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. RELATION OF FUNGI TO SOIL PROCESSES. The great cycle of changes occurring in the soil whereby organic matter is gradually transformed and again made available as plant food is entirely dependent upon micro-organisms. Until a decade ago it was thought that bacteria were by far the most important group concerned in the bringing about of these changes, but recent studies have shown that, in at all events certain arcs of this great organic cycle, the fungi have, perhaps, an equal part to play. The life of fungi in the soil may, for our purposes, be considered from three points of view--their part in the decomposition of carbon compounds, their nitrogen relationships, and their work in the mineral transformations of the soil. CARBON RELATIONSHIPS.