(ectophytic mycorrhiza) A form of mycorrhizal association in which there is a well developed mycelium forming a mantle on the outside of the root. Such associations are found on most trees and often a tree will not grow properly unless the appropriate fungus is present. It is thus a common practice when planting young trees to inoculate the soil with mycorrhizal fungi. These are frequently members of the * Agaricales. Infected roots commonly show a characteristic coralloid form of growth in which the lateral roots fail to elongate and instead branch repeatedly forming a swollen mass. Initially some of the hyphae penetrate the cortex and form an intercellular mesh-work termed the Hartig net. The outer mantle, which replaces the piliferous layer of the root, subsequently arises from this net. It has been demonstrated that mycorrhizal roots take up nutrients better than uninfected roots, while the fungi obtain carbohydrates and possibly B-group vitamins from the tree. See also endotrophic mycorrhiza , vesicular-arbuscular mycorrhiza. |
