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summary The extension rates of Clitocybe nebularis (Batsch ex Fr.) Kummer strains on 2% malt agar were only 30–40%, of those, up to 3.4 mm d−1, observed in woodland at equivalent exponential mean temperatures. Extension of mature field systems was accomplished by mycelial annuli or arcs 30-40 cm wide, differentiated into a leading edge of mycelial cords followed by a zone of dense, diffuse mycelium which bleached litter components, and a trailing edge of greyish, Used mycelium. Disruption of mature annuli by natural obstacles or experimental re-orientation within the mycelial band resulted in regression of the affected segment of mycelium. Localized lysis following encounter with an obstacle by immature patches of mycelium with a diameter of 30–50 cm, led to polarized development of the residual mycelium. Strains from different fruit bodies were somatically compatible when paired on 2% malt agar if sampled from the same ring, but incompatible if from different rings, resulting in mutual antagonism and formation of a persistent demarcation zone. By contrast, collision between adjacent systems in woodland culminated in mutual obliteration of the interaction fronts. C nebularis was non-combative when paired against other decomposer basidiomycetes on 2% malt agar, being either replaced or deadlocked but not replacing mycelia of these fungi. The implications of these observations are discussed in terms of emerging concepts of ecological strategies, foraging theory and polarity in mycelial collectives.

summary Ratios of nuclear genotypes observed in conidia from heterokaryotic strains of Heterobasidion annosum (Fr.) Bref., obtained from pairings between sympatrically derived, sib-related and non-sib-related homokaryons, commonly deviated from 1:1. Ratios were temporally stable, and the genotypes examined could be ranked in a strict dominance hierarchy, linked both to the relatedness of the association partners and to the growth rates of the parental homokaryons. Parental homokaryons and sibrelated heterokaryons produced conidia with a mean number of nuclei of about two, whereas non-sib-related-d heterokaryons produced conidia that were predominantly uninucleate. Moreover, whereas conidia containing more than one nucleus germinated most rapidly when derived from homokaryons or sib-related heterokaryons, uninucleate conidia germinated more readily if derived from non-sib-related heterokaryons. In a study of naturally occurring heterokaryons, distribution patterns of the number of nuclei in conidia were found to be similar to those of the homokaryons. The possible interpretation of these findings in terms of inter-nuclear genomic conflict is discussed.