HYPOGEOUS FUNGI
Authors: HAWKER LILIAN E. 
Published: January 2008

Hypogeous fungi are those soil fungi which produce macroscopic fruit‐bodies partially or completely embedded in soil or humus. While showing a superficial similarity correlated with habitat, they include members of the Basidiomycetes, Ascomycetes and Phycomycetes.

The edible truffles have been known from very early times, and speculations as to their nature are found in Greek and Roman literature. Other groups, which are not edible, were described later. The monographs of Vittadini (1831, 1842) and L. R. & C. Tulasne (1851) are the starting‐point for all modern work on these fungi.

The true truffles and various related forms are included in the Tuberales and show obvious relationship with the Pezizales. Young fruit‐bodies of the more complex species, such as Tuber, resemble mature ones of the simpler forms, such as Gyrocratera, or of the Pezizales. The group is best divided into four subfamilies (Knapp, 1950‐2): Pseudotuberaceae, Geneaceae, Eutuberaceae and Terfeziaceae.

The hart's truffle, Elaphomyces granulatus, has long been known and was valued as an aphrodisiac. Together with other species of the genus Elaphomyces, the related Ascoscleroderma and the Australian Mesophelia, it is included by Dodge (1929) in the family Elaphomycetaceae of the Plectascales.

The hypogeous Basidiomycetes, which were formerly grouped in the family Hypogeous fungi Hymenogastrales (or Hymenogastraceae of some authors), have not yet been satisfactorily classified owing to lack of detailed knowledge of fruit‐body development for all except a few species. It is clear, however, that they do not form a single homogeneous group. They may be provisionally divided into the groups Proto‐gastraceae, Hysterangiaceae, Rhizopogonaceae (or Melanogastraceae), Hymenogastraceae and Hydnangiaceae. The epigeous Secotiaceae probably provides a link with the Agaricaceae, and relationships have been traced between the Hydnangiaceae and the Russula‐Lactarius group of agarics and between the Hysterangiaceae and the epigeous gasteromycetous group of the Phallales.

The genus Endogone formerly included species producing zygospores, chlamydospores or sporangia in definite fruit‐bodies. The method of zygospore production is such that there is no doubt that the zygosporic species are best classified as a separate family, the Endogonaceae, within the Mucorales. The chlamydosporic species are of similar general structure and are probably related to the zygosporic forms, since Thaxter (1922) demonstrated the presence of both types of spore within the same fruit‐bodies of E.fasciculata. Cultural studies by Kanouse (1936) showed that a zygosporic species (E. sphagnophila) produced sporangia resembling those of Mucor. Kanouse suggested that the forms producing sporangia aggregated in loose hyphal wefts, formerly attributed to Endogone, are in fact more closely allied to Mortierella and should be transferred to the new genus Modicella and excluded from the Endogonaceae. Some other little‐known genera producing chlamydospores aggregated in definite fruit‐bodies are included in Endogonaceae by Thaxter.

Most hypogeous fungi are found only in woodlands. Fruit‐body formation is, in general, favoured by a light, well‐drained soil, with a slightly alkaline hydrogen‐ion concentration, although a few species occur in acid soils. Growth is inhibited or checked by extreme cold or by drought.

Considerable circumstantial evidence exists for a mycorrhizal association between many, but not all, species of hypogeous fungi and trees or other plants. In a few examples there is definite proof. Other species are probably true parasites on roots of higher plants.

Very few species have been isolated in artificial culture and very little is known of their nutritional requirements. Those which have been investigated resemble other mycorrhizal species in requiring organic nitrogen and vitamin B1.

Attempts have been made, particularly in France, to stimulate the development of edible truffles in known truffle‐grounds and to establish them in new places. Some success has been claimed by inoculation of the soil of young tree plantations both with soil from known truffle grounds and with pieces of mature fruit‐bodies.

No attempts to produce mycelium by germinating spores of hypogeous fungi have so far been successful. Various authors have depicted what they claim to be early stages of germination, some of which have been shown to be in error.