Judy Pastor, age 14, of Pittsburgh, Pa., for her question:

What is mycelium?

Suppose you had to find a name for a plant made entirely of rooty filaments. The experts were faced with this wordy problem when they needed a term to describe the bodies of certain fungus plants. They solved it by creating the word mycelium from the Greek word for mushroom.

Mushrooms and toadstools pop up from the ground on dewy mornings and sometimes after mild summer showers. They seem to come from nowhere, but this cannot be true. We call each stubby umbrella a mushroom or a toadstool as though it were an entire plant. But this is not true either. Actually, the stubby umbrella that pokes above the ground is just one part of the fungus plant.

The main part of a mushroom plant is buried out of sight in the soil. It is a tangled mass of rooty fibers forming a spongy mixture with the rich dirt. This mesh of pale threads is the nycelium and mycelium is the main plant body of most members of the fungi family. Its rooty threads are called hyphae, and each fleshy hypha does far more work than a root on a non fungus plant.

A new mushroom sprouts from a tiny spore, and almost at once its fine threads of mycelium begin to finger their way through the moist Earth. The young fungus may grow below ground for a year or several years, spreading its network of fibers far and wide. Its tflycelium soaks up moisture and chemicals from the soil, but the pale threads contain no green chlorophyll to create food for the growing plant. It cannot make its needed plant sugars from air, water and sunlight.

For this reason the fungus can grow only in super rich soils. It depends for food upon rotting logs and leaves and other substances that were created by living green plants. The chemicals in these prefabricated plant foods are dissolved in the moisture of the soil and absorbed by the mycelium. The entire fungus plant could live and die without poking one shoot above the ground. Truffles and certain other fungi never see the light of day. But the mushroom mycelium pokes up stubby shoots when time comes to hand on life to a new generation. The fat little umbrellas that we call the mushrooms are only the fruiting bodies of the main plant, which is the buried mycelium.

Many members of the fungus family live as parasites within the living tissues of other plants. These uninvited guests may be rusts or mildews, and like other fungi they spread forth masses of mycelium. When these hungry threads spread through living tissues, they damage the cells and often destroy the host plant. The rusty spots on a green leaf may be caused by the fruiting bodies of certain parasitical fungi. The black spots on rye and willow may be caused by the strangling mycelium of fungi growing within the host plants.