Relation of Soil Fungi to Living Plants.
Older classifications of fungi frequently divided these organisms into four categories—parasites, saprophytes, facultative parasites, and facultative saprophytes, but the further mycological studies are carried the more clearly it is seen that these groups are entirely artificial. There are probably few fungi that cannot, under particular conditions, invade living tissues, and it only seems a question of time before at all events the vast majority of fungi will be grown on synthetic media in the laboratory. From our present point of view the importance of this lies in the fact that fungi living saprophytically in the soil may, given the right conditions or the presence of some particular host plant, become parasites or symbionts, and conversely well-known pathogens may live a saprophytic existence. Thus Cucumber Leaf Spot is caused by Colletotrichum oligochætum, and Bewley[3] has repeatedly isolated this fungus from glasshouse manure and refuse of various kinds. In his early studies, Butler[13] isolated many parasitic species of Pythium from Indian soils, and the presence of P. de Baryanum as a soil saprophyte has been confirmed by Bussey, Peters, and Ulrich.[11] De Bruyn[17] has recently found that most species of Phytophthora, including P. erythroseptica and P. infestans may live as saprophytes in the soil, whilst Pratt[53] has isolated from virgin lands and desert soils various fungi, which cause disease in potatoes. In 1912 Jensen[29] gave a list of twenty-three “facultative parasites” isolated from soil, and these are but a moiety of those which could be listed to-day.
Furthermore, it was shown by Frank[24] many decades ago that forest humus is not merely a mass of the remains of animals and plants, but that a considerable part of its organic substance is made up of fungus hyphæ, which ramify and penetrate in all directions. Evidence is rapidly accumulating that this is also true of most other soils containing organic matter. It is well known that many of the higher plants live in symbiotic or commensal relationship with these humus fungi, which are present in the host tissues as mycorrhiza, and further studies only serve to show the widespread and fundamental nature of this relationship. Thus many Basidiomycetes[50] (species of Tricholoma, Russula, Cortinarius, Boletus, Elaphomyces, etc.) possess a mycorrhizal relationship with various broad leaved trees, such as beech, hazel, and birch[57] and with various conifers and certain Ericales. Other Ericales show this relationship with species of the genus Phoma,[62] many orchids, with species of Rhizoctonia[2] (or Orcheomyces[10]), whilst Gastrodia elata contains Armillaria mellea.[36] Certain species of Pteridophyta and Bryophyta are also known to certain mycorrhizal fungi. Of the numerous fungi taking part in these mycorrhizal relationships, only a small number have yet been identified, but there is little doubt that perhaps the majority of these organisms must be regarded as true soil forms.[14], [45] The mycological flora of the soil thus plays an important part in the life of many higher forms of vegetation, and this relationship is a very fruitful field for study.