lo] ENVIRONMENTAL FACTORS 297 



organisms, and so forth. We have seen how climatic factors, such 

 as temperature and precipitation, are of supreme importance in 

 determining the general character of the vegetation over wide areas ; 

 here and in the next section we shall deal with the edaphic and 

 biotic factors, respectively, which are apt locally to modify the 

 conditions and vegetations of these major climatic belts. Especially 

 has it long been recognized, and utilized in agricultural, horticultural, 

 and forestral practice, that differences in the soil are often largely 

 responsible for differences in vegetation within the same climatic 

 region : consequently they are of great significance in plant 

 geography. 



Soil may be considered as the unconsolidated superficial material 

 of the earth's crust, lying below any aerial vegetation and undecom- 

 posed litter, and extending down to the limits to which it affects 

 the plants growing about its surface. Beneath the soil lies the 

 subsoil or unaltered rock. Though usually composed primarily of 

 material derived from the parent rock, the soil has come into being 

 largely through interaction of this ' substratum ' with climate and 

 living organisms. Thus its texture may be dependent largely on 

 water- and frost-action and other ' weathering ' tendencies, while its 

 content of humus (partially decomposed organic matter) results from 

 the contributions and activities of inhabiting plants and animals. 

 For soil is a veritable ' microcosm ' or little world — with its own 

 physical structure, chemical composition, atmosphere, flora, and 

 fauna. And characteristically it exhibits a perpetual series of actions 

 and reactions between organisms and environment. In it about 

 one-third (by volume) of the bodies of higher plants spend their 

 lives — influencing, and being influenced by, the particular conditions 

 obtaining in the soil. These conditions, through the subterranean 

 organs of plants, in turn influence their living aerial parts. 



Soils that are undisturbed by agriculture and other factors com- 

 monly become stratified into layers or ' horizons ' at different 

 depths, which often have very different compositions as well as 

 natures. Such ' profiles ' are, however, largely destroyed by cultiva- 

 tion. Normally, three main horizons or groups of horizons are 

 exhibited, namely, the upper or ' A ' zone of extraction of soluble 

 salts and fine-grained materials, the middle or ' B ' zone of their 

 concentration, and, below, the ' C ' zone where neither extraction 

 nor accumulation has occurred at all extensively. The characteristic 

 profiles so formed are to a considerable extent climatically engendered, 

 and so mature soils can be classified broadly into climatic types or 



