THE CONTINENTAL FLORA OF SOUTH SWEDEN 329 



(Plate 13), and Ranunculus illyricus (Plate 13) are ver)' peculiar. Here it may 

 also be observed that many South or Central European species have isolated 

 occurrences on Oland and Gotland (cf. above p. 276). 



The occurrences on Oland and Gotland are attached to isolated minor distri- 

 bution areas in North-Eastern and Central Germany, which, like those of Oland, 

 rest on ecological conditions that arc exceptionally favourable to the species. 



True, the flora of Oland and Gotland lacks a number of species found in the 

 North German centres of occurrence of steppe species (e. g., Stipa pennata and 

 capillata, Aster amellus, Scorzonera purpurea, Campanula sibirica and bononiensis 

 etc.); but it must be considered remarkable that the Baltic has not been better 

 able to prevent the species from reaching Oland and Gotland. [It must be 

 pointed out, however, that on the other hand Oland and Gotland lack steppe 

 species that have a wide and even distribution in North Germany (Trifolium 

 alpestre, Thesium ebracteatum, FLryngium campestre, Dianthus carthusianorum etc.)]. 

 The species here called ■>-> Siberian-i> are especially characteristic of the flora of 

 Oland and Gotland. (Add to the just mentioned species Potentilla fruticosa, be- 

 longing chiefly to a vegetation type, which will be treated later on in Chapter x.) 

 Their occurrences on these islands are the only important ones outside the 

 Siberian and North American distribution-areas. They may be explained as being 

 caused by an immigration from localities in Central Europe, which in their turn 

 should be looked upon as relics from a wider European distribution during some 

 pre-glacial or inter-glacial epoch. During the continental periods of the post- 

 glacial epoch they must have had a wider distribution than at present; and 

 thus we can more easily imagine their immigration to Oland and Gotland. — The 

 occurrence of these Siberian species on Oland and Gotland, as in their other 

 isolated European localities, is of very great interest from several points of view; 

 and thus there should be much to add here, but space does not permit it. 



Of the remaining isolated occurrences of steppe species on Oland, the occur- 

 rence of Plantago tenuijiora is probably the most striking one (Plate 13). 



Its appearance on the Alvar of Oland would seem to be the only one outside 

 the Pontic steppes. It is so much the more remarkable, as the plant in its dis- 

 persal is wholly dependent on its fructification. It can easily be observed that 

 the frequency of the species varies a great deal during difterent years, and that 

 this is connected with the state of the weather, but in spite of this the species 

 has been able to spread over almost the whole of the island. Hence we have 

 here another example of the great climatic amplitude of steppe species. 



But if Plantago tenuiflora thrives so well on Oland in the present climate, it 

 may be questioned whether its immigration thither must be placed in some con- 

 tinental period, during which an immigration step by step into Oland would ex 

 hypothesi take place, thanks to now vanished occurrences in Central Europe. 



