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192 FOLKE BJORKBACKgrow (S. plumulosum, S. subfulvum, S. teres, S.warnstorfianum) .In the upper (proximal) parts of the £larks, andin those fen communities which are jammed inbetween close-lying strings, the vegetation is of apoor fen type, with Oarex magellanica and SphagnumLindbergii prominent. The transitions to stringvegetation are correspondingly different in species. composition, with large bright yellow-greenishcarpets of Oalliergon stramineum, constituting adominant feature of a Calliergonetum straminei.Pollen-nalytical investigations have shown thatthe bog elements and the strings in the £lark complexesare relatively young formations (BooBERG1930, G. LuNDQVIST 1951 a, ToLONEN 1963). Thepresent surface morphology of this kind of mireswas developed as late as 1000 B. C. or later, roughlycorresponding to RY Ill in GRANLUND's (1932)system, although the "recurrence surfaces" maynot always be synchronous (G. LuNDQVIST 1963 b,pp. 95.;:_96).Of other kinds of mixed _mire, only the islandtype should be mentioned. It is here representedby a conglomerate of poor fen and large fuscumislands with Rubus chamaemorus as field layer dominantthat occupies some of the upper parts ofthe mire. In the poor fen grow e.g. Betula nana,Eriophorum vaginatum, Oarex pauciflora, Sphagnummagellanicum and S. parvifolium.PURE FEN ELEMENTS .-As a third componentof this mire complex occur pure fen elements whichhave been little investigated and will be mentionedbut briefly.There are some areas of continuous poor fensimilar to the type just mentioned as · occurringbetween fuscum hummocks. These poor fens arefound where there is some seepage between slopingbog areas. The occurrence of Oarex rostrata, M enyanthestrifoliata and other indicators of weak mine-rotrophy shows that these areas differ from thebog. Also Sphagnum magellanicum in this mirebehaves similarly, being absent in the true bogcommunities.The large "ribbed" mixed mire spoken of abovepasses at its lower end into a £lark fen where thehigh-hummocky "ribs" (strings) are replaced byequally broad but much lower structures of a richfen character. They also act as dams but constituteonly low thresholds easily overflowed or percolatedby the flush of electrolyte-rich water. This combinationof good mineral nutrition, firm peat andfairly good oxygen supply at dry seasons, whenthe surface is well above the water table, providessuitable habitat conditions for many more plantsthan do the usual mire environments. The typicalcommunity that builds up the low dams is formedby eo-dominant Trichophorum caespitosum andMolinia coerulea, and harbours many other species,e.g.Oarex dioecaTrichophorum alpinumParnassia palustrisPotentilla erectaSelaginella selaginoidesTofieldia pusillaSphagnum centraleS. subsecundumOampylium stellatumDrepcmocladus revolvens(sens. lat.)The intervening £larks are here developed asmud-bottom communities (SJORS 1948a, p. 64),the bottom layer of Scorpidium scorpioides or locallySphagnum platyphyllum being scanty. Among vascularspecies Oar ex livida, J uncus stygius, Droseraanglica, Utricularia minor and U. intermedia arefrequent, and in occasional deeper £larks a truewater plant, Sparganium minimum, is even found.There are also some few local occurrences ofextremely rich fen vegetation in this mire, harbouringe.g. Schoenus ferrugineus and in other placesSaxifraga hirculus, but Oarex jemtlandica andOrchis Traunsteineri have not been found..Acta Phytogeog1·. Suec. 50