Marasmius rotula is a species of fungus in the Marasmiaceae family of mushrooms. It is also commonly known variously as the pinwheel mushroom, the pinwheel Marasmius, the little wheel, or the horse hair fungus.
This eastern species is often overlooked because it is so tiny; the caps max out at two centimeters in diameter, and are usually half that size or smaller. It is found in hardwood forests from spring to fall, growing from sticks and other woody debris. Marasmius capillaris is very similar, but grows from leaf litter.
The attachment of the gills is by means of a tiny "collar" that circles the stem. Additional distinctive features include the dark, wiry stem; the pleated white cap that has a flat top when viewed from the side (the cap of Marasmius capillaris, in contrast, is evenly rounded); and the absence of a distinctive taste or odor.
Other "pinwheel" species include the orange Marasmius siccus, the pink Marasmius pulcherripes, and the rusty Marasmius fulvoferrugineus.
Marasmius rotula is considered inedible but not poisonous.
Ecology: Saprobic on sticks and woody debris in hardwood forests; growing alone or gregariously (or in clusters); spring through fall; widely distributed east of the Rocky Mountains.
Cap: .5-2 cm; convex, but soon with a central depression; pleated; appearing to have a flat top and squarish sides when viewed from the side; smooth; dry; brownish in the depression, whitish elsewhere.
Gills: Attached to a tiny "collar" that encircles the stem; white to yellowish white; distant.
Stem: 1.5-8 cm long; 1-2 mm thick; equal; dry; shiny; wiry; pale at first but soon dark brown to black except at the apex; base sometimes with stiff hairs.