Fungus Mycelium of the Agaricus bisporus species. Mycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelia are found in soil and on or within many other substrates. Agaricus bisporus is also known as the common mushroom, button mushroom, white mushroom, table mushroom, champignon mushroom, crimini mushroom, Swiss brown mushrooms, Roman brown mushrooms, Italian brown, Italian mushroom, and the cultivated mushroom.