Although ericoid mycorrhiza are mainly restricted to one order, the Ericales, other kinds such as the ectomycorrhiza and vesicular-arbuscu-lar mycorrhiza are not restricted in the same way to a taxonomic group. Vesicular-arbuscular mycorrhiza are found widely in pteridophytes, gymnosperms and angiosperms of all growth habits. Ectomycorrhiza are mainly restricted to arborescent plants of angiosperms and gymnosperms but not to particular taxonomic groups. Some of these can also develop vesicular-arbuscular mycorrhiza, e.g. some Rosaceae and some Salica-ceae can be either ectomycorrhizal or vesicular-arbuscular mycorrhizal, or both.
Turning to actual specificity; species of host to species of fungus—this is extremely loose. Amongst vesicular-arbuscular fungi (Endogonaceae) most have been found to form functional mycorrhiza with many species of host and few are restricted. More is known of the ectomycorrhizal fungi, for they can be tested in culture. Very few are genus-specific, some are restricted to gymnosperms, some to a single family, but most are of very wide host range. These matters, which clearly have important implications for the efficiency of functioning and of ecological behaviour, are discussed by Harley and Smith, S. E. (1983) and will be considered briefly below. It should be noted that the low specificity poses difficulties in the understanding of the possible fungus-to-host recognition mechanism.
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