Over the past decade much progress has been made in an effort to determine which fungi are mycorrhizal symbionts. It is now apparent that one ectomycorrhizal or ectendomycorrhizal host is often able to form mycorrhizae with a wide variety of higher fungi. Evidence is presented which supports the hypothesis that fungal succession occurs as the host matures. In addition, fungi which are normally ectomycorrhizal may sometimes form endomycorrhizae on different hosts or under altered ecological conditions. There has also been much evolution in fruiting-body form and mode of spore discharge especially under severe montane or desert conditions. As a result patterns are emerging in the Homobasidiomycetes at the order, family, and generic level in the evolution of the mycorrhizal habit. Spore morphology, habitat, and distribution are listed and discussed in relation to the biological role of the fungi.
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