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218 JIM LUNDQVISTan unstable, often somewhat calcareous substratemoistened by water seeping from the cliff, andgood light conditions.The more demanding plants are only found . inthe south of Lappland. Beside the elm (here mostlya shrub) we note for instance Polygonatum odoraturn,Carex Pairaei, Corydalis fabacea, Astragalus glycyphyllus,and Lonicera xylosteum. The poor, acidsubstrates of eastern Lappland are as a rule occupiedby less demanding southern plants, viz. Silenerupestris, Sedum annuurn, Fragaria vesca, Potentillaargentea, and Veronica officinalis. In the extremesouth Epilobium montanum and Circaea alpina arealso known from such substrates. A few southernplants, like Pteridium aquilinum, Carex digitata,Pyrola media, and P. chlorantha are also to be foundin other types of vegetation. Especially on or nearshores of lakes and rivers we occasionally findadditional southern species (cf. S. RuNE).The isolated mountains that are situated not farto the east of the Caledonian border may be quite .rich in species, e.g. Lulep Istjakk just south of theArctic Circle in the middle of La ppland. On thismountain we find a total number of about lOOspecies, among which Polygonum dumetorum, Arabishirsuta, Arabidopsis thaliana, Turritis glabra, Erysimumhieraciifolium, and M yosotis stricta are themore demanding elements of the flora. Many ofthese plants _are favoured by a more or less calcareoussubstrate which does not occur further east.In the upper parts of the scree, the most characteristicplant community is a thermophilous Betulapubescens-Populus tremula wood. In wet habitatsit shows great affinities to the meadow birch woodof the mountains (Lactucion alpinae, NoRDHAGEN1943), but there are in. dry meadows some differentialspecies present ( Tortula ruralis, W oodsia ilvensis,and Poa glauca). Especially where there is a richwater supply from the cliff we find tall herbssignificant for the above-mentioned alliance, viz.Matteuccia struthiopteris, Dryopteris filix-mas, Aconitumseptentrionale (not in the extreme north andeast), Filipendula ulmaria, Geranium silvaticum,V aleriana sambucifolia, and Lactuca alpina. Togetherwith some shrubs such as Ribes spicatumvar. lapponicum, Rubus idaeus, and Rosa majalis,they may constitute a lush vegetation that maybe quite hard to penetrate, almost a "jungle". Itis in small patches of more open vegetation thatwe frequently find the above-mentioned southernspecies. Other good habitats for these plants arethe cliff itself and those parts of the talus slope,below the wood, where the material is fine-grained.The coarse, usually bouldery lower talus slope isas a rule very poor in higher plants. Thus we findgreat contrasts within a few metres on these screes.Other exposures in these mountains may showa luxuriant vegetation rich in species only whenthe substrate is rich in electrolytes and unstableenough to be replaced successively by new calciferousmaterial. Towards the north there is atendency among the "southern" plants to becomealmost confined to favourable substrates.Rich south-facing slopes occur also within theScandes, extending through the subalpine and lowalpinebelts, but naturally we find but fragmentsof a "southern" flora. The flora of the south-facinghills and mountains brings up many problems whichhave for a long time stimulated an eager discussionamong botanists in this country. An ecologist mayask whether the southern plants can endure poormineral substrates if the percolating water is richenough in electrolytes.As noted above the history of the southern floraof Lappland yields other problems. When did itsettle in Lappland? Scientists generally agree thatmany southern species must be relics from thePost-glacial warm period, when these plants wereprobably more evenly distributed over Lappland.During the later cool period the pretentious elementsof the flora had to retreat, leaving behind alimited number of survivors on the relatively warmand also edaphically favourable south-facing hillsand mountains.Acta Phytogeog1·. Suec. 50