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272 MẠGNUS FRIESleached. The subsequent formation of a mor (rawhumus) cover and the podsolization finally excludedthese plants from large areas definitely as effectivelyas did the invasion of closed forest. Only onthe areas of almost soil-free calcareous bedrockcalled aZ . var, and related types of very shallowsoils, mainly on Oland and Gotland and in Vastergotland,some relics from the Late-glacial "steppetundra" still survive, such a.S the mainly alpinevascular plants Poa alpina and V iscaria alpina(both have also other occurrences in the lowland),and some mosses and lichens with their maindistribution in alpine regions. Further the speciesH elianthemum oelandicum and Artemisia oelandica,endemic to Oland, and Arenaria gothica, endemicto Gotland and Vastergotland, are supposed to beLate-glacial relics. This might also be applicable toseveral more or less continental plants on alvarground, for instance Potentilla fruticosa, Globulariavulgaris, Plantago tenuiflora, and Artemisia rupestris,and ta, rich-fen species as Euphrasia salisburgensis,Bartsia alpina and Pinguicula alpina (cf. BENGTPETTERSSON's paper about Gotland and Oland inthis volume).The fossil finds- of arctic or tundra plants inSweden are almost totally restricted to the southernfourth of the country, including the islands ofOland and Gotland, i.e. mainly south of theCentral-Swedish Moraines formed during theYounger Dryas period (see maps in TRAL.AU 1963).The lack of fossil finds between this area and theScandes (Scandinavian mountains), where manyof the Late-glacial tundra plants now live, isconsidered more as evidence for a different originof the arctic-alpine element in the mountain florathan for immigration from the south.On the basis of the present distribution of theso-called west-arctic high-mountain species, whichare absent in the Alps, and for other reasons,biogeographers regard some restricted areas on theNorwegian west and north coasts as having beenrefugia, where these species and probably otherarctic and alpine species as well have survived thelatest glaciation (NORDH.AGEN 1935, 1936 b, 1954;Du RIETZ 1935b; N.ANNFELDT 1935, 1947; HoLMBOE1937; G. BJORKMAN 1939; ARWIDSSON 1943; E.D.AHL 1946; SELANDER 1950 a; and others). How-ever, geological data have been presented that areregarded incompatible with the theory of glacialsurvival. The origin of the mountain flora of theScandes is still a matter of discussion, with argumentspro et contra; cf. articles in SvENSK NATUR­VETENSKAP, Yearbooks 1957-58, pp. 108-151; 1959,pp. 116-142; 1961, pp. 81-96 (summaries in English),and in NORTH ATLANTIC BIOTA AND THEIRHISTORY (1963).During future studies in this important problemof possible survival on refugia, special interest willprobably be directed towards the late Full-glacialand early Late-glacial geography of the coastalareas in question, and to the North Sea continentand adjacent land as a potential source area forplants that later became restricted to the alpineregion in the Scandes. Furthermore, the idea of"shifting ice culmination, alternating ice coveringand ambulant refuge organisms" (R. DAHL 1964; cf.LJUNGNER 1949) may involve a productive holdof the problem in the fields of geomorphology, .glaciology, and meteorology. In the present situationmore fossil material would be of great help,but the possibilities of finding suitable samplingsites are small indeed.The Post-glacial, early partThe most important vegetational change in thelandscape after the ice retreat was the invasionof forest. The former tundra was rapidly occupiedby trees and forest plants forming new communities.In many areas, the ground was then graduallycovered by mor (raw humus) or in moist places evenpeat. The edaphic conditions became more andmore unfavourable for those plants which requireopen mineral soil and much light. This radicalchange, initiated about 8000 B.C. (8300 B.C. inSkane according to T. NILssoN 1964 a, p. 30),defines the beginning of the Post-glacial time (theHolocene). Obviously the climate changed from thearctic-subarctic type of Younger Dryas to temperateconditions as early as the first centuriesof the Post-glacial, although locally, near the iceborderand the ice-dammed lakes, the summersought to have been rather cold.The rapid ice retreat from the Central-SwedishMoraines northward during the Preboreal time indi-Acta Phytogeogr. Suec. 50