Fruiting Body Forms
Mushrooms come in many shapes, including coralloid forms, bracket fungi, puffballs, and crustlike resupinate forms (Figs. 5-6). Gross morphology of fruiting bodies was the basis for the nineteenth-century classification of Agaricomycetes and other Fungi by Elias Fries (1874). The Friesian taxa are no longer regarded as natural entities (monophyletic groups, also called clades), but these groupings remain useful for categorizing fruiting body forms and they are emphasized in many useful field guides (e.g., Barron 1999; Bessette et al. 1997; Phillips 1991). Fries made a basic distinction between those fungi that produce their spores internally, which he called Gasteromycetes, and those that produce their spores externally, which he called Hymenomycetes. Gasteroid forms are now understood to have evolved repeatedly from hymenomycetes (note the lower-case "h", indicating that this is an informal, descriptive term referring to mushrooms with external spore-bearing structures, rather than a formal name for a taxon). The most common gasteroid forms are puffballs (Fig. 5) and false-truffles (tuberlike fruiting bodies that are formed underground). Puffballs and false truffles have evolved independently in multiple clades of Agaricomycetes. Uniquely-evolved gasteroid forms include bird's nest fungi, stinkhorns, earthstars (see the title illustration), and the "cannon-ball fungus", Sphaerobolus stellatus (Brodie 1975, Coker and Couch 1928, Miller and Miller 1988).
In some hymenomycetes, the hymenium is initially formed inside the fruiting body, but later becomes exposed as the cap expands (Fig. 5). It has been suggested that a developmental arrest in such forms could lead to the evolution of gasteroid forms (Bruns et al 1989; Hibbett et al. 1994b, Thiers 1984).
Figure 5. The hymenium in the hymenomycete Agaricus (left, © D. Hibbett) is initially concealed by veils. Several groups of puffballs, including the giant puffball Calvatia (right, © M. Binder), are closely related to Agaricus and may have been derived by paedomorphosis.
The Friesian classification divided the "Hymenomycetes" according to the configuration of the spore-bearing surfaces (the hymenophore). Thus, all gilled forms were placed in the Agaricaceae (Fig. 5, left), all poroid forms were placed in the Polyporaceae, all toothed forms were placed in the Hydnaceae, and so on. The taxa just named are still recognized in modern taxonomy, but in much more limited scope than the original Friesian concepts. It is now well understood that the hymenophore types that Fries used to define his taxa have evolved over and over (Hibbett 2007). Thus, there are non-gilled forms in the Agaricaceae (Fig. 5), as well as many gilled forms outside of the Agaricaeae. Commonly used terms like agaricoid, polyporoid, hydnoid, and gasteroid, are derived from the names of the Friesian taxa, but they are purely descriptive and refer to polyphyletic groups (Fig. 6).
Figure 6. Fruiting body diversity in Agaricomycetes. From top left: Auriscalpium vulgare (Russulales), with a toothed or hydnoid hymenophore © Taylor F. Lockwood; Fomitopsis pinicola (Polyporales), a poroid bracket fungus © Mike Wood; Phlebia chrysocrea (Polyporales), a resupinate form © D. Hibbett, Ramaria botrytis (Phallomycetidae), a coralloid form © Taylor F. Lockwood. The Auriscalpium, Fomitopsis and Ramaria images are from MykoWeb and FUNGIPHOTO.COM; used with permission.
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