This family is very rich in species (more than 8000 have been described), and to it belong all the “Mushrooms” and “Toadstools.” The _fruit-bodies_ present very various forms; they are generally fleshy, very perishable, seldom leathery or corky, in the last case often perennial. The _basidia_ are more or less _cylindrical_ and bear _generally_ 4 (seldom 2, 6 or 8) _sterigmata and basidiospores_. The hymenium in the fully-formed fruit-bodies lies free on the surface: in orders 1 and 2 and a portion of order 6 it is from the commencement exposed, fruit-bodies _gymnocarpic_; orders 3–6 have _hemiangiocarpic_ fruit-bodies (p. 157). In the first order the basidia (or the hymenium) are developed immediately from the mycelium (Fig. 163); the fruit-bodies of orders 2 and 3 present a higher grade of development, and have between the mycelium and hymenium a special hyphal-tissue, a _stroma_, which is crustaceous, club-like, or coralloid, etc., and in general bears the hymenium on the largest part of the free, smooth surface. In the forms most highly developed (orders 4–6) a new tissue--the _hymenophore_--is introduced between the stroma and hymenium, which appears on the under side of the fruit-body in the form of warts, projections, tubes, folds or lamellæ (Figs. 166, 167, 174 _bc_). _Paraphyses_ are frequently found in the hymenium, among the basidia. In the Hymenomycetes few examples of _conidia_ can be recognised at first. More frequently _chlamydospores_ are found, particularly _oidia_. The _mycelium_ is richly branched, generally colourless, often perennial; it lives in humus or decaying wood, and is seldom parasitic. The hyphæ generally have clamp-connections and unite, sometimes, to form a rhizomorpha (Fig. 177) or sclerotia with coloured, pseudo-parenchymatous covering. [Illustration: FIG. 163.--_Exobasidium vaccinii._ I Hypertrophied stem of _Vaccinium vitis idæa_; II leaf with gall-like swelling; III section of II; IV transverse section: _m_ mycelium between the parenchymatous cells; _p_ hypodermal cells; _e_ epidermis with basidia in various stages of development; V epidermis with germinating spores; VI and VII spores germinating in water (IV-VII × 620).] Order 1. =Tomentellaceæ.= To this order belong the simplest of the Hymenomycetes. The basidia (Fig. 145) arise free and irregularly from the mycelium; a _hymenium_ is _entirely absent_ or _very slightly formed_ (in _Corticium_ it attains its highest development); _fruit-bodies_ are _also wanting_.--In general they form flaky, membranous or leathery coverings on bark and wood. Some are parasites.