MYCORRHIZA

cross-section of root with ectomycorrhiza, a form of mycorrhiza 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 belowcross section of root showing ectomycorrhiza, a kind of mycorrhiza 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:

MYCORRHIZA SOCIALISM

In the early 1990s mycologist Suzanne Simard and her team at Oregon State University discovered that cobwebby networks of mycorrhiza could connect not only many trees of the same species but also trees of different species. They encountered birch connected to fir trees by up to ten different species of fungi. Moreover, birch trees growing in bright sunlight seemed to be subsidizing fir trees in the shade by sharing sugars via their mycorrhiza network.

  • Ectomycorrhizae can interconnect different host plants: When mycorrhizal hyphae from different host plants -- even from different host-plant species --  meet as they travel through the soil, often they fuse, so that a flow of carbohydrates and other nutrients can move from one mycorrhizal system into another, and even from one host plant to another. A dominant tree, then, by way of the network of fused mycorrhizal hyphae below it, may provide nutrients to tree seedlings, shrubs and herbs in its shade, thus diversifying and enriching the local ecosystem.