
Marasmius oreades

Marasmius oreades, also known as the fairy ring mushroom or fairy ring champignon, is a mushroom native to North America and Europe. Its common names can cause some confusion, as many other mushrooms grow in fairy rings, such as the edible "Agaricus campestris" and the poisonous "Chlorophyllum molybdites".
Distribution and habitat.
"Marasmius oreades" grows extensively throughout North America and Europe in the summer and autumn (fall) (June–November in the UK), or year-round in warmer climates. It loves grassy areas such as lawns, meadows, and even dunes in coastal areas.
Description.
"Marasmius oreades" grows gregariously in troops, arcs, or rings (type II, which causes the grass to grow and become greener). The cap is across; bell-shaped with a somewhat inrolled margin at first, becoming broadly convex with an even or uplifted margin, but usually retaining a slight central bump- an "umbo"; dry; smooth; pale tan or buff, occasionally white, or reddish tan; usually changing color markedly as it dries out; the margin sometimes faintly lined.
The bare, pallid stem grows up to about tall and in diameter.
The gills are attached to the stem or free from it, fairly distant (rather a distinctive character), and white or pale tan, dropping a white spore-print. The spores measure 7–10 μm × 4–6 μm; they are smooth, elliptical, and inamyloid. Cystidia absent. Pileipellis without broom cells.
This mushroom can be mistaken for the toxic "Clitocybe rivulosa" which lacks an umbo, is white to grey in color, and has closely spaced decurrent gills.
Edibility.
"Marasmius oreades" is a choice edible mushroom. Its sweet taste lends it to baked goods such as cookies. It is also used in foods such as soups, stews, etc. Traditionally, the stems (which tend to be fibrous and unappetizing) are cut off and the caps are threaded and dried in strings. A possible reason why this mushroom is so sweet-tasting is due to the presence of trehalose, a type of sugar that allows "M. oreades" to resist death by desiccation. When exposed to water after being completely dried out, the trehalose is digested as the cells completely revive, causing cellular processes, including the creation of new spores, to begin again.
Ecological uses.
"Marasmius oreades" can be used for the biological remediation of bismuth in polluted soils.