The most common, commercially grown mushroom is the white button mushroom, Agaricus bisporus. Mushroom farms consist of special planting beds in buildings with controlled temperature and humidity. The beds are filled with soil mixed with material rich in organic matter. They are then inoculated with mushroom spawn—a pure culture of the mushroom fungus grown in large bottles on an organic-rich medium. The mycelium grows and spreads throughout the soil mixture for several weeks. Mushroom formation is induced by adding a layer of casing soil to the surface of the bed. Mushrooms appear on the surface of the bed through a process known as a “flash” with the mushrooms collected immediately after flashing, while they are still fresh.