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106 BENGT PETTERSSONbufonius, Rorippa islandica, Atriplex and Chenopodiumspp. as well as hemicryptophytes, e.g.Potentilla anserina.THE FoREDUNE.-Above the foreshore a ridgeof sand, the foredune, is often formed on exposedshores and built up by sand-collecting plants, aboveall Ammophila arenaria and Elymus arenarius. Thisridge may be characterized as a yellow dune, andis consequently mostly devoid of any cover oflichens and bryophytes. The mobility of the sandis considerable.Starting from the edge of the foredune someperennial species grow out over the foreshore, butpart of this vegetation is regularly destroyed by theabrasion that may affect the foredune as well andcut out a steep bluff along many stretches of thecoastline. Among the perennial species can be mentionedAgropyron junceiforme and M inuartia peploides,fast-growing plants that send out longcreeping rhizomes through the loose sand.Sometimes small scattered islets of dunes havebeen built up on the foreshore, in front of the foredune,through sand collected around the highgrowingstands of Elymus or Ammophila derivedfrom shoots which have become detached from theforedune. More seldom there is a spread by meansof seeds, the soil being too unstable for this.Many field observations support the theory thatthe abovementioned high-growing grasses are spreadmainly vegetatively. A good indication is offeredby the intergeneric hybrid x Ammocalamagrostisbaltica, closely resembling one of its parents, Ammophilaarenaria. The hybrid often occurs abundantlyand is vicarious for the latter species on theforedune, even in places where the other parentspecies, Oalamagrostis epigeios, does not occur.A m mop hila and Elymus have widened their rangein recent time in many districts (BENGT PETTERS­SON 1958). This may be due to the fact that thegrazing on the shore vegetation has ceased on manycoasts. On Gotska Sandon a foredune whichreaches a height of over 10 m has been built upduring half a century, after the sheep had beenremoved from the island. This is an illustrationof the geomorphological significance of these species.Both species have been planted since long ago inorder to stabilize dunes, and their natural range isuncertain.On the foredune or equivalent lower ridges andsand patches ·there are comparatively few otherperennials, many of which have spread ratherrapidly during this century. Lathyrus maritimus(cf. CEDERGREN 1947) is one of the most conspicuousand interesting species among those which haveenlarged their range considerably during the lastfew decades, especially within South Sweden. Itsrange comprises most of the coasts of Norden, butwith large gaps. The southern Petasites spurius isalso spreading to new, very scattered loclities,probably through pieces of the · rhizomes (cf.LUTHER 1955, p. 16).During the last three decades a new memberof the Swedish flora, Lactuca tatarica, has spreadalong many shores on Gotland and now partlydominates the vegetation on the foredune. Lactucatatarica was first found on the south Baltic shoresin the beginning of this century. It had probablycome to the Baltic from the steppes of SouthEastEurope with transports of grain (BENGT PETTERS­SON 1958). The occurrence of this species is closelyrestricted to the sandy beaches but it has a widerecological amplitude than Elymus and A mmophila,successfully competing with these grasses underfavourable conditions and growing farther downon the foreshore, as well as farther landwards in acompletely closed vegetation.Eryngium maritimum, a species which is widelydistributed on the coasts of Western Europe andthe Mediterranean (BENGT PETTERSSON 1958, mapp. 150), seems to some extent to depend on thecontent of lime in the soil, because it has a northernboundary running from northern (Hand overGotska Sandon to Dago (Estonia), thus mainlyfollowing the limit of distribution of the Cam bro­Silurian strata (however, the course of the isohalinescoincides as well). Eryngium often grows as singleindividuals on fixed sand and in colonies in opensand mixed with gravel and pebbles. Its occurrenceon the coasts of the Baltic seems to be rather unstable,and many previous reports are not validtoday.Important but less conspicuous elements of theActa Phytogeogr. Suec. 50