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the symbiotic association of the mycelium of a fungus with the roots of a higher plant. The association is referred to as ectotrophic (external) in those cases in which the fungus entwines the ground tissue of the ends of young roots and penetrates the intercellular spaces of the outer layers of bark. Endotrophic (internal) mycorrhiza is characterized by the implantation of mycelium (the hyphae of the fungus) inside the cells of a plant. Ectotrophic mycorrhiza characterizes many trees (oak, spruce, pine, birch) and shrubs (willow), as well as some low shrubs (Dryas) and herbaceous plants (viviparous bistort). The young roots usually branch and their ends thicken; the growing parts of the roots are enveloped by the thick, solid fungal sheath, from which emanate the fungal hyphae. The hyphae penetrate the soil and the intercellular spaces of the root through one or more layers of bark, forming the Hartig’s network. As a result, the root hairs atrophy (euectotrophic mycorrhiza). In the low shrub Arctous alpina and the herb Pyrola rotundifolia, the fungal hyphae penetrate not only the intercellular spaces but the cells of the bark as well (ectoendotrophic mycorrhiza). Ectotrophic mycorrhizae are formed most often by hymeomycetes (Boletus, Lactarius, Russula, Amanita) or, less frequently, by gasteromycetes. More than one species of fungus may participate in the formation of the mycorrhizae on the roots of a single plant. However, as a rule, in plant communities only certain mycorrhizal fungi are symbionts of given species of plants.
Endotrophic mycorrhizae do not cause a change in the shape of the roots, and the root hairs generally do not atrophy; the fungal sheath and Hartig’s network do not form, and the fungal hyphae penetrate the cells of the cortical parenchyma. In plants of the families Ericaceae, Pyrolaceae, Vacciniaceae, and Empetraceae, the fungal hyphae form nodules in the cells, and these nodules are digested later by the plant (ericoid mycorrhiza). Phycomycetes of the genera Endogone and Pythium participate in the formation of ericoid mycorrhizae. In plants of the family Orchidaceae, the fungal hyphae from the soil penetrate the seed and form nodules, which are then digested by cells of the seed. This type of mycorrhiza is characteristic of imperfect fungi (genus Rhizoctonia) and, less frequently, basidial fungi (genus Armillaria).
The phycomycetous type of mycorrhiza, in which the fungal hyphae permeate the epidermal cells of the root and localize in the intercellular spaces and in the cells of the middle layers of the cortical parenchyma, is the most widespread. It occurs in many annual and perennial herbs, shrubs, and trees of the most diverse families.
Mycorrhizal associations are beneficial to plants. The developed mycelium increases the absorptive surface of the root and intensifies the entry of water and nutrients into the plant. Mycorrhizal fungi probably decompose certain organic compounds of the soil that are not accessible to the plant and manufacture vitamin-like substances and growth activators. The fungi, on the other hand, use certain substances (possibly carbohydrates) that they extract from the roots of the plant. When forests are planted on soils that do not contain mycorrhizal fungi, small quantities of forest soils are applied; for example, when planting acorns, soil from an old oak grove should be added.
REFERENCESKursanov, L. I. Mikologiia, 2nd ed. Moscow, 1940.
Shemakhanova, N. M. Mikotrofiia drevesnykh porod. Moscow, 1962.
Lobanov, N. V. Mikotrofnost’ drevesnykh rastenii, 2nd ed. Moscow, 1971.
Katenin, A. E. Mikoriza rastenii Severn- Vostoka Evropeiskoi chasti SSSR. Leningrad, 1972.
B. P. VASIL’KOV