#map lichen

Rhizocarpon riparium

Streamside map lichen, riparian map lichen

Recognizing a member of the subgenus Rhizocarpon is pretty easy. That bright yellow-green thallus comes from the presence of rhizocarpic acid, and is truly unique amongst lichens, along with the distinctive black prothallus and black, angular apothecia. But trying to tell the individual species of the Rhizocarpon subgenus apart? That's a whole other story. Basically every paper and description for this species I have found says there is variation and overlap between R. riparium and other members of the group. So R. riparium might not even be like, an actual thing. But this isn't really a concern for YOU, the nonlichenologists out there. Afterall, the most stable characteristic to identify R. riparium from look-alikes appears to be a greenish pigment in the hymenium of the apothecia, only visible in microscopic cross-section. And I doubt y'all will be doing a lot of that. So I will just be happy if you go out and see a lichen like this and go "Oh look! A Rhizocarpon! Isn't she gorgeous???" To do so, head to your nearest high-elevation (1370-3535 m), sun-exposed, siliceous rock and take a gander. Shouldn't take you long to find one.

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Crustose lichens found growing on rock, and sometimes parasitising other lichens. The colonies usually have a black border of fungal hyphae - leading to the geographic name. Most common in arctic and alpine environments, but also found throughout temperate, subtropical, and sometimes tropical biomes.

A genus of uncertain size - the species are variable in appearance, and many have been synonimised, and molecular evidence strongly suggests that some currently nested within the genus should be split off. Current estimates put the count at somewhere between 75 and 400.

Rhizocarpon cookeanum

Cooke's map lichen

Rhizocarpon disporum

Single-spored map lichen

This crustose lichen has an areolate (tile-like) thallus of warty to globose, pale to dark gray to browns areoles (tiles) interspersed with black apothecia, giving the surface a distinctive map-like appearance. But to distinguish R. disporum from other similar looking members of the same genus, you gotta get closer--A LOT closer. Using a microscope to examine an apothecial cross section, you gotta check the size (large), shape (ellipsoid), structure (muriform), color (brown to black), and number (1) of spores produced in the asci (sac-shaped, reproductive structures in the apothecia) to differentiate this guy. It's fun, trust me. R. disporum grows on exposed siliceous rock in dry, continental habitats.

images: source | source

Rhizocarpon atroflavescens

Darkening yellow map lichen