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168 SVEN FRANSSON60"Cetraria lJelisei •80 KMFig. I. South-Central Sweden withthe borderland between North andSouth Sweden in the biogeographicalsense. The geomorphologicalbackground to this division, viz.the southern limit of the northernupland, called the N orrland terrain,is marked by the irregularline from NE to SW (S. DE GEER1910, 1919). Cetraria Delisei, alichen on mires and mountainheaths, represents the northernplant-geographical element (dots);from HASSELROT 1953, with additions.The ash (Fraxinus excelsior}represents the southern, comparativelywarmth-demanding flora.Its northernmost occurrences, generallyon calcareous sites, arecombined by the broken line,based on maps by HALDEN 1928,1951 a and b, HARD AV SEGERSTAD1952). However, even south of theline the ash is rare or lacking inlarge areas. By M. Fries.This is the type of upland landscape that one meetsrather suddenly when one travels north from thelowland plains near Lake Vanern or around LakeHjalmaren and the western part of Lake Malaren,or goes northwest from the coastal lowland thatoccupies the province of Uppland and continuesalong the coast of the Bothnian Sea. Not far fromthe topographical border itself, the "chequer plateaux"rise to 300 or 400 m, but the bottoms of the"broad valleys", at about lOO m, are only slightlymore elevated than the lowland plains into whichthey open out. The phytogeographical and topographicalboundaries do not coincide completely,the greatest difference being on the Bothnian coastwhere the lowland continues north through Halsinglandbut the phyt.ogeographical borderline,according to various different criteria, must bedrawn to meet the sea no farther north than Gastrikland.The geomorphological boundary accordingto S. DE GEER (op. cit.) is marked on the maps,Figs. 1 and 2.It is the general rise in land level that makes theclimatic shift so abrupt, but the brokenness of thetopography of the upland may also exert some influenceat least with regard to local climate. Thusthe existence north of the border of cool northfacingslopes, of climatically favoured steeps facingother directions, and of flushed areas with movingwater near the surface all affect vegetation typesand the occurrence of plant species near their limitsof distribution.If the valleys are excepted, the topographicalborder of the upland can often be drawn at about200 m altitude (M. FRIES 1948, p. 58, HARD AVSEGERSTAD 1952, p. 55, SJORS 1954, p. 127), whichis only slightly above the former highest coastalline in this part of Sweden. This coincidence markedlyincreases the difference in the appearance ofthe landscape. Above the topographical border, thehills are mostly covered by glacial drift (this till isricher in fine material at higher elevations); belowit small hilltops showing clean-rinsed bare rocksare seen everywhere, and in addition extensive tillareas have been wave-washed superficially. On theActa Phytogeogr. Suec. 50