This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
In this genus the plants are tough and fleshy or membranaceous, leathery and dry. They do not easily decay, but shrivel up in dry weather, and revive in wet weather, or when placed in water. This is an important character in distinguishing the genus. It is closely related to Collybia, from which it is difficult to separate certain species. On the other hand, it is closely related to Lentinus and Panus, both of which are tough and pliant. In Marasmius, however, the substance of the pileus is separate from that of the stem, while in Lentinus and Panus it is continuous, a character rather difficult for the beginner to understand. The species of Marasmius, however, are generally much smaller than those of Lentinus and Panus, especially those which grow on wood. The stem in Marasmius is in nearly all species central, while in Lentinus and Panus it is generally more or less eccentric. Many of the species of the genus Marasmius have an odor of garlic when fresh. Besides the fairy ring (M. orea-des) which grows on the ground, M. rotula is a very common species on wood and leaves. It has a slender, black, shining stem, and a brownish pileus usually with a black spot in the depression in the center. The species are very numerous. Peck, 23rd Report, N. Y. State Mus., p. 124-126, describes 8 species. Morgan Jour. Cinn. Soc. Nat. Hist. 6: 189-194, describes 17 species.
Marasmius oreades Fr. Edible. - This is the well known "fairy ring" mushroom. It grows during the summer and autumn in grassy places, as in lawns, by roadsides, in pastures, etc. It appears most abundantly during wet weather or following heavy rains. It is found usually in circles, or in the arc of a circle, though few scattered plants not arranged in this way often occur. The plants are 7-10 cm. high, the cap 2-4 cm. broad, and the stem 3-4 mm. in thickness.
The pileus is convex to expanded, sometimes the center elevated, fleshy, rather thin, tough, smooth, buff color, or tawny or reddish, in age, or in drying, paler. When moist the pileus may be striate on the margin. The gills are broad, free or adnexed, rounded near the stem, white or dull yellowish. The spores are elliptical, 7-8 µ long. The stem is tough, solid, whitish.
Marasmius oreades. Caps buff, tawny, or reddish.
This widely distributed fungus is much prized everywhere by those who know it. It is not the only fungus which appears in rings, so that this habit is not peculiar to this plant. Several different kinds are known to appear in rings at times. The appearance of the fungus in rings is due to the mode of growth of the mycelium or spawn in the soil.
Having started at a given spot the mycelium consumes the food material in the soil suitable for it, and the plants for the first year appear in a group. In the center of this spot the mycelium, having consumed all the available food, probably dies after producing the crop of mushrooms. But around the edge of the spot the mycelium or spawn still exists, and at the beginning of the next season it starts into growth and feeds on the available food in a zone surrounding the spot where it grew the previous year. This second year, then, the plants appear in a small ring. So in succeeding years it advances outward, the ring each year becoming larger. Where the plants appear only in the arc of a circle, something has happened to check or destroy the mycelium in the remaining arc of the circle.
It has been noted by several observers that the grass in the ring occupied by the mushrooms is often greener than that adjoining. This is perhaps due to some stimulus exerted by the mycelium of the fungus on the grass, or possibly the mycelium may in some way make certain foods available for the grass which gives an additional supply to it at this point.
Fig. 129 is from plants (No. 5503, C. U. herbarium) collected in a lawn, October 25, 1900, Ithaca.
Illustrations of some fine large rings formed by this fungus appeared in circular No. 13 by Mr. Coville, of the Division of Botany in the U. S. Dept. Agr.