Is a mushroom the whole fungus, or just its fruit?

A mushroom is not the whole fungus. It is only the fruiting body: the fleshy, spore-bearing part that a fungus pushes up, usually above ground on soil or another food source. The fungus itself is the mycelium, a mass of branching, thread-like hyphae that grows out of sight, and mushrooms grow from it. Think of an apple: the apple is not the tree.

The mycelium can be so small that you could never see the colony, or it can be enormous, spanning thousands of acres in some Armillaria fungi. A mushroom only appears when the conditions are right. It begins as a pinhead-sized lump, less than two millimetres across, called a primordium, which forms within the mycelium. That lump swells into an egg-like button, and the button opens into the cap and stem we know.

The cap has a job: it carries the spores. Underneath it, the gills produce microscopic spores that help the fungus spread, and on most mushrooms they fall in a fine rain of powder from beneath the cap.

Here is the part that fools people. Many mushrooms seem to appear overnight, and the idea even gave English the phrases to mushroom and to pop up like a mushroom. In reality, every mushroom takes several days to form its primordium. What is quick is the last step, when the mushroom absorbs fluids and expands. The fruit may last only a short time, but the mycelium underneath can be long-lived and massive.