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172 SVEN FRANSSONany case no two species in an ecological group showexactly identical limits of distribution. From existingdata on the distribution limits of vascular plantsit is even difficult to prove the existence of aborder zone. On the other hand, many species arelikely to show a decrease or increase in frequencyat the same discontinuity of terrain or climate. Thisis perhaps the most striking feature of the borderland,and justifies the concept of a boundary ornarrow border zone better than does the doubtfulgrouping of area limits. Unfortunately, frequencyis much more difficult to determine than area ofdistribution.A few examples of distribution and frequencygroups may suffice, although several hundreds ofvascular species and numerous non-vasculars areinvolved. Maps are found in HULTEN (1950), withmore details in the provincial floras (e.g. ALMQUIST1929, 1949a, HARD AV SEGERSTAD 1952).SouTHERN PLANTS.-Among southern ligneousplants, many reach the southern outskirts of theborderland but do not penetrate into it, e.g. V iscumalbum (WALLDEN 1961), Crataegus spp., or show adistribution limit quite close to the large lakesexcept in the east, for instance Prunus spinosa (A.HAMRIN, see Fig. 3c). Rhamnus cathartica reachesthe borderland in the east but not in the west. Theoak (Quercus robur) itself has not such a welldefinedborderline as indicated on the older maps;there exist oakless areas south of the border andmany finds of .seedlings have been made north ofit, indicating recent dispersal chiefly from plantedornamental trees. Ash (Fraxinus excelsior) wasstudied by H.ALDEN (1928) and found to have awinding limit (Fig. 1); it oversteps the oak-line inmany sections due to preference of lime (as well ascultural influence). The other demanding broadleavedtrees and shrubs (Ulmus glabra ssp. scabra,Acer platanoides, Tilia cordata, Oorylus avellana,etc.) penetrate far into the Boreal zone, takingsome advantage of its broken topography, but theirfrequency is greatly reduced already in the borderland;G. ANDERSSON (1902) drew a limit for commonoccurrence of the hazel which has often beenreproduced but should be somewhat revised in itsdetails (cf. E. PETTERSSON 1956) .Herbaceous species that follow similar patternsare numerous, but in most cases there are so manyoutpost occurrences or irregularities that it is noteasy to place the boundary lines in a definite order.Some species do not reach the borderland at all,but come rather close to it (e.g. Mercurialis perennis). As mentioned, a large group penetrates onlyslightly into the borderland (examples Lathraeasquamaria, Selinum carvifolia, Sanicula europaea,Oarex montana). Still more numerous are speciesthat are absent or rare above the border zone inthe west but in the eastern part appear far moreto the north, i.e. in Dalarna and eastern Norrlandor even in Jamtland. For instance, Adoxa moschatellinais not rare in the Dalalven valley, occurs inHalsingland, Medelpad and Jamtland, and is evenfound far more to the north in Norway and Finland.Corresponding types exist among eutraphentsouthern water or lake-shore plants (G. SAMUELS­SON 1934), many of which halt at the fringe of theborderland (Sium latifolium) or only reach it in itseastern part (Lemna gibba, Potamogeton crispus). ·However, in many cases there are also occurrencesin the Dalalven valley (Acarus calamus, probablyoriginally introduced) or farther north, in southeasternNorrland, too (Hottonia palustris, Oenantheaquatica, Hydrocharis morsus-ranae, Iris pseudacorv,s;the latter two also on the north and east sidesof the Gulf of Bothnia). The strong tendency toreach far north in the eastern parts of their Fennoscandianareas is remarkable among water plants;we find such extreme examples as Potamogetonlucens, P. zosterifolius and Lemna trisulca, which donot reach the borderland at all in the west, have awell-defined boundary in Vastmanland but in Norrbottenoccur far north of the Arctic Circle. Examplesof species barely reaching the borderland in thewest and centre and having similar occurrence upto Norrbotten and northern Finland are Ceratophyllumdemersum, Stratiotes aloides, Butomus umbellatusand Elodea canadensis.It seems likely that many water plants are moredependent on summer heat than on length ofgrowth season. Provided water temperature is highenough, the long daylight in the north may compensatefor the shorter growth season. As all thesewater plants are dependent on a high nutrient.Acta Phytogeog1·. Suec. 50