"I could study a single piece of bark for hours," Thoreau wrote in his journal. He meant, of course, a piece of bark covered with lichens.
I don't own a really first-rate lichen field guide, but the college library does have a copy of Lichens of North America by Irwin Brodo, Sylvia Sharnoff and Stephen Sharnoff (Yale University Press, 2001), which has got to be one of the most beautiful nature guides ever published. Unfortunately, at a hefty 9 pounds, it's not a book you'd carry into the field. Maybe Bailey, Greg and I could take turns lugging it around.
Most folks know that a lichen is an alga and a fungus living together for mutual benefit. The alga makes nutrients with sunlight; the fungus provides the alga with a steady water supply and a chance to live in habitats -- dry rocks, exposed tree bark -- where it could not survive on its own.
What we seldom hear about is the dicey nature of the symbiosis. The fungi feed on the algae they have enticed or trapped into collaboration, sucking their vitals, sometimes killing them. It is only because the alga cells reproduce faster than they are consumed that a lichen can exist at all.
Sort of makes me think of the symbiotic relationship of teacher and student. Over the years I have fed on the youth and enthusiasm of my students, while they still had youth and enthusiasm to feed on; and every year a new batch of youthful, enthusiastic students appeared on the doorstep. Let's hope they got something in return from the crusty old fungus.