E. V. WulfE — 38— Historical Plant Geography 



consequently, migrated presumably from the east. Lastly, the third 

 category embraces species distributed within the limits of the islands in 

 two isolated areas— western and eastern. From these data Palmgren 

 draws the conclusion that the first category of species, constituting the 

 great bulk of the flora of the islands, migrated to the islands from the 

 Scandinavian peninsula, from Sweden; the second category from 

 Finland or the eastern section of the Baltic seacoast; and, lastly, the 

 third category from both directions. 



But, in opposition to the foregoing, Eklund (1931) shows that in 

 southwestern Finland there is found a very great diversity of habitat 

 conditions, particularly of edaphic conditions. This diversity is very 

 clearly reflected in the distribution of plants. In the western part of 

 this region there are the best soil conditions, shown, first of all, in the 

 fairly high content of Hme in the soil. Here the flora is richest. From 

 this locality in all directions the soil grows poorer, accompanied by an 

 impoverishment of the flora. Hence, the decrease in the frequency of 

 occurrence of species from west to east is to be explained not by the 

 greater distance from the place from which they migrated, as Palm- 

 gren assumed, but by ecological causes, expressed in this case by the 

 indicated differences in soil conditions. Eklund remarks that in the 

 Aland Islands there may be observed a decrease in the frequency of 

 occurrence of species and an impoverishment of the flora from west to 

 east, while in Uppland there is just the reverse— an impoverishment 

 from east to west. In both cases this impoverishnient is to be ex- 

 plained by one and the same cause, by a decrease in the content of 

 hme in the soil. Consequently, Palmgren's conclusions with respect 

 to the direction of migration and the chief country from which the 

 Aland Islands derived their flora, based on the decrease in frequency 

 of occurrence of species from west to east, are in the given case in- 

 correct, since he did not take ecological conditions into account. 

 Eklund comes to the conclusion that the islands were populated with 

 species from an entirely different direction than Palmgren supposed. 

 The present areas of many species do not constitute the maximum 

 territory that they may possibly be capable of occupying. The fur- 

 ther expansion of these areas has been curtailed by obstacles that have 

 up to the present prevented these species from continuing to spread. 

 By the artificial introduction of plants into new habitats outside their 

 natural area it is frequently found that a species may grow under a 

 considerably wider range of ecological and geographical conditions. 

 This shows that each species has, besides its actual area, a "potential 

 area" (Good, 1931). This circumstance is of exceptional practical 

 importance in the introduction and regional allocation of new crops. 



Boundaries of an area: — The limits of distribution of a species, the 

 boundaries of its area, formerly very ineptly termed "vegetation lines", 

 are determined by the reaction of the species to any of numerous 

 factors or combinations of these factors. Among the most obvious 

 causes hindering the dispersal of a species are purely mechanical 

 obstacles, such as mountains, seas, deserts, etc. Only in rare cases 

 does a plant, by the mere dissemination of its seeds and their transport 

 by chance agents, succeed in overcoming such obstacles and extending 

 its area beyond them. 



