Two other types of mycorrhizae are found in the heather and orchid families. In heather (family Ericaceae), the fungus secretes enzymes into the soil that convert materials, particularly nitrogen-containing compounds, into forms that can be taken up more readily. In orchids (family Orchi-daceae), the seeds contain a mycorrhizal fungus that is required for seed germination. Within the seed, the hyphae absorb stored carbohydrates and transfer them to the plant embryo.
Some plants, such as those of the mustard family (Brassicaceae) and the sedge family (Cyperaceae), lack mycorrhizae. In addition, most plants growing in flooded soils (or under hydroponics) do not form mycorrhizae nor do plants grown where conditions are extremely dry or saline. Also, plants growing in very fertile (i.e., nutrient-rich soils) have less-developed mycor-rhizae compared to plants growing in nutrient-poor soils.
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