Article Source ~ Science Direct
Nicholas P. Money, in The Fungi (Third Edition), 2016
Cords and Rhizomorphs
The use of the term ‘organ’ to describe the multicellular structures of fungi is subjective. Animal organs are differentiated structures that perform specific functions. A mycelium may be viewed as an organ by this definition, but the degree of differentiation among its hyphae is very limited. A higher degree of differentiation is achieved through the formation of cords by mycelia of wood-decay and ectomycorrhizal fungi and it seems justifiable to describe the more complex of these structures as organs. We will return to this issue of terminology when we consider the structure and function of fruit bodies later in the chapter.
Strands, cords, and rhizomorphs vary in complexity from bundles of hyphae, whose cell walls adhere to one another to produce slender cylinders of unpigmented cells on the surface of a culture dish, to fat pipes with a diameter of a few centimetres that can extend for hundreds of metres. The larger pipes are formed from hundreds of thousands of hyphae, develop a complex internal anatomy, and are sometimes tipped with a rounded, mucilaginous cap. Variations in the internal structure of these organs make it difficult to discriminate between strands, cords, and rhizomorphs. The term rhizomorph may be useful to designate the larger of these invasive organs that have an identifiable tip that pushes through the soil.
Cord is the preferred term for other linear organs without an organised tip.