The term mycorrhiza refers to a symbiotic relationship that can occur between a fungus and the roots of a plant. This is usually a mutualistic relationship, meaning that it has a positive effect on both the plant and the fungus. In some uncommon cases, however, the relationship can have mild detrimental effects on the plant and shift the balance of the relationship in favor of the fungus. In most cases of mycorrhiza, nutrients are shared between the plant and the fungus to the benefit of both parties. It is important to note that this exchange of nutrients is bi-directional; both the plant and the fungus receive nutrients and essential inorganic minerals from each other.
Mycorrhiza can form in many different forms with varying effects. Sometimes the fungus grows around and between the roots and only superficial contact and sharing can occur. In other cases, the symbiosis occurs at a cellular level and a much deeper and more detailed level of material sharing is possible. In some cases, the fungus is largely restricted to the area directly around and between the plant's roots, while in other cases, the fungus branches out and extends deeper into the soil. The types of relationships possible tend to depend on the soil quality and on the types of fungi and plants present in a given area.
Plants can respond in several different ways to mycorrhiza. Generally, it is beneficial to the plant as it can aid the plant in growing or reproducing more abundantly. In some cases, however, it can depress the plant's growth. This occurs especially in cases in which the fungus is completely dependent on the plant for some kind of nutrient. The response of the plant mostly depends on the nutrients needed by the plant and the fungus; significant overlap can lead to one of the two lacking the nutrients needed to flourish.
There are three primary classifications for mycorrhiza. Endomycorrhiza is a broad class that generally describes a relationship in which parts of the fungal cells penetrate the cellular membranes of some of the plant cells. In ectomycorrhiza, fungal cells form a structure that resembles a net over the root tips on the plant. Ericoid mycorrhiza, the third major classification for mycorrhiza, describes a relationship in which fungal cells coil in and around the other layer of cells in the plant's root. Generally, the fungi involved in ericoid mycorrhiza do not extend very far into the surrounding soil.