But it also likes nutrient enrichment, and you can occasionally find P. dubia on eutrophicated basic substrates, just as you can occasionally find P. caesia on eutrophicated acidic substrates.
P. caesia and P. dubia can also overlap in form. P. dubia tends to lack the bluish tinge and spotty white pseudocyphellae of P. caesia. But P. caesia doesn’t always look obviously bluish, and P. dubia can sometimes have faint pale flecks on its lobes. And though the two species do have different ‘looks’ overall, such distinctions can be difficult to discern in the field, especially with imperfect specimens.
Fortunately, a simple spot test can separate P. caesia and P. dubia. Both produce atranorin, and therefore they both test K+ yellow and K/UV(dry)+ yellow. But P. caesia contains atranorin in both upper cortex and medulla, while P. dubia contains it in upper cortex only.
So if you encounter a lichen that you think is P. caesia, but you’d like to rule out P. dubia definitively, test your specimen very carefully with K. It’s always an ethical imperative to test with the smallest amount of reagent possible, but in this case it’s also a practical necessity. You’ll need to be certain which part of the lichen you’re testing, as the yellow reaction can ‘bleed’ into other parts.
I find it helpful to use a cocktail stick to place a very small drop precisely on the part of the lichen that I intend to test. Use a sharp knife, or even better a razor, to expose the medulla. You can also test a soralium instead, since medullary hyphae make up the fungal component of soredia.
If you discover that your lichen is P. caesia, savour the moment. To be sure, it’s a very common lichen in Britain. But though it might seem just a humble adornment to an unloved slab of concrete in your town, it instantly connects you with far away places from Greenland to Antarctica, where other lichens just like it cling to bare rocks on lonely shores.
LAB NOTES
Physcia caesia can normally be identified without microscopy.
SPECIMENS
Physcia caesia : England : VC20 Hertfordshire : TL3510 : March 2025 : On wooden bench
Physcia caesia : England : VC25 East Suffolk : TM3874 : February 2025 : On eutrophicated flint cobble
Physcia caesia : England : VC28 West Norfolk : TF9528 : October 2024 : On concrete bridge parapet
Physcia caesia : England : VC25 East Suffolk : TM3389 : April 2024 : On concrete pavement
