Chap. I] THE FORMATIONS 177 



and rocks, whilst S. incanus exclusively occupied spots where the soil 

 was deeper. The hybrid between the two species was confined to the 

 place between these two habitats. 



Such a grouping is determined much more rarely by chemical than 

 by physical differences in the substratum, since the latter usually change 

 much more rapidly and are more varied than the former. 



Much more marked than in the cases of the above description are the 

 effects of the soil in edaphic formations, where the type of vegetation 

 is determined, not by the climate, but by the soil, so that it remains 

 essentially the same in both woodland and grassland districts. Climate 

 exercises merely a differentiating action in edaphic formations, just as 

 does the soil in climatic formations. 



The presence of plenty of water in the soil, due to the infiltration of 

 continuous supplies of water, determines the existence of certain edaphic 

 formations, but it is the mechanical texture of the substratum that 

 determines the existence of others. Chemical differences in the soil 

 have at most merely a regulating or differentiating action. Only large 

 quantities of easily soluble salts, especially common salt, or of free 

 humous acids can efface the climatic character of vegetation and evoke, 

 for instance, xerophytic formations in a hygrophytic climate. 



ii. EDAPHIC FORMATIONS DUE TO TELLURIC WATER. 



In the neighbourhood of water the soil remains constantly moist, down 

 to a considerable depth ; this is the case even in a grassland climate, 

 where the rain merely wets the superficial soil. Accordingly, we find 

 the banks of rivers and lakes stocked with woods as far as infiltration 

 extends. These are often mere bush- woods, but not unfrequently developed 

 as luxuriant forests {fringing forests) not inferior to those of the best 

 forest climate (Fig. 93). Such edaphic woodlands are evidently always 

 distinguishable from climatic ones by their dependence on collections 

 of water, whether this be superficially exposed to the air, or, as in oases, 

 subterranean. 



Stagnant water determines the origin of those formations that are 

 termed swamps, which are again subdivided into several groups, of 

 which the peat moors, and the mangroves of tropical coasts, are the 

 best characterized. Swamp-formations are but slightly affected by 

 atmospheric precipitations, and therefore exhibit essentially the same 

 vegetation both in a woodland climate and in a grassland climate ; on 

 the other hand, their two most striking forms, moor and mangrove, 

 depend on the supply of heat, the latter for reasons not yet known, 

 the former because the chemical processes on which the formation of 

 peat depends come into play at a low temperature only. 



SCHI.MPER N 



