THE CONTINENTAL FLORA OF SOUTH SWEDEN 241 



IV. Siberian species, for instance, Artemisia laciniata Willd. and rupestris L., 

 Potentilla fruticosa L., Carex obtusata Liijcbl.; Potentilla rupestris L., which 

 has a rather great distribution in Central Europe. 



The majority of the continental species of tiie luiropean flora belong to the 

 types of distribution which we call Pontic and Ponticosarmatian — Central Euro- 

 [)can. These species are to a large extent xerothermous. In view of their distri- 

 bution and mode of occurrence outside the limits of P2astern Europe they are 

 closely connected with the xerothermous species of Southern or Central Europe. 

 The Pontic — South European and the Pontic — (Southern- and) Central European 

 types evidently form a transition. 



The distribution-area of the species in Europe is often characterized by a 

 boundary line running mainly in the direction NE — SW. In many cases the 

 outermost localities lie on a line which runs from the north-cast of Russia almost 

 due south-west, south of the Baltic Sea, down to the south-east of France; while 

 in other cases the boundary line makes a great westward bulge over southern 

 Scandinavia and a more or less pointed indent in the north-west of Germany 

 (see, for instance, Plates 17 and 18). 



Very pecuHar are the markedly isolated occurrences by which certain species 

 are represented in the flora of South-Western Europe (see, for instance, Plate 14). 



The Pontic-Baltic type of distribution reckons only a few representatives, but 

 nevertheless merits attention. Like species belonging to this type, some species 

 of other types of distribution have a peculiar and extensive distribution in the 

 lowlands near the south of the Baltic — e. g. Petasites spurius (Retz.) Rchb. 

 (mainly Pontico-sarmatian), Senecio palustris (L.) Hook. (Baltic), Scolochloa festu- 

 cacea (Willd.) Link., and Cnidium venosum (Hofl"m.) Koch. The species may be 

 distributed far to the west (e. g, Carex ligerica Gay, Koeleria glauca (Schkuhr) 

 DC, and Senecio palustris (L.) Hook., but they are absent in Central Europe. 



The reason for this distribution would seem properly to be sought in dispersal 

 conditions. As has been especially pointed out by Loew (1878), Scholz (1905) 

 and Preuss (1912), the river valleys of North Germany, ancient or modern, with 

 their shore-slopes of loess, marl or sand, would seem to form extremely important 

 routes of migration in an east-west direction. It is also highly probable that 

 species on the German Baltic coast have great possibilities of such a dispersal. 

 According to Scholz (1. c.) and Preuss (1. c), it is now easily perceptible how 

 at the present time species migrate from Russia to the north-east of Germany 

 along the Vistula — e. g. Artemisia .scoparia L., Corispermum Marschallii Ledb., 

 and Rumex ucranicus Fisch. It is conceivable, therefore, that the species now distri- 

 buted over western Balticum also have immigrated along the Vistula, and that from 

 there they have spread further along the river valleys or the coast. — There are 



