Although a rather confused area, due to the use of a number of terms by different authors, attempts have recently been made to clarify the terminology relating to linear mycelial organs. In the ectomycorrhizal context the terms used over the years have included "hyphal bundle" (e.g. Masui, 1926), "mycelial cord" (e.g. Dell et al., 1989), "rhizomorph" (e.g. Agerer, 1988) and "strand" (e.g. Foster, 1981). It is probably fair to say, however, that "strand", either unqualified (e.g. Garbaye and Bowen, 1989), or qualified by "fungal" (e.g. Ashford and Allaway, 1985), "hyphal" (e.g. Duddridge and Read, 1984), "mycelial" (e.g. Fox, 1987) or "mycorrhizal" (e.g. Dighton and Mason, 1985), has been favoured by the majority of ectomycorrhizal workers in recent years. This appears to be founded (Bowen, 1973; Read, 1984) on the separation of rhizomorphs and strands on the basis of the presumed differentiated meristem in rhizomorphs of Armillaria mellea sensu lato (Garrett, 1970).
Rayner and Todd (1979), however, in discussing linear mycelial organs produced by saprotrophic basidiomycetes, argued that "strand" describes a single filament—thus a single hypha would constitute a "mycelial strand". "Mycelial cord" was proposed as a more appropriate term, and gained wide acceptance in this context (e.g. Thompson, 1984). Our understanding of linear mycelial organs has been further advanced by Rayner et al. (1985), who demonstrated that all linear mycelial organs, including those produced by A. mellea sensu lato, differentiate behind a front of normal apically-extending hyphae. While still advocating the use of both "rhizomorph" and "mycelial cord", these authors suggested the adoption of qualifying terms such as "apically dominant", "apically spreading" or "apically diffuse" to describe the outgrowth patterns at the growing front. (See Fig. 1, Chapter 12, Jennings, this volume.)
More recently, Cairney et al. (1989) highlighted structural similarities common to all differentiated linear mycelial organs irrespective of the degree of hyphal aggregation at their growing front, stressing that only a single, qualified term is required in their description. "Rhizomorph", qualified as described by Rayner et al. (1985) along with a term, such as "simple" or "complex" (see Fig. 1), to describe the degree of internal differentiation attained in the mature organ, was thus regarded as most appropriate.
Although generally smaller and composed of fewer hyphae, the differentiated linear mycelial organs associated with ectomycorrhiza
Fig. 1. Schematic illustration of mature linear mycelial organs in transverse section showing relative hyphal arrangement and size, (a) "Simple" rhizomorph (little differentiation between cortex and medulla; large diameter "vessel" hyphae poorly defined from other hyphae). (b) "Complex" rhizomorph (clear distinction between cortex and medulla; distinct large diameter "vessel" hyphae in medulla), (c) Undifferentiated mycelial "cord" (loose aggregate of hyphae of similar dimensions; large diameter "vessel" hyphae are absent). Based on Cairney et al. (1989, 1991).
should be regarded as both structurally and functionally equivalent to those produced by saprotrophic basidiomycetes (Cairney, 1991b; Cair-ney et al., 1991). The continued use of "strand" in the ectomycorrhizal context cannot be justified and will only serve to confound such similarities. It is essential, therefore, that we adopt a common nomenclature, and I encourage ectomycorrhizal workers to use "rhizomorph", qualified as outlined above, in describing all differentiated linear mycelial organs produced by ectomycorrhizal fungi.
The undifferentiated linear hyphal aggregates (see Fig. 1) which are associated with the mycorrhiza of certain species (e.g. Dermocybe crocea; (Uhl and Agerer, 1987) are neither structurally nor functionally equivalent to the differentiated rhizomorphs so far discussed (Cairney, 1991b; Cairney et al., 1991). Although I have recently suggested that either "cord" or "strand" might be appropriate here, "cord" (sensu Rayner and Todd, 1979) probably best describes these undifferentiated aggregates.
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