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194 YNGVE NORDQVISTwestern mountainous parts of Jamtland but occurlocally wherever there is a suitable hydrotopography,e.g. in the lower parts of long slopes. Thecomplex element consisting of "flarks" (see SJORS,above) separated by low bands ("ribs" or "strings")with cyperaceous or grass fen vegetation are alsomore frequent in the western mires with theirgreater excess of water, whereas high bog-hummocky,ridge-shaped "strings", which as featuresof "mixed mires" have a continental distribution,are met with only locally, chiefly on large expansesof mire (e.g. on Gisselasmyren and Algfloarna, seeBJORKBACK).pH AND ELECTROLYTE CONCENTRATION.-0wingto the finely divided, calcareous Cambro-Silurianmaterial usually abundant in the tills, the miresgenerally show high contents of calcium and a highconductivity in their water whenever this is derivedfrom adjacent mineral soils. Such high electrolytecontents even characterize whole river systems incases where they are entirely derived from Cambro­Silurian areas (e.g. Bjornan and Halaan). Riversrunning through, carrying water from areas westof this district, have originally a low conductivitybut are greatly enriched during their passage (SJORS,personal communication).High contents of electrolytes are constantlyaccompanied by high pH values (the reverse is notalways true: SJORS 1952, pp. 251-252). Althoughmeasurements have been confined to pH, conductivity(several sources, partly unpublished) and Cacontent (WITTING 1949) it is obvious that Ca 2 + is,among the cations, responsible for the bulk of thehigh electrolyte content. It is also the only importantneutralizer of the peat acidoids and frequentlyoccurs in such great excess that it is precipitatedas CaC0 3 . Thin temporary sheets are frequent onmud- bottoms, and highly calcareous marl, algalmud, or lime-peat is found below many fens andon the bottom of tarns and lakes. Some shallowlakes even have deposited thick, white, chalkylayers of CaC0 3 of great purity. Both fossil andrecent, usually porous and not very hard calcaroustufa deposited by springs and containing embededOratoneurum moss, leaf impressions, etc., occursfrequently in the topographically more varied partsof central Jamtland (SERNANDER 1915-16, 1925 b,MALMSTROM & AsPLUND 1925). The tufa springsfeed often quite strongly sloping calcareous fens ofgreat floristic' and sociological variability (Du RIETZ1933a, pp. 67-68, WITTING 1949, pp. 727-728,SJORS 1948b, 1950a, p. 27); this vegetation will notbe dealt with further here.The following small table is extracted from WIT­TING's work. The Osterasen site is a nature reserveon Crown land, including an undrained wet areaof extremely rich fen (as well as less rich areas notincluded here); the Odensala fen (near Ostersund)is a small strongly sloping site with tufa formation,originally very rich floristically (e.g. Carex hostiana,Ophrys insectifera) but unfortunately damaged fromthe expansion of urban development. WITTING'ssamples were taken on July 28 and 29, 1948.OsterasenOdensalaSample no . ... 60 61 68 67 58 59pH 6.6 7. 1 7.2 7.6 7.5 7.7Ca, as mg per I 25 62 64 80 108 108Conductivity · 106 145 261 358 384 592 557From the recently drained Forsflon, originally afeebly sloping, not very wet mire with manywooded small islands and variable rich fen vegetation(including large patches with Oypripediumcalceolus), pre-drainage values (MALMER & SJORS1955, p. 56) varied between 46 and 74 mg Ca per lat a conductivity between 198 and 286 ·10 - 6. Othervalues for conductivity obtained in various sitesin Jamtland by several workers also fall withinthe Osterasen range.VEGETATIONAL GRADIENTS. -The terminologyused below is chiefly taken from SJORS (1948 a,1950 b) but a few terms are new, and the expression"direction of variation" has been changed into"vegetation gradient" (SJORS l963 b, p. 38).Several vegetation gradients are discernible onthese mires but there is not enough material for aconsequent division to be carried out. With regardto the gradient from poor fen to rich fen, the poorertypes (\vhich are subordinate as to areal extension)Acta Phytogeog1·. Suec. 50