The Encyclopedia Americana (1920)/Mycorriza
MYCORRIZA is a general name for a group of subterranean fungi that spread their growing, feeding part (mycelium) through the loose damp soil (humus) of the floor of a forest or wood-lot. They are symbiotic with a great variety and number of green-leaved plants, including forest trees of several families. The first root of one of these affected plants, just born from the seed, remains free, but as fast as the lateral roots push out they are enveloped in the mycelium of the fungus, and their ramifications become entangled in it as they spread, and this connection continues throughout life. Wherever a root or rootlet extends underground the fungus accompanies it. The mycelial filaments may form a dark-brown, felt-like coating over the whole surface of the root, or, in other cases, depending on the species, may make a net-work of spider-web-like mesh. At various points the spore-producing hyphæ proceed outward through the soil from the mycelium; they look like rootlets and seem to perform a similar service. This mycelium (called “spawn” by mushroom growers) represents an unknown number and variety of species of fungus, and it abounds in the humus of forests and uncultivated heaths where the top-soil is the result of vegetable decay. The coating of roots by this mycelium is to be seen wherever plants grow, but it does not affect every kind of plant. It is restricted to the flowering plants, among them all the Pyrolaceæ, Vaccineæ (whortleberries), and Arbutae, and most if not all the heath family (Ericaceæ), rhododendrons, daphnoids, a great number of conifers and all the Cupuliferæ — a group that includes the oak, beech, alder, chestnut and many other forest trees; also many isolated trees, as willows and poplars, and the genista and several other familiar garden herbs.
This association of the mycorrhiza with the flowering plants is in each case a partnership of mutual benefit. Covering the root with a mantle it prevents, it is true, the absorbing pores from performing their function of withdrawing from the soil the dissolved elements that constitute the food of the plant, but in compensation the fungus takes on itself this function, absorbing the required nutriment and delivering it to the plant. It is enabled to do this because the mycelium has the power of secreting “those special enzymes, or ferments which render soluble the organic ingredients they touch.” In return for this service it receives from the plant-root nourishment for itself, which has been elaborated in the leaves and distributed to every part of the plant, including its uttermost rootlet. This, indeed, is only following the custom of all fungi, which, having no chlorophyl with which to derive nourishment from the air through chemical dissolution and recombination, must get it from vegetable sources. This association of the mycorrhiza with plant-roots is therefore really a parasitism, yet it seems not only harmless but decidedly beneficial — in fact, those plants participating in this association will not do well, and perhaps will fail to grow at all, in a soil wholly free from suitable mycorrhiza. Hence the special value of the addition of wood-mold to garden or window-pot soil is that it brings with it this advantageous, and sometimes indispensable, mycorrhiza mycelium.