Physcia adscendens
Hooded rosette lichen

Description: Thallus pale gray to pale greenish gray, spotted with white maculae and sometimes slightly pruinose, forming small clusters of ascending lobes, many of which expand at the tip to produce inflated and hollow, helmet-shaped soralia, which are formed from a separation of the upper and lower cortices and contain greenish, granular soredia; lobes mostly under 1 mm broad except for the helmets, which can be up to 2 mm across; long, white, mostly unbranched cilia (some with darkened tips) grow from lobe margins and tips; lower surface white. Apothecia not common.

Chemistry: Cortex PD-, K+ yellow (atranorin), KC-, C-,
      Medulla PD-, K-, KC-, C-.

Habitat: On bark, twigs, and wood of a variety of trees, less frequently on rock, in well-lit or slightly shaded sites.

Comments: This is the only clearly "hooded" Physcia. Physcia tenella, a common bark dwelling, ciliate lichen along the east and west coasts, has lip-shaped soralia on the lobe tips but does not form hollow hoods or helmets.

Physcia aipolia
Hoary rosette lichen

Description: Thallus pale to dark gray, conspicuously spotted with white maculae; lobes narrow and radiating, flat to slightly concave or upturned at the tips, sometimes overlapping, 1-2(-3) mm across, without any soredia or isidia; lower surface white to pale brown with many pale rhizines. Apothecia very common, 1-2(-3) mm in diameter, very dark brown but typically with a heavily white “frosting” of pruina, (pruinose) giving the lichen its English name; spores (16-)18-25 x 7-12 µm, with angular cells.

Chemistry: Cortex PD-, K+ yellow (atranorin), KC-, C-.
      Medulla PD- or + pale yellow, K+ yellow, KC-, C-.

Habitat: On bark and wood of different kinds of trees in open habitats.

Comments: Physcia stellaris is a very similar species with about the same range as Ph. aipolia, but it usually lacks conspicuous white spots (maculae), has flat or somewhat convex lobes, and has a K- reaction in the medulla.

Physcia millegrana
Mealy rosette lichen

Description: Thallus pale gray, spotted with white maculae; lobes thin, appressed to somewhat ascending, 0.3-1(-2) mm broad, margins (especially the tips) finely divided and finally dissolving into granular soredia; lower surface white, with pale rhizines. Apothecia very common, under 1 mm in diameter, dark brown, often lightly pruinose.

Chemistry: Cortex PD-, K+ yellow (atranorin), KC-, C-.
      Medulla, PD, K-,KC-, C-.

Habitat: On bark, especially deciduous trees, as well as wood and occasionally granitic rock.

Comments: This is among the most common bark-dwelling lichens in eastern North America, even occurring close to urban areas on cultivated as well as wild deciduous trees. Physcia dubia has its soredia in lip-shaped soralia at the lobe tips, and the margins are never finely divided.

Physcia stellaris
Star rosette lichen

Description: Thallus pale gray, darker gray in the center, more or less smooth and uniform although sometimes with some white spotting, especially on older parts of the thallus; lacking pruina, soredia, isidia, or lobules; lobes radiating out (like a star) but sometimes crowded, flat to convex, 0.5-1.5(-3) mm broad; lower surface white to light brown, with fairly abundant pale to dark rhizines. Apothecia common, 0.7-3 mm in diameter, dark brown, often frosted with a white pruina; spores 16-22 x 7-10 µm, with angular locules.

Chemistry: Cortex PD-, K+ yellow (atranorin), KC-, C-.
      Medulla K-.

Habitat: On bark of many kinds, but especially poplars, alders, and elms; rarely on wood or rock.

Comments: The negative K reaction of the medulla is the most reliable way to distinguish Ph. stellaris from Ph. aipolia, although the convex lobes and lack of conspicuous white spots (maculae) can also be helpful.