Introduction

All the evidence that is available indicates that translocation of solution through both stranded, i.e. producing rhizomorphs, and unstranded mycelium of S. lacrymans occurs by the pressure-driven flow of solution (Jennings, 1987). The pressure is generated osmotically. Essentially, the volume flow of water across the outer membrane of the mycelium growing on the food source, as the result of a water potential gradient across that membrane, generates the pressure which is dissipated as a flow of solution through the mycelium, most particularly in the growing front. In mature rhizomorphs the flow of solution almost certainly takes place in the vessel hyphae (Eamus et al., 1985).

It is highly likely that translocation in all rhizomorphs occurs by the same mechanism, irrespective of the degree of their internal complexity.

The circumstantial evidence is strongly in favour of the mechanism for rhizomorphs of A. mellea (Jennings, 1987). The data for water movement in mycorrhizal rhizomorphs produced by Suillus bovinus between pine seedlings (Brownlee et al., 1983) are in keeping with the above view. But compelling evidence of the probable ubiquity of pressure-driven translocation of solutions in all rhizomorphs comes from the investigations on translocation within mycelium of S. lacrymans (Jennings, 1987), which indicate that the process is the same irrespective of the degree of differentiation of the mycelium. In the very young mycelium, the larger diameter hyphae are clearly a very important route for translocation (Brownlee and Jennings, 1982b) and there is indirect evidence that such hyphae are of the "armbone" kind in terms of their shape (Eamus et al., 1985; Hornung and Jennings, 1981).

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