Within this single, yet not particularly large, group of fungi, several biological phenomena are demonstrable. There is good evidence that the majority of British boletes are mycorrhizal; several species are known to be associated only with one species of tree or group of closely related tree-species. Thus _Suillus grevillei_ and _S. aeruginascens_ (Secretan) Singer grow in association with larch trees; _S. luteus_ and _Boletus badius_ in contrast grow in association with pine trees; _Leccinum scabrum_ with birch trees; _L. aurantiacum_ (Fries) S. F. Gray, with poplar trees and _L. quercinum_ (Pilát) Green & Watling, with oak trees. _Boletus edulis_ can be separated into several distinct subspecies which are associated with different trees; the two commonest subspecies are those associated with birch and with beech trees. It is well known that although present in this country during the warmer periods of the Ice-Age, larch neither survived the intense cold of the last advance of the ice nor migrated back into Britain after the ice had melted. Thus all larches which we see in Britain have been planted by man. There is little doubt that mycelia of many fungi were introduced along with these plants very probably including the mycelium of the larch-bolete. A similar pattern can be seen with other introduced trees, although not to such a marked degree, e.g. spruce trees. The beech tree, however, is native to the south of England, unlike the larch returning to this country after the ice had melted; it has been planted extensively outside its former range in northern areas of the British Isles taking with it its associated fungi. There is some evidence that some stocks of beech and fungi have been introduced from continental Europe in comparatively recent times. A parallel, yet inexplicable association is found between the bolete _Suillus bovinus_ (Fries) O. Kuntze and its close relative _Gomphidius roseus_ (Fries) Karsten where the mycelium of two fungi are found intertwined forming a close association! Parasitism although rare is also found amongst the boletes, and an uncommon parasitism at that--a fungus on a fungus; for example in Britain although infrequent _Boletus parasiticus_ Fries grows attached and ultimately replaces the spore-tissue of the common earth-ball (_Scleroderma_, see p. 192). Those fungi which grow on dead and decaying substrates are called saprophytes and although the greater number of higher fungi would be included in this class of organisms the character is infrequent amongst the boletes. One British example of this type of fungus is the rare _Boletus sphaerocephalus_ Barla which grows on woody debris.