316 



RIKARD STERNER 



Fig 



the fact that on the arable plains the 

 species have at their disposal numerous 

 dry and unshaded localities. Concerning 

 certain features of their distribution on 

 the plains, however, there are other causes 

 as well. One is the higher lime percen- 

 tage of tlie soil in certain districts (see 

 fig. 4, p. 264). We find the steppe flora 

 most numerously represented in places 

 where the soil has a high lime percen- 

 tage: Skane, Uppland, parts of Vaster- 

 gotland and of Ostergotland. In Vaster- 

 gotland the importance of the lime per- 

 centage in the soil is very striking: the 

 occurrences of steppe species are located 

 in the more calcareous districts, especially 

 Falbygdcn, but they are missing on the 

 wide plain to the west of Falbygden 

 12. The whole distribution of Polygala co- where the clay and the moraine are com- 

 mosa in the Scandinavian North. paratively poor in lime. (See further 



O: uncertain occurrences; cl'. ]>. 400. liter on D "" '' I ) 



We should also take into consideration 

 the fact that the climate on the plains may favour steppe species. It appears 

 from the tables and isothermal and isohyetal maps on pages 256 — 259 that 

 certain plain districts, especially in Uppland and ()stergotland, have a higher 

 temperature and a lower precipitation in summer than the surrounding woodland 

 and hill districts. 



It may further be noticed that, in accordance with their steppe distribution, 

 many species are adapted to an open flat country in their dispersal equipment. 

 Iheir seeds or dispersal units of other kinds are equipped for a dispersal by 

 wind and consequently they can move widely about above the woodless, flat 

 plains (cf. Sernander 1901 b). Moreover the human activity directly facilitates the 

 distribution in various ways. ■ ■■, ~ 



Finally great importance must be attached to historical causes. Sernander 

 (1908, p. 219 ff.) has pointed out that the rich herbaceous hillside flora on the 

 hillocks and os slopes of the Upsala arable plain may partly be a relic fioni 

 the time v\hen the plain had just risen out of the sea. For this upheaval of the 

 land took place, for the most part, during the Sub-boreal period when the climate 

 was favoiuable for the distribution of xerothermous species. The iirst flora in 

 the localities mentioned must have been rich in such species. 



