. The continental element in the flora of south Sweden. Plants. 316 RIKARD STERNER. 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 the 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
. The continental element in the flora of south Sweden. Plants. 316 RIKARD STERNER. 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 the 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 Falbygden, but they are missing on the wide plain to the west of Falbygden where the clay and the moraine are com- paratively poor in lime. (See further later on p. 321.) 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 Ostergotland, 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. Their 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 Jiistorical causes. Sernander (1908, p. 219 fl".) 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 from the time when the plain had just risen out of the sea. For
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