Lichen are comprised (symbiotically) of a species of fungi, and a species that photosynthesizes (usually algae, but sometimes bacteria). The two species are distinct, but cannot survive without each other. They live almost anywhere, and sometimes, in their interdependent partnership, put out fruiting buds that transmit a particular type of lichen to other locales.
They have been evolving for over 400 million years; certain types of micro-organisms live only on lichen. People have eaten them for thousands of years, and used them for dyes more recently. It is only the last 15 or so years that biologists have come to understand the different types of lichen, and how lichen interact with ecosystems.
So: they reflect the coalescence of two biological realms; they produce their own unique colors and (sometimes)
fruiting bodies. They are distinct from both fungi and algae — a true cooperative coalescence: the basic building
blocks of lichens cannot live without each other.
Reindeer survive on lichens; and now, we are discovering that lichens are sensitive to changes in the composition of our atmosphere and may prove an important “indicator species”.
Note: both realms must be present: Psychology, and Ecology! And the result must be boundaried, or “contained” — something totally unique. They are receptacles for both energy from the sun, and energy from the earth, and they have served humans for thousands of years!