ABSTRACT
The fungi constitute a large, heterogeneous, and ubiquitous group of organisms characterized by a filamentous, multinuclear mycelium, which forms a system effectively exploiting the resources of the substratum. The walls of the hyphae, remaining in direct contact with the substratum, exert an important function in a free-living fungus. They contain a set of enzymes able to degrade extracellular nutrients, making them available for absorption. They may produce organic acids, which can take part in the transformation of insoluble salts of needed elements. The narrow, filamentous nature of fungi allows them to exploit tiny soil pores and the surface of soil aggregates not accessible for plant root hairs. Hyphae are also important for long-distance transport of absorbed elements, especially when symbiotically associated to plants as in mycorrhizas or forming lichens with algae or cyanobacteria.
