Saturday, Paula DePriest led a workshop of mostly frozen participants to Carderock on the Maryland side of the river. Our subject: rock and tree lichens (the soil lichens being inaccessible due to snow cover). Soil, rock, or bark is only the substrate (although certain species do have a preference); the lichen gets no nourishment from it. In all cases, sugars are generated by algae held in the body (thallus) of the lichen, which comprises a fungus. Depending on your point of view, the alga is a captive of the fungus or (Dr. DePriest’s preference) is domesticated by the fungus.
Rocks in this area often are commonly home to a Mid-Atlantic specialty, Flavoparmelia baltimorensis. It’s the pale yellow-green (yellow to a lichenologist) lichen at the lower left of this badly-focused image, along with Lepraria sp. at the right and Pertusaria sp. at the left.
The good news is that there are about three dozen taxa of lichens of interest in the D.C. area. You can write a key (as Dr. DePriest has) to all of them that takes up only two pages. The bad news is that you need at least a hand lens to apply the key.