236 RIKARD STERNER 



Naturally no sharp border can be established between the meridional and the 

 boreal species, as forest-oases occur in the whole of the steppe district as well 

 as steppe-like associations in edaphically suitable localities, such as calcareous 

 hillsides and rocky escarpments facing south high up in North Russia. (See espe- 

 cially Pohle 1903 and Korshinsky 1886 and 1888). It should also be noticed 

 that steppe species have in a considerable way extended their range in Central 

 Russia, thanks to the woods being cut down or thinned by the hand of man. (See, 

 e. g., Flerov 1902.) As a matter of fact, there are also a considerable number 

 of species that are so evenly distributed that it would be impossible to class 

 them with either the meridional species or with the boreal ones. It will there- 

 fore be necessary to set up two more groups. ( )ne comprises species that are 

 comparatively evenly distributed in the oak-zone of the boreal district and the 

 steppe region; the other comprises species that are widely distributed in Subarctic 

 Russia also. The former may be called MeridioBoreal, and the latter I'biquiious. 



Besides these five main groups now mentioned there is yet another of (juite 

 a different character. It consists of species with their chief distribution in con- 

 tinental Siberia. They do not occur in Eastern Europe, but have a few very 

 isolated occurrences or small areas of distribution in other parts of Europe, 

 above all Central Europe. This group of species may be called Siberian. 



Hence continental species might in the first instance be classified as follows: 



I. Meridional species: in Eastern Europe chiefly distributed in the steppe 

 zone. 



II. Meridio-Boreal species: in Eastern Europe distributed in the steppe zone 

 and the oak zone. 



III. Euboreal species: in Eastern Europe distributed in the oak zone. 



IV. Subarctically Boreal species: distributed throughout Eastern Europe north 

 of the steppe districts. 



V. Ubiquitous species: distributed in almost the whole of Eastern Europe. 



VI. Siberian species. 



It should be of interest to see how the species in a district on the boundary 

 between the Russian steppe antl forest districts are divided between these groups. 

 An analysis of the Choripetalae in the flora of the comparativeh' well-explored 

 government of Kazan gives the following results: 



Meridional species: 67. 



Meridio-Boreal species: 43. 



Euboreal species: 29. 



Subarctically Boreal species: about 20. 



Ubiquitous species: about 125. 



The great number of ubiquitary species is remarkable. A pretty conspicuous 

 characteristic of the East European flora is the fact that ubiquitary species are 



