MYCORRHIZA. The roots of land plants live in soil, com peting therein for water and nutrients with a crowded population of micro-organisms. They are surrounded by a rich and varied fungus flora many members of which are potential parasites, but the majority of roots are relatively immune to such parasitic attack and continue to flourish side by side with their fungus com petitors. To the interaction of these three phenomena—the com petitive struggle for food, the capacity of many fungi to penetrate living tissues and the properties of resistance to such invasion possessed by roots in common with organisms in general—is probably due the evolution of mycorrhiza, that curious and interesting partnership between the roots of vascular plants and fungus mycelium now known to affect a vast number of the higher plants and to be of great importance in their lives.
Mycorrhizae or fungus-roots (ub/cns, a mushroom) (i4a, a root) are formed alike by wild and cultivated plants, by species from the high mountains and by those from salt marshes at sea level, in the vegetation of tropical forests as in that within the arctic circle. The phenomenon was comparatively well-known, especially to foresters when, in 1885, the German botanist, Frank, coined the new name, `mycorrhiza,' to register his own view that the root with its associated mycelium constituted an independent organ of great importance in plant nutrition. He reported an invariable and intimate association of fungus mycelium with the roots of forest trees, especially oak, beech, hornbeam and other members of Cupuliferae, and certain conifers, such infection being, in his opinion, not parasitic but of definite service to the trees in relation to the absorption of water and nutrient salts. On a contemporary view associated especially with the name of Robert Hartig, the root fungi of trees were mischievous parasites hindering root action and conferring no benefit upon the hosts.
By the end of the nineteenth century it was realized by botanists that the mycorrhizal habit was widespread, affecting not only trees but an immense number of herbaceous species belonging to different families.
Distribution and Structure.—Mycorrhiza is formed by ferns, club mosses, conifers and an ever growing list of flowering plants belonging to many different families. It is constant and conspicuous in certain groups, e.g., Orchidaceae, Ericales, Empe traceae and forest trees generally, and is invariably well developed in species lacking chlorophyll, e.g., Neottia Nidus-avis, Corallo rhiza spp. and a number of tropical orchids belonging to this class, members of the Monotropoideae, and certain species belonging to the families Gentianaceae and Burmanniaceae. Structures closely resembling the mycorrhizal tissues of living species have been described in fossils of Coal Measure age. It is specially char acteristic of the vegetation of humus soils, viz., those of moorlands, woodlands and heaths, but may be well developed in garden soil and in the most diverse situations elsewhere. It may be regarded as an annual phenomenon affecting all or a proportion only of the young absorbing roots of the current year.
It is assumed that the fungi concerned are widely distributed in soil, and with few exceptions invasion of the roots takes place from this source. The effect of infection is strikingly different from that in ordinary parasitism, there being no indication that the tissues suffer any permanent injury from the more or less extensive infestation by mycelium to which they are subjected. The external appearance of the root may be altered, as in trees, where arrest of growth accompanied by profuse branching results in the production of dense clusters of rootlets, brown or vari ously coloured according to the kind of mycelium present. Some times the appearance of the roots is not altered in any way. There is great variation in different plants and at different seasons in respect to the relative abundance of mycelium, its distribution upon and within roots and the details of structure of individual mycorrhizas.