Only certain algae and fungi can get together to form a lichen. When it is possible, a fungus creates a framework, or thallus, to shelter both itself and its algal partner.

Each fungal-algal union forms a unique type of thallus body. Scientists use this structure to help identify and name different lichens.

So far, more than 20,000 lichen species have been I.D.'d. Many are quite beautiful. Lots of different kinds of fungi can become "lichenized" by algae over time. But only 20 to 40 different kinds of algae are now known to merge with fungi to become lichens.

Once joined, an alga begins using sunlight to make sugars as food for both itself and its fungal partner. The fungus' job includes protecting them both from intense sunlight and other stresses.

One way that lichens spread is by forming little, powdery packets. These are made up of a few algal cells wrapped up in fungal threads called hyphae that are blown around in the wind. This way, they're already together upon landing on a suitable new surface, or substrate.

Air pollution and the substrate's texture mainly determine where lichens can live.

For example, a car parked long enough under a lichen-covered tree will become covered with lichen, too. This can happen if the car's paint has become roughened and it's parked out in the country.

How do lichens enlarge? They just spread out . . . very slowly. The growth rate of lichens has been determined by observing those that develop on gravestones. It's obvious by the date when a stone was "planted," and the lichen probably started enlarging soon after. They grow about a quarter inch per year.

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