Mycorrhizae:
Some soil fungi form mycorrhizae (intimate associations between fungi and roots of plants). The fungi acts as additional roots, helping the plant acquire nutrients. Probably, most plants have mycorrhizae, but some mycorrhizae associations, such as those between basidiomycete and beech, birch and pine trees, are particularly abundant in forest soils of temperate regions. The fungal mycelium penetrates the outermost layers of the tree root, but most of the mycelium remains just outside the root forming a sheath that can be up to 40mm thick. Most mycorrhizae forming fungi can not be cultivated in the absence of the plant with which they normally associate. Presumably, the plant supplies some essential nutrients to its fungal partner. The plant can survive without the fungus, but not very well. Such plants are yellow and stunted when grown without mycorrhizae are added they become vigorous and deep green. The fungus supplies the plant with mineral nutrients that are usually in short supply in forest soils.
Some orchids are completely dependent on their mycorrhizae partners. The fungus supplies them with organic growth factors as well as mineral nutrients. These orchids will grow with our mycorrhizae fungi if supplied with certain organic nutrients, including vitamins.
