I felt a quickening of hope for the Republic when a friend told me that these days lichens are hip. Everyone who's building a stone wall has got to have them. And everyone is building a stone wall.

Now lichens, at first blush, would seem improbable objects of passion for the home-and-garden cognoscenti. These curious, sometimes grotesque symbiotes hang out in rough neighborhoods where more refined species fear to tread. Shady boulders, wind-blasted cliffs, rotting wood, and spooky old tombstones — few spots on Earth have not been colonized by one sort of lichen or another.On second thought, this quirk of fashion makes sense (at least, as much as fashion ever does). For lichens grow at such a glacial pace, and each type is so picky about the micro-habitat where it likes to grow, that those gray-green splotches on the rock beside your driveway signify permanence, solidity, a durable connection to a particular piece of ground — much more so than, say, a hedge of slow-growing yew, or artfully weathered siding on the barn.

Here in Maine we are probably more aware of the prevalence of lichen than folks in other places, largely because, for much of the year, we have little else to look at. Stroll through a patch of woods at any time from late autumn through early spring, and the world around you, if not buried in snow, is a minimalist composition of browns and grays.

But look more closely, and a fantastic miniature landscape appears: ghostly green and mustard yellow, splotches that might be curdled cream and others like crimson spatters of blood. Bristling from tree limbs, flapping like fleshy leaves from the sides of boulders, poking through fallen needles, and creeping over every surface in sight, lichens are truly all around us. How odd, then, that in general we fail to notice.

Their shapes are as varied as their colors. The names we give them tell the story. Wolf moss. Rock tripe. Old-man's beard. Puffed shield lichen. British soldier lichen (stiff, upright, adorned in red). Reindeer moss (which not only looks like a mass of tiny antlers, but provides winter sustenance for reindeer and caribou, who snuffle through the snow to get at it). Such diversity reflects an underlying biology that has variously baffled, awed, and stimulated observers since at least the time of the ancient Israelites — or so say present-day scholars, who believe the "manna from Heaven" provided to Moses and the Israelites was a whitish, edible lichen, Lecanora esculenta, that still provides forage for Middle Eastern sheep.

A sea change in biological thinking has occurred since I was taught that lichens belong to the plant kingdom, as partnerships between fungi and algae. Today the living world is seen as comprising five kingdoms rather than two; plants and animals have been joined by fungi, bacteria, and simple beings called protists or protoctists. Lichen, in this scheme, represents a cross-kingdom alliance between a fungus and a plant (if the partner is an alga), or a cyanobacterium (the things we used to call blue-green algae) — or, in rare cases, all three. The fungal partner builds a durable outer structure, tolerant of environmental extremes, while the photosynthetic life-mate produces food for both.

Individual lichens may be 4,000 years old, which would place them among the oldest living things on Earth. Viewed against such a time scale, the life span of a mighty oak is a passing interlude. For this reason, the state of lichens in an ecosystem such as the Maine woods gives us a reliable indication of the health and the antiquity of the system as a whole — more accurately, in fact, than traditional approaches like estimating the age of standing trees. And that makes this clan of quirky, often uncomely organisms a true natural treasure.