Chapter VIII —129— Natural Factors 



But even in their case a study of their geography reveals that it is char- 

 acterized by definite regularities (Ascherson, 1905; Setchell, 1935). 

 This has been confirmed by a number of papers on the geography 

 of lichens (Degelius, 1935; Schindler, 1936-1938; Minayev, 1936, 

 1938; SuzA, 1925), mosses (Herzog, 1926), and ferns (Christ, 1910). 



Barriers to Dispersal: Their Influence on Plant Distribution: — 



It is necessary to draw particular attention to the existence and signif- 

 icance of barriers (physiographic, climatic, edaphic, biotic) to the dis- 

 persal of plants, barriers which are often insuperable even in case of the 

 slow, gradual, age-long advance of plants. 



1 . Physiographic Barriers. — Among the barriers hindering a plant's 

 free dispersal over the globe physiographic barriers are of major im- 

 portance. Chief among these are, for terrestrial plants, bodies of water, 

 primarily, of course, seas, and, for aquatic plants, bodies of land. 

 Kerner (1898) correctly points out that a considerable number of 

 American plants found their way to Europe not by being carried over 

 by birds of passage nor by the aid of sea or atmospheric currents but 

 simply by man's intervention, and that the boundaries set by the sea 

 are actually insuperable but, at the same time, temporary, i.e., of 

 significance only as long as the present distribution of land and sea is 

 preserved. If at any time a land-bridge had arisen connecting Europe 

 and America, many plants could have migrated over this bridge, and 

 the plants which, as is known, found their way from America to Europe 

 with man's aid might have spread thither without his assistance and 

 become distributed throughout Europe. Environmental conditions 

 would no more have hindered their establishment in Europe than they 

 did after their introduction by man. Precisely the opposite takes 

 place, i.e., migration of land plants is stopped when two regions 

 formerly united {e.g., Spain and North Africa) become separated by the 

 juncture of two water basins (in the given case, the Mediterranean Sea 

 and the Atlantic Ocean). 



Another major obstacle in the way of the dispersal of plants we find 

 in mountains, the vegetation on the opposite sides of a mountain range 

 often being sharply distinct. Mountains, besides constituting a purely 

 mechanical barrier, hinder the dispersal of plants as a result of the 

 efifect they produce on climatic conditions. Even in case seeds are 

 accidentally carried from one side of a mountain range to the other, 

 the differences in climate create for the immigrant plants habitat con- 

 ditions to which they have not been accustomed. 



2. Climatic Barriers. — Climatic conditions constitute one of the 

 chief factors in the distribution of plants over the earth's surface, de- 

 termining in most cases the limits of the distribution of species. Due 

 to the close dependence of plants on climatic conditions, climatic and 

 vegetation zones closely correspond. The ability to become adapted to 

 entirely different habitat conditions is exceedingly limited, in conse- 

 quence of which climatic boundaries constitute very real barriers to 

 plant dispersal. It should be noted that in this regard not only is the 

 climate as a whole of significance but each separate climatic factor. 

 In some cases the major role is played by temperature conditions; in 

 others by the length of day (short- and long-day plants); in stiU 



