Mycorrhiza

It is still convenient to recognize two main types of structure and use the descriptive names given to them by Frank in 1885; the ectotrophic type, specially characteristic of trees, in which mycelium completely invests the tip and younger portion of the root as with a sheath of varying thickness, branches from which penetrate between the cortical cells to form a continuous net work, while individual cells remain relatively immune from infec tion; and the endotrop/tic type, showing a variable amount of mycelium externally without formation of a sheath, combined with a more or less profuse distribution of hyphae within the cells, sometimes irregularly distributed in the cortical tissues, sometimes localized in definite zones. The individual cells that suffer invasion become filled with coiled or branched hyphae continuous with those which form an irregular mesh upon the external surface of the root and permeate the soil around it. In a majority of fungus-roots these intracellular hyphal com plexes disintegrate, dwindle and disappear, the soluble by products presumably passing to the host cells.

These two types of mycorrhiza are not sharply distinguished from one another as Frank believed. It is known now that those of the ectotrophic type often exhibit relatively heavy infection of individual cells, and it seems probable that the structural differences are related to the character of the mycelium and to soil conditions rather than to any fundamental distinction in the association as a whole.

The Mycorrhiza of Orchids.

The mycorrhiza of orchids is of the endotrophic type and mycelium may be widely distributed throughout the root cortex or confined to certain regions. Two contrasting types of cell are observable : Pilzwirthzellen or "host cells," containing active hyphae, and Verdauungzellen or "diges tion-cells," enclosing only their disintegrated remains subsequent to digestion. In the former the mycelium fills the cells with skeins or "pelotons" of characteristic appearance and probably abstracts nutrient materials from the host; in the latter these substances and possibly others brought in from the soil are placed at the disposal of the root.

The researches of a French botanist, Bernard, initiated a new method of experimental inquiry and demonstrated the far-reaching character of the relationship in orchids. The difficulties experi enced by growers in germinating seeds of orchids had long been known. Bernard isolated the root fungi from a number of species and showed, by "pure culture" methods, that germination of seed and development of the seedling are bound up with infec tion by the appropriate root fungus at a critical stage of de velopment. In nature, this symbiotic type of germination is en sured by the presence of mycelium of the root fungus in the soil about the roots of the parent plant ; in horticultural practice, artificial cultures have been successfully utilized by L. Knudson

for the same purpose. It has been discovered that the stimulus normally provided by the appropriate fungus can be replaced artificially by carefully adjusting the chemical constitution and reaction of the seedbed, and such asymbiotic methods of secur ing germination have been successfully applied by horticulturalists in the case of certain orchid species_ There remains much that is obscure in the relation of fungus and host in this group of plants. The nature of the problems awaiting solution is sufficiently indicated by the invariable asso ciation of particular fungus strains with individual orchid species, and by the fact that orchid growing is concerned not only with the cultivation of natural species but with the raising of new hybrids.

Mycorrhiza in Ericaceae.

In northern Europe certain mem hers of the family Ericaceae, e.g., various kinds of heath (Erica spp.), and heather or ling (Calluna vulgaris) are a constant fea ture of the vegetation of humus soils—moorland and woodland— on which they often monopolize large areas to the almost complete exclusion of other plants.

Described by Frank as obligate mycorrhiza plants, it was early recognized that members of this group might exhibit specialized relations with their root fungi. The mycorrhiza is of the endo trophic type with certain characteristic features, in some respects showing points of resemblance with that of trees. The young roots are excessively fine, the outermost layer of cells in each forming a definite mycorrhizal zone. In favourable soil conditions prac tically every cell in this layer is filled with a densely branched complex of fungal hyphae from which strands of mycelium extend outwards into the soil. The growth of mycelium on the outside of the roots is rather more profuse than in orchids, and, in certain species, may resemble the sheath-like condition found in ecto trophic mycorrhizas. As in orchids, the complexes of mycelium within the root cells are subject to rapid and complete digestion followed by disappearance of the resulting products.

In ling (Calluna vulgaris) and certain allied species an obligate relation unique of its kind has been revealed by experimental research. In these plants the mycelium which infests the root spreads throughout the tissues of the vegetative shoots, flowers and fruits reaching eventually the seed-coats of the developing seeds. These are shed bearing with them the mycelium of the fungus partner and at germination the emerging seedling is im mediately subject to invasion.