The word "mycorrhiza" is built from classical Greek roots. Myco means "fungus" and rhiza means "root," so the word mycorrhiza literally means "fungus-root." When the hyphae of certain fungi form specialized sheaths around the roots of certain plants, that fungal root-coating is the mycorrhiza.
Having mycorrhiza on its roots improves a plant's ability to deal with droughts, to acquire mineral nutrients, to store carbohydrates, and more. These benefits are further discussed below.
In the image at the right, the thicker pine-tree root is covered with mycorrhiza while the slender root at the right is not. In the drawing below you see a cross section of a root with mycorrhiza. The fungal sheath, which constitutes the main part of mycorrhiza, is a mass of hyphae encasing the root. Notice that some hyphae penetrate between the root's outer cells, but they do not invade the cells themselves. Also note the hypha growing away from the root, thus giving the mycorrhiza more surface area for absorbing water and nutrients.
Mycorrhiza forming an exterior sheath on roots are called ectomycorrhizae. Some trees with ectomycorrhizae are species of pine, oak, beech, spruce, maple, juniper, willow and elm. Mycorrhiza with cells not forming a sheath but rather living deep inside the root are endomycorrhizae. Plants with endomycorrhizae include the legumes, grasses, tomatoes, apples, strawberries and peaches.
Though some plants cannot survive without mycorrhiza -- certain orchid species, for instance -- most plants can get along without it. Mycorrhiza simply improves their ability to survive. The hyphae of many kinds of fungus, including some of our most common mushroom producers, form mycorrhiza. Some fungus species form mycorrhiza on the roots of a broad range of plant species, while others "infect" only a few. Similarly, some plants can have any of several fungus species forming the mycorrhiza on their roots. Douglas Firs form mutually helpful (symbiotic) relationships with around 2000 fungal species!
Here are some specific ways in which mycorrhizae help plants: