For most Basidiomycota in terrestrial ecosystems the predominant body form is the mycelium, comprising an interconnecting series of apically extending tubes —hyphae. Hyphae provide a large surface:volume, ideal for secreting enzymes for extracellular digestion of resources (Chapter 2), and for subsequent uptake of small molecules. Mineral nutrients, carbon and energy sources are presumed to be taken up largely at hyphal tips, be they embedded within an organic resource or foraging externally for new resources, and translocated from these sources to sites of demand (sinks; Chapter 3). Nutrient acquisition and other aspects of physiology are affected by the local environment (Chapter 2), and mycelia exhibit remarkable physiological and morphological plasticity. Moreover, since mycelial activity in one region can be supported by supply of water and nutritional resources from elsewhere, growth can sometimes occur in inhospitable places and adverse conditions. The interconnectedness of mycelia is of crucial significance to the organization and ecological roles of fungi (Rayner et al., 1995).

In terrestrial ecosystems, the organic resources on which saprotrophic Basidiomycota depend are usually discrete, varying in size from small to large plant fragments, e.g. bud scales, leaves and large woody components. These resources are distributed heterogeneously in both space and time. For example, the relatively homogeneous carpet of forest floor leaf litter comprises spatially discrete leaves, input largely over a 6-8 week period in autumn by broadleaf deciduous trees, or more evenly during the year by many conifers. Branches are patchily distributed on the forest floor, falling throughout the year, though often with larger inputs following high winds. To survive saprotrophic fungi must be able to capture these discontinuously dispersed resources. Some Basidiomycota can only achieve this by production of sexual and asexual spores or sclerotia, and have been categorized as 'resource-unit-restricted', whereas 'non-resource-unit-restricted' Basidiomycota can also spread between organic resources as mycelium. Spores, although allowing rapid spread, sometimes over long distances, contain only relatively small food reserves from which to produce a mycelium for invasion of the organic resource upon which it has landed. Sclerotia often provide larger resources and also allow survival in time. Growth as mycelium, in contrast, allows the fungus to draw upon a much larger supply of nutrients.

This chapter considers mycelia growing within organic resources, and the ways in which they search and colonize them when discontinuous. It also examines the significance of network architecture, and the costs and benefits of large mycelial networks.

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