The mycelium mostly grows in soils rich in humus or dung, on decaying trees and similar objects. Many species, _e.g. Tricholoma personatum_ and _Marasmius oreades_, form the so-called “fairy rings.” The fruit-bodies in these species are confined to a larger or smaller surface on which they are very regularly arranged in a ring. The reason for this is found in the radial growth of the mycelium, so that the oldest portion, or the starting point, is found at the centre of the ring, and the younger ones, on which the fruit-bodies are formed, at the circumference. The older hyphæ gradually die, and at the same time, the radial growth continuing, the ring of fruit-bodies becomes larger and larger. The “fairy-rings” are marked not only by the fruit-bodies, but also by the more vigorous growth and darker colour of the grass upon these spots. Some species are _parasites_. An example is presented by _Armillaria mellea_, a remarkable and very destructive Fungus in woods and forests (Figs. 176, 177). ~In addition to the filamentous, white mycelium, it has also black, or black-brown, horny, root-like mycelium-strands (rhizomorpha) which were formerly considered to belong to a special genus of Fungi described under the name “_Rhizomorpha_.” The mycelium lives parasitically on the Conifers and other trees, forcing its hyphæ into the bark and between the bark and wood, and thence penetrating into the wood so that the tree is very severely attacked. It may also live saprophytically, and clusters of fruit-bodies are often found on old stumps and stems, on old timber, and in the rich soil of woods. The rhizomorpha, living underground, can extend for considerable distances and infect the roots of neighbouring trees, and spreads in this way the diseases known as “Harzsticken” and “Bark-Canker,” which are very destructive to young trees.~ [Illustration: FIG. 174.--_Psalliota campestris. A_ Tangential section of pileus showing lamellæ (_l_). _B_ Portion of gill more highly magnified; _t_ trama; _hy_ hymenium with basidia and basidiospores; _sh_, subhymenial layer. _C_ A portion of the same more highly magnified; _s′ s′′ s′′′ s′′′′_ various stages in the development of basidiospores; _q_ paraphyses.] The chief characteristics by which the numerous genera are separated are the presence or the absence of the two kinds of veils, the nature of the fruit-body, the form, branching of the lamellæ, and their position and relation with respect to the stem, the shape of the pileus, the colour of the spores, etc., etc. A knowledge of the colour may be obtained by placing the pileus with the lamellæ turned downwards on a piece of white or coloured paper, so that the spores, as they fall off, are collected on the paper, and the arrangement of the lamellæ can then be clearly seen. [Illustration: FIG. 175.--_Cantharellus cibarius_ (reduced).]