In 1867, Simon Schwendener startled the scientific world when he announced to the Swiss Naturalists' Club that lichens were not the distinct organism they had long been thought to be, but rather were formed of two separate organisms:  a fungus and an alga. Leading lichenologists were outraged at the radical idea; not for 50 years would Schwendener's theory be accepted. But once it was, lichens were seen as a classic example of a mutually-beneficial symbiosis:  The alga, able to photosynthesize, provides food for the fungus; the fungus provides shelter and water to the alga.

Some 20,000 kinds of this unique association grow on earth, living in almost every environment, from within Antarctic rocks to the surface of desert soils. Lichens can withstand years of desiccation by simply becoming dormant. They revive again with miniscule amounts of water - in the desert, humid night air suffices. While dormant, lichens can also stand searing heat and extreme cold; some species can survive temperatures as high as 180ºF and as low as 100ºF below freezing. Some arctic lichens are estimated to be over 4,000 years old, among the world's oldest plants.

What looked to early botanists like a single plant is actually a highly-organized association of hundreds to millions of microscopic algal cells woven into a fabric composed of fungal filaments, or hyphae. The fungus forms the thallus, or body of the li

chen; the algae, arranged in a thin layer near the top of the thallus, absorb solar energy and produce food.

Lichens come in three different forms. Crustose lichens often form colorful patches on rocks, adhering so tightly to their growing substrate that they cannot be separated. The bodies of fruticose lichens, such as beard lichens hanging in clumps from tree branches, are formed of many slender stalks. Foliose lichens, common on the surface of soils in forests and on tree trunks, get their name from the upcurled edges of their thalli, which resemble drying leaves.

New lichens grow from soredia, specialized pieces of the parent thallus consisting of a few algal cells enveloped by fungal hyphae. Soredia are designed to break away easily with the action of wind or rain. The fungi of some lichens also reproduce by themselves, producing spore which grow into new fungi, or if they encounter a suitable alga, into another lichen.

When Simon Schwendener announced his two-separate-organisms theory, he further asserted that the fungus captured the alga and lived off its food-making ability. New research supports this once-poo-poohed kidnapping theory:  When a fungus finds an alga, it interferes with the algal food-production process, turning the algae into "slaves" that make and secrete food for the fungus; yet the algae do not appear to benefit.

Next time you see a lichen, examine it closely - its life is not as simple as it seems.