1. Decaying wood
Decaying wood is particularly appropriate for obtaining rhizomorphs of A. mellea. The best material is fallen timber 30 cm or more in diameter, clearly covered by mature black rhizomorphs. This covering should be stripped off and pieces of timber around 100 cm long incubated in a water-saturated atmosphere. After a period of time, rhizomorphs grow out of the wood and can be harvested for experimental purposes. Rhizomorphs with cream-coloured apices should be used; those with dark brown apices should be discarded. To my knowledge, this procedure has not been used with other fungi.
Figures 2a, b and c illustrate how rhizomorphs might be produced from a wood inoculum such that they grow over a non-nutrient surface, in this instance Perspex. Section VI indicates how the capability to produce rhizomorphs in this manner can be used for studying the translocation of nutrients along them. Rhizomorphs can be harvested from such systems for experimental treatment or chemical analysis.
Mycelium growing on nutrient agar may develop rhizomorphs (Fig. 2d). The anatomy of the rhizomorphs can be investigated, but otherwise they cannot be used to much advantage experimentally. The exception to this is when the rhizomorphs can be induced to grow into agar, as is the case for A. mellea, when they can be several centimetres in length. This is possible because oxygen can diffuse to the apex from the air above the agar via the central space within the rhizomorph (Smith and Griffin, 1971). Experimentally, tubes (6-12 cm i.d.) are filled with nutrient agar to within 20 cm of the top (Granlund et al., 1985; Cairney et al., 1988a). The concentration of agar used (<1.5% w/v) should be such that the gel is semi-liquid. The tubes should be inoculated from agar plate cultures with plugs which have diameters slightly larger than the tube to minimize sinking. Growth at 20 °C gives rhizomorphs 15-25 cm in length after 8-15 weeks.
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