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Stipa pennata and its companions in the flora of V iistergotland 165Vastergotland (cf. maps in STERNER 1922 andHULTEN 1950). SERNANDER (op. c.) was the first todistinguish a group of companion species of Stipa.He discussed problems associated to the distribution·Of the following species, all of which have beenfound at the new Stipa locality: Potentilla rupestris,Dracocephalum ruyschiana, Polygala comosa, Galiumtriandrum and Pulmonaria angustifolia. Some ofthese species have been found by me on new localitiesand have a wider distribution in Falbygdenthan previously known (FRIDEN 1948, 1959 a).Polygala comosa reaches its Scandinavian northwesternlimit at Aneberg. Galium triandrum andthe more frequent Pulmonaria angustifolia reachthe southern slopes of Billingen to the north. Prunellagrandiflora, frequent in the kame area at.Asaka and Nas and rare around Alleberg, has anorthern outpost on Kinnekulle. The area of Potentillarupestris is isolated but fairly wide, as it occursin all the neighbour provinces, and near Oslo. Aremarkable companion of Stipa is Dracocephalumruyschiana, the single representative in Vastergotlandof the cold-resistant steppe flora of EastEurope and Siberia. This species reaches far northwestinto the climatically continental interior Norway.Its disperse Swedish occurrence centers onFalbygden and an area to the south of it, includingthe upper valley of the river Atran, which showscertain geobotanical similarities with Falbygdenand has for instance several localities of Pulmonariaangustifolia (WESTFELDT 1954).HISTORY OF THE STIPA FLORA.-Other isolatedwestern outposts-e.g. Pulmonaria angustifolia inBretagne and southernmost England-are evidenceof a wider, presumably continuous distribution ofthese species during an earlier, climatically favourableperiod. It seems likely that most of thesespecies (and also Potentilla rupestris), nearly allabsent from Oland and Gotland, reached Falbygdenfrom the SSW, very likely before the land connectionvia Denmark was broken in the early Postglacial(ALBERTSON 1941 b, p. 80). However, themajority of steppic species no doubt immigrateddirectly from the east, but for example Prunellagrandiflora and Galium triandrum (also near Oslo)may have used both routes. SERNANDER (1908-09,pp . 206-207) thought the first immigration wasduring the Boreal, but as this light-demandingvegetation might have had difficulties at the timeof maximum forest development (i.e. his Atlanticperiod), he concluded that the plant association ofthe Stipa steppe was fully constituted only sincethe Sub-boreal period (op. c., p. 208).An early immigration was advocated with greatconsistency by ALBERTSON (1941 b, pp. 79-80):"The invasion would thus have begun during thefirst part of the Post-glacial warm period or evenearlier, in Late-glacial time, when the tundra vegetationbegan to be intermingled with warmthdemandingplants". This was remarkably prophetic,as most of the now available evidence about steppicelements in the Late-glacial flora and aboutwarmth-demanding plants of the Allerod period was.unknown in 1941. The steppe species were believedto have survived the period of dense forests on· refuges of open vegetation, e.g. on cliffs (op. c., p.81). Similar opinions were later vindicated by SE­LANDER (1955, pp. 370-371).It has been commonly agreed that the dispersalof steppic elements has been favoured by culturalinfluences. The area under consideration was earlyinhabited and constituted a centre of Neolithicculture, involving construction of numerous megalithicPassage graves (Chamber-tombs, etc.). Earlyagriculture has been verified, e.g. with pollen analysis(M. FRIES 1958a). A much discussed declinein the Ulmus pollen curve occurs also in this area.It may indicate "coppice cutting" (SJOBECK 1964,p. 35) for twigs used for cattle winter forage, andpartial clearing for agriculture and grazing. Hencean open landscape was locally formed, notably onelevated, fairly dry ground, suitable for many herbsand grasses. Later, during the latter part of theIron Age, an increase in Juni:perus pollen accordingto M. FRIES may indicate the spread of forest.clearance and development of pasture grounds.The prospering trade and increased intercommunicationbetween countries particularly in the Vikingand ·crusade times (cf. BENGT PETTERSSON 1958,p. 24) greatly favoured the dispersal of plants, notleast of Continental origin. SJOBECK (1951, pp.53-56) has indicated the existence of an easterncommunication route towards the Falbygden areaActa Phytogeog.r. Suec. 50