If you've ever wondered why the verb "mushroom" means "to grow or spread rapidly," a walk in the Piedmont woods these days will provide a graphic explanation.

All summer, while we suffered from severe lack of rain, our soil got drier and drier. Even normally-moist humus in deciduous woodlots was parched by no rainfall and low humidity. In many habitats herbaceous plants and saplings shriveled, but for me the organisms affected most conspicuously were the fungi. Nowhere this summer in the thickets and woods around Hilton Pond was I able to find a single mushroom or toadstool, and there was a dearth of other fungi that typically grow in profusion.

When rains finally came in early August, our ubiquitous red clay again began absorbing moisture, and thunderous deluges that hit York County over the past few weeks saturated the upper layer of soil. That's all the fungus family needed to trigger its growth mechanisms, and suddenly there were mushrooms all over the place.

Mushrooms and other fungi make up one of five major groups, or kingdoms, of living things, the others being animals, protozoans, bacteria and blue-green algae, and the green plants. For many years, botanists claimed fungi were actually plants because they had cells with thick walls, but now taxonomists classify them separately for several reasons. The most obvious difference is that green plants photosynthesize their own food with sunlight and chlorophyll, while fungi lack this special green pigment and depend upon outside food sources.

Most fungi are saprophytes that survive on dead organic material like tree stumps and fallen leaves, but some are parasites requiring live hosts. In either case, fungi are unable to produce their own food and must draw nutrients from things on which they grow.

All true mushrooms are saprophytic, living mainly on well-rotted plant parts just beneath the surface of the ground. For most of the year, the "mushroom" consists of branching root-like structures called hyphae that live in total darkness and grow by absorbing and processing nutrients. Hyphae convert carbon compounds from their food sources into more hyphae, and this subterranean fungus colony may extend several feet during a warm growing season.

Dry soil slows hyphal growth rates but when it rains things start to happen quickly. Somehow sufficient moisture--perhaps in conjunction with factors such as temperature or the ratio of day length to night length--tells the hyphae it is time for a change in lifestyles. Instead of making more hyphae, the underground fungus builds very different cells that burst from the soil, shooting up rapidly to form familiar mushroom structures.

During the past two weeks, many species of mushrooms have erupted from the earth, often appearing in large numbers overnight; I know of few organisms that grow as fast. I watched toadstools in the Hilton Pond woods sprout six inches in 24 hours, and have heard of others that grow even faster. It is interesting to note that mushrooms now popping up have been around all along, hiding out as hyphae in the leaf litter.

Actually, mushrooms are just structures that allow hyphae to get from one place to another in a hurry, and they develop only when conditions are appropriate for the formation of reproductive spores. To the naked eye, the mushroom (or toadstool) is a cylindrical stipe or stalk that elongates as an umbrella-like cap unfolds. Beneath the lower surface of the cap may be thousands of tiny pores or hundreds of thin radial partitions called gills; these structures bear the several millions of microscopic spores produced by each mushroom.

Most of these spores die and decompose, but a precious few may be swept away by raindrops or transported by ground beetles, small mammals, and naturalists' pantlegs. Spores that end up in favorable locales may germinate, often during the same moist conditions that caused mushrooms to form in the first place. Others may settle onto dry spots and remain dormant for many years, waiting on a few requisite water droplets.

Each germinating spore becomes a tiny hyphal strand that colonizes new territory for the parent mushroom. The hyphae grow and spread radially, forming underground structures rather like many-spoked bicycle wheels. Often, mushrooms sprout only from the outside rim of the hyphal circle, forming a "fairy ring." As mushrooms die around the circle's circumference, the hyphae grow outward until the next reproductive season; thus, each successive ring is bigger than the last. I have seen horse-pasture mushroom circles nearly 50 feet in diameter, and it's no wonder such a phenomenon led to fantasies of fairies in moonlit magic rings.

Many local mushrooms are edible, but some of these are noxious to the taste. A few others have toxic spores, and each year novice mushroom gatherers succumb to lack of wisdom and inability to make proper field identifications. Most mushrooms that get to the table are grown commercially in carefully-supervised underground monocultures, and the chance of dying from store-bought mushrooms in this country is essentially nil.

Despite their appeal to the palate, the main value of mushrooms has nothing to do with gastronomy. Mushrooms, and the saprophytic fungi in general, are important ecologically as decomposers of dead organic matter. Were it not for fungi, we soon would be up to our eyebrows in dead trees, dried grasses, and other organic debris. As mushrooms sprout from lawns, people tend to spa

The best mushroom crop I ever saw is growing right now in old manure on my compost pile. Cow droppings provide an extremely rich source of nutrients that are converted by mushrooms into more mushrooms, all of which will die, decay, and be absorbed by organisms somewhere on the food chain.

In this way, cow manure gets changed back to basic elements--mostly carbon, hydrogen, and oxygen--and the cycle goes on and on, thanks to fast-growing fungi that populate our Piedmont woods and lawns.