324 . R 1 K A R D S T E R X E R 



an immensely barren nature and with several occurrences of northerly (for in- 

 stance Betula nana) and Atlantic species (for instance Erica tetralix). 



To these factors makin<^ a hindrance to the occurring of steppe species in 

 Smaland there are to be added considerable disadvantages in the temperature 

 conditions. As may distinctly appear from the accounts given above on pp. 

 256 and 257, the coast regions and lowlands around the South Swedish highland 

 have a considerably more genial climate than the latter. 



With regard to the steppe species now being treated, the causes of the South 

 Swedish distribution-limits are thus to be sought in the following phenomena: 



1. The distribution of the arable land; 



2. The distribiitiou of calcareous soils, which, esi)ecially in Vastcrgotland, causes 

 very sharply defined boundaries. 



3. The occurrences of ufell-developed os-slopes diud broken coiaitryxn north-eastern 

 Smaland. 



4. The strongly varying precipitation conditions, especially the contrast between 

 the north-eastern and the south-western parts of the South Swedish highland. 



The less directly displayed intluence of irregularities in the temperature con- 

 ditions should also be ascribed a certain importance. 



With regard to the distribution-limits of the remaining steppe species, whose 

 South Swedish distribution is not restricted to a small number of occurrences, 

 the following may be said : 



A few species have a pretty remarkable distribution area along the coast of 

 South-Eastern Sweden. Melica ciliata (Plate 10; in the interior j;art of the coast, 

 chiefly on limestone) [Poa bulbosa (often a colonist)], and Isatis tinctoria and Silene 

 viscosa, which arc sea-shore plants. — This distribution type will be treated 

 later on in connection with similar ones of other continental species (Chapter x). 



Regarding Silene viscosa, however, a few words will here be mentioned. 



The peculiar character of its habitats on the Baltic sea-shores has been above 

 (p. 292) slightly treated. Not less peculiar is, how'ever, its area of distribution 

 in this region, the only one outside the steppe districts (Fig. 17). It may con- 

 ceivably be that the islets and skerries hardly exposed and often strongly nitro- 

 philous in their soil, form suitable habitats, the species in them getting lid of 

 the struggle for space with other species. Another cause of the distribution 

 area might be found in the climatic conditions, the scanty precipitation (it must, 

 however, be observed that this factor is to a great extent counterbalanced by the 

 lower saturation deficit in the dampness of the air) and the higii temperature 

 during the latter part of the summer. The most important factor, however, seems to 



