In spite of the more general distribution of many bryophytes, dramatic disjunctions exist, many of them similar to those shown by vascular plants. Various explanations have been offered to explain these disjunctions including continental drift, long-distance dispersal, and the fragmentation of a once more continuous distribution. No single hypothesis is sufficient to accommodate all species within any disjunctive pattern. The most serious difficulty is tlie inadequacy of exploration of considerable areas of the globe.Various bryophyte disjunctions are mapped in 51 distribution maps, and details of sexual patterns and dispersal mechanisms are assessed.Most bryophytes are widely distributed. In the Northern Hemisphere more than 60% of the flora of arctic and boreal regions is made up of the same species.Within this wide range, however, each species has highly specific requirements and some are exceedingly local.Becavise bryophytes have air-borne diaspores their means of dissemination would appear to guarantee a wide distribution of all species. That disjunctions exist at all would seem somewhat anomalous, yet such disjunctions do exist, some of them very dramatic. The explanation of these disjunctions has led to numerous intriguing hypotheses, many of which have been derived from similar studies of flowering plant disjunctions.In North America tlie disjunctions that have received the greatest attention

Two major disjunctions are recognized between the bryofloras of Europe and North America: Amphi‐Atlantic and Western Europe—Western North America. Each of these has specifically segregated floras related to climatic regimes and correlated with different historical development. An interpretation of the disjunctions is based on reconstruction of past continental interconnections coincident with reconstructed climates and an understanding of the biology of the bryophytes.

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Bryological research in boreal and arctic North America is in very preliminary stages. Although the flora is moderately well documented, details are lacking in much of the region. Greenland, Alaska, and Ellesmere Island are better understood than the rest of the area. Much of the information has been accumulated as casual collections and observations and incidental to other research. Bryophyte cover in arctic regions is less than that of vascular plants; in boreal regions wetlands are often dominated by bryophytes and open forests have extensive moss carpets. Turfs dominate the growth forms in the arctic while in boreal regions whorled-branched turfs, wefts, and compact mats become the predominant growth forms. Bryophytes are important in plant community structure and dynamics of both boreal and arctic regions, but detailed studies are few. Cytology of arctic and boreal bryophytes in North America rests on a single paper, thus any generalizations are hazardous. Physiology of bryophytes in northern North America has been inadequately documented. The sexuality, reproductive cycles, growth rates, and metabolic activities of bryophytes are areas that could yield intriguing results. Reproduction in bryophytes in northern regions appears not to be greatly different from that of more southern regions. In spite of the shorter growing season and the terrain and climate favoring wind dispersal, this had not led to an increase in the incidence of asexual reproduction in spite of the fact that more than 60% of the bryophytes are dioicous. Bryogeographic patterns are similar to those of the vascular flora but the presence of western North American taxa in the easternmost arctic and their absence in intervening areas is highly suggestive of eastern refugia. Glacial refugia are supported by the bryophyte distributions; their presence in unglaciated Alaska–Yukon, Ellesmere Island, and parts of Greenland seems best documented. Thirty-six maps are given showing bryophyte distribution patterns in the region under discussion.

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