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150 ILS MALMERFig. l. The wide Tingvalla bog in Dalsland shows the pattern of hummocks and hollows. Aug. 4, 1952. Photo G. Lohammar.greatest areal percentage is in the SW, somedistance inland. In most districts they are concentricallydomed and surrounded by narrow £ensknown as laggs (OsvALD 1930). In these cases thehydrological boundary between bog and fen, themineral soil water limit, is very distinct. Eccentricallydomed bogs, which predominate in the mosthumid parts, often abut upon large fen areas. Inthese cases it is not everywhere possible to establisha clear boundary between bog and fen, and the truebog character of large seemingly bog-like areas canbe questioned.The following treatment refers to the mire vegetationin Nemoral and Boreo-nemoral Sweden, i.e.the southern deciduous and southern coniferousforest regions, up to the oak-line. It follows theprinciples worked out by Du RIETZ (1949a, 1954b).The fundamental classification refers to the vegetationof the whole site. Among the vegetationalgradients or "directions of variation" in the mirevegetation (SJ6Rs 1948a, l950b) above all the poorto-richgradient (MALMER 1962a, p. 46) is used.Within the units (which represent "zonations" inthe sense of MALMER, op. c., p. 51) there is often avariation in the vegetation according to wetnessfrom hummocks to mud-bottoms, sometimes sociologicallymore distinct than any other gradient.Bog and poor fen vegetationThe various communities· within the bog vegetation(Ombrosphagnetea, Du RIETZ 1954b) representingthe poorest types of mire vegetation aresociologically closely related to similar types of fenvegetation (Sphagno - Drepanocladetea, Du RrETZl954b) but differ through the absence of exclusivefen plants (Du RrETZ 1949 a, 1950a, b, c), in SouthSweden e.g. Menyanthes trifoliata, Narthecium ossifragum,Carex lasiocarpa, C. rostrata, C. pauciflora,Eriophorum angustifolium and Sphagnum apiculatum.For a more thorough discussion, see MALMER1962a (pp. 35-36 and 79-82). The vegetation boundaryresulting from the near coincidence of thedistribution limits of these and other fen indicatorswhere a fen abuts upon a bog has been called thefen plant limit (SJORS 1948a) or "Mineralbodenwasserzeigergrenze"(Du RrETZ 1954b). On mireswhere the bog margin has a slope towards the fen,this boundary obviously coincides with a distincthydrological mineral soil water limit as mentionedabove. If the hydrological transition between bogand fen is gradual, the sociological conditions arealso more complicated (OLAUSSON 1957, MALMER1962a).The complexity of the two main types of SouthSwedish bog vegetation, wooded and woodless, wasoutlined in OsvALn's work (1923) on Komosse. Hethorougly described the regeneration, stagnationand erosion complexes, etc., of the bogs. Later(1925c, 1930, 1937) this work was followed by accountsof some regional features. The sociology wasworked out on a floristic basis yet with a personalmethodical touch by Du RrETZ, and published infull by him in 1949a, although with his permissionused in print earlier by several of his pupils.Wooded bog vegetation is most important in theeastern parts of South Sweden, where it often coversActa Phytogeogr. Suec. 50