Answer
Mycorrhizae greatly improve plants’ ability to absorb soil mineral nutrients which are required for growth and development today, and this would have been especially important for the first land plants which lacked true roots and were therefore even less able to absorb soil nutrients. The evidence for early mycorrhizal associations comes from the fossil record of some of the earliest land plants and also from genetic work in which the transfer of genes for mycorrhiza formation from primitive liverworts to advanced flowering plants allowed the latter to form mycorrhizae, showing that those genes must have been present in the most primitive of land plants.
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This question is really asking you to summarize the material on p.654 about the history of mycorrhiza formation.
