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Dematiaceous hyphomycetes are a taxonomically diverse artificial assemblage of conidial fungi that have dark pigments known as melanins in their walls. Select dematiaceous hyphomycetes from the Coniochaetales, Pleosporales, and Helotiales obtained from arctic and alpine bryophytes and vascular plant roots were chosen for further study. Eight isolates, all from roots of Saxifraga oppositifolia, represented a new species of Monodictys, M. arctica. This species, characterised by large, darkly pigmented, multicelled, often branching conidia, was most closely related to members of the Leptosphaeria, making M. arctica potentially the first non-pycnidial anamorph associated with that genus. In alpine glacial forefields, the primary colonisers are bryophytes, which form an organic grout between the rocks. These mosses trap organic particles. Three pleosporalean fungi were isolated from this grout and examined to see if they could degrade moss. All three removed the outer, cellulose rich layer of the cells, but not the inner, polyphenol rich layer. I propose that the mosses disarticulate after degradation and the fragments are trapped in the grout, increasing soil organic matter and aiding pedogenesis. This first soil ostensibly facilitates colonisation by vascular plants, the roots of which are colonised by dark septate endophytes (DSE), a group known for asymptomatically colonising vascular plant roots and thought to be quasi-mycorrhizal. Cadophora luteo-olivacea, Cadophora melinii, Leptodontidium orchidicola, Phialocephala fortinii and Coniochaeta nepalica were examined for their abilities to degrade moss. All showed that colonisation patterns varied along with the formation of conidia and sclerotic structures, which could serve as inocula for establishing species of vascular plants. Cadophora and Phialocephala are delineated by phialide complexity, so I decided traced the origin and evolutionary history of that character on a phylogenetic framework. Based on this reconstruction, phialide complexity is a good character for separating the two genera. I propose that the sclerotium of the outgroup taxon, which develops phialides that produce spermatia, evolved into the phialidic heads of Phialocephala, which resemble sclerotia, and then into the solitary phialides of Cadophora. Traits in the sexual fungi associated with these asexual taxa support the phylogenetic placement of the asexual fungi. / Ecology