ABSTRACT
Endosymbiosis is an important process of biological evolution where simple cells combine to produce more complex cells. Through this process major evolutionary change has occurred giving rise to the eukaryotic kingdoms of organisms from prokaryotes. The process of endosymbiosis is ongoing and occurs within the fungi. The major groups of terrestrial fungi (Ascomycetes, Basidiomycetes, and Zygomycetes) are believed to have evolved from an aquatic chytridiomycete that associated with early terrestrial plants, either as a saprophyte or as a parasite of living plants (Pirozynski and Malloch, 1975). The earliest land plant fossils contain fungal remains that may be interpreted as chytridiomycetous and zygomycetous in nature (Kidston and Lang, 1921; see Chap. 2 in this volume). These Early Devonian fungal remains have been interpreted as endosymbionts (Pirozynski and Malloch, 1975), but are probably saprophytes due to the degraded condition of host tissues in which mycelium is encountered (Kidston and Lang, 1921). It is evident that some time after plants colonized land, symbiotic associations began to develop between plants and terrestrial fungal groups. Fossils containing roots from the Triassic were found to contain fungi (Zygomycetes) of the ecologically important symbiotic association, known as arbuscular mycorrhizae (Stub-
50 White et al.
