For hundreds of years, naturalists didn't quite comprehend what they were. Originally these odd forms were thought to be part of the plant kingdom, which is why we still see lichens collected by botanical gardens. Eventually, microscopy enabled scientists to identify lichens as composites of mutually beneficial fungi and algae. Because fungi take the more dominant role and cultivate photosynthesizing algae for food, in return providing them a shady, moist, vitamin-rich environment, scientists have classified lichens based on their fungi species. Their identity came into dispute again when blue-green algae, a frequent component of lichens, were reclassified as cyanobacteria, a kind of bacteria that obtain energy through photosynthesis. But as it turned out, whether the fungi were harvesting algae or cyanobacteria, the symbiotic modus operandi of the lichens proved to be the same. Perhaps Trevor Goward, the lichen curator at the University of British Columbia Herbarium, describes them best. "Lichens," he says, "are fungi that have discovered agriculture."

My walks in the Ozark woods impressed upon me the lichens' diversity and the confusion they can create. Sometimes their identity was clear. A lichen species known as British soldiers sports distinctive, bright red caps atop green stalks. Old man's beard can run more than three feet long and hangs from trees in the manner of Spanish moss. But without a microscope to see green clusters or strings of photosynthetic organisms running like arteries through the fungal flesh, you cannot always tell you are seeing a colony of lichens. A colony might look like a plant, an uncomplicated fungus, or even a patch of rust: here a fence speckled in autumnal reds and yellows, there some orange lace spread on a stone.