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186 HUGO SJORSing group but its indicators are only partly of anexclusive character and besides largely different invarious parts of the area. In consequence, a subclassificationof the northern rich £ens is a verydifficult task; for the two principal extremely richfen districts, viz. the Jamtland Cambro-Silurianand some parts of the Scandes, see the contributionsby NoRDQVIST and by A. PERSSON, respectively.Another Swedish classification system is used byMALMSTROM (1923 and several later works) andothers. It differs with regard to the definition offen and bog (in this respect being closer to currentBritish usage of the two words).Other systems include those of N ORDHAGEN(1936 a, 1943) and of CAJANDER (1913) and later investigatorsin Finland. The NoRDHAGEN system iswell applicable. It includes one typical rich fenalliance of the Scandes (Caricion atrofu.scae-saxatilis)and another of the upland and lowland (Schoenionferruginei). Furthermore there is a mesatrophicallia!lce (Caricion canescentis-Goodenowii)but there occur also mesotrophic communities of adifferent character, e.g. the "moderately rich fen"and "intermediate fen" of non-calcareous expansesof sloping fen. The next community of N ORDHA­GEN's is the Stygio-Caricion limosae, poor in speciesand with a scanty bottom layer, but occasionallywith such mosses as Scorpidium that in thesystem of Du RIETZ are regarded as indicating"rich fen". Finally, the Leuco-Scheuchzerion is analliance of wet-growing oligotrophic vascular plantsand Sphagna; it includes one strongly hygrophyticand one less hygrophytic fraction. The fuscumhummocks are dealt with separately by N ORD­HAGEN (Oxycocco- Empetrion hermaphroditi, inthe subalpine region).The well-known Finnish mire system has fourmain divisions, the dwarf shrub mires, the "whitemires", the "brown mires" and the wooded swamps.As each type is hghly variable, a gret number ofsubdivisions have been proposed, and it would takeus too far to discuss these intricate problems here.Most of these subdivisions have their counterpartson the Swedish side of the Gulf of Bothnia, and itseems even likely that the Swedish mires, althoughless extensive, are even more differentiated. Thusthe true alpine and subalpine mires, the stronglysloping fens, and the highly calcareous sites haveonly less typical counterparts in Finland. In thefour-fifths of North Sweden that have a brokentopography with a great influence on the mires, adirect use of the Finnish system seems quite difficult,although its four main divisions are fullyapplicable. Neither the NoRDHAGEN system nor theFinnish one in its original version took into accountthe clear difference between ombrotrophic and poorminerotrophic vegetation, but recent authors inFinland (e.g. RUUHIJARvi) begin to take up thisvegetational gradient and discuss its importance inFinland.For the flat areas in northern Sweden, notablyits extreme north-east, the Finnish system is excellentlyapplicable and possibly superior to any of thetwo Swedish systems with regard to the desirabilityof giving broad but many-sided ecological descriptionsof mire vegetation and its environment.There are many more vegetational gradients discernibleon the mires than that from ombrotrophicto rich minerotrophic vegetation. Some are partlytaken into account in the mentioned alternativesystems. Thus NoRDHAGEN's Stygio-Caricion limosaeand two subdivisions of Leuco-Scheuchzerionwere distinguished along a gradient from wetmud-bottom via soft carpet to firm tussocky oreven lawn-like vegetation. This vegetational gradientwas further expanded by SJORS (1948a,1950 b) who also added another important vegetationalgradient (from "mire expanse" to "miremargin" and spring vegetation) further worked outfor northern conditions in particular by A. PERSSON(1961). A slightly different approach was used forsloping mires in Finland by HAvAS (1961), whoworked from groups of species with related ecologicalrequirements rather than from a system ofintercrossing vegetational gradients.BoG COMMUNITIES.-The extent to which ombrotrophicbogs occur in northern Fennoscandia wasunderestimated by earlier authors (e.g. GRANLUND1932). Through the regional works by RuuHIJARVI(1960), EuROLA. & RuuHIJARVI (1961) and EuROLA(1962) their distribution in Finland is now wellknown. Indeed, examples of ombrotrophic bog areknown to occur in nearly all parts of Fennoscandia,Acta Phytogeog1·. Suec. 50