Chapter III —39— On Areas 



Among other factors limiting the extent of an area — and for the 

 plant usually insurmountable — are climatic conditions. The latter, 

 creating the climatic boundaries of an area, may limit the distribution 

 of a species both horizontally (to the north, south, east, and west) and 

 vertically (altitudinally). Climatic boundaries are not determined by 

 any one climatic factor but by all of them taken together, in conse- 

 quence of which a study of the climatic boundaries of an area and the 

 elucidation of the rdle of individual climatic factors meet with very 

 great difficulties. The latter are all the greater because the reaction of 

 species to climatic phenomena is closely linked with their biological 

 characteristics, as a result of which their climatic boundaries are charac- 

 terized by extraordinary diversity. Nevertheless, a study of the areas 

 of plants provides a basis for determining the most important climatic 

 factors affecting their distribution. 



Altitudinal climatic boundaries are the result of a particularly com- 

 plex combination of causes, often very difficult to fathom, the most 

 important of all being insufficient warmth (inadequate sum of tem- 

 peratures above the minimum temperature required for the given 

 species). In addition to the latter, insufficient humidity, intensity of 

 the sun's rays at high altitudes, strong heat radiation, eternal snow or 

 late melting of snow, height above sea level depending on the latitude 

 of the locality, and other factors also play a part in determining the 

 altitudinal climatic boundaries of an area. 



Until the end of the nineteenth century evaluations of climate and 

 also of the cHmatic boundaries of the areas of plants were made chiefly 

 on the basis of temperature data. At the beginning of the twentieth 

 century it became clear that atmospheric humidity, as a factor deter- 

 mining the boundaries of the distribution of plants, was of predominant 

 importance. In many cases, as, for example, for most evergreen plants, 

 temperature plays only an indirect role in the limitation of the distri- 

 bution of a species, the chief factor being humidity conditions. Hence, 

 in determining the boundaries of areas, both these factors should be 

 taken into account (Gams, 1931). 



The boundaries of an area may be determined not only by climatic 

 causes but also by edaphic causes or by a combination of edaphic, 

 climatic, and geographical causes. Lastly, competition with other 

 plants may create an insurmountable obstacle to the further distri- 

 bution of a species. 



In many cases the boundaries of areas cannot be explained by any 

 cause at present in force, due to the fact that these areas were formed 

 under the influence of conditions in past epochs, often in other geo- 

 logical periods. A study of such areas and the establishment, on the 

 basis of such a study, of the history of distribution of a given species 

 constitute one of the chief tasks of historical plant geography. 



The boundaries of areas may, then, be subdivided into three main 

 types: first, boundaries set by physical barriers impassable for the 

 given species, such as seas, straits, rivers, mountains, deserts, etc.; 

 second, boundaries determined by ecological conditions; and, third, 

 boundaries determined by competition among species. Moreover, an 

 area may be in a state of expansion, in case the dispersal of the species 

 is still in progress, or, on the other hand, it may be in a state of con- 



