CliAi'. I] THE FORMATIONS 



^11 



ind locks, whilst S. incanus exclusively occupied spots wiiere 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 

 Dy physical diilcrences in the substratum, since the latter usually change 

 nuch more rapidh^ and are more varied than the former. 



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

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

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

 ssentially the same in both woodland and grassland districts. Climate 

 xercises merely a differentiating action in edaphic formations, just as 

 loes the soil in climatic formations. 



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

 ontinuous supplies of water, determines the existence of certain edaphic 

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

 letermines the existence of others. Chemical differences in the soil 

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

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

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

 or 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 

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

 "here the rain merely wets the superficial soil. Accordingly, we find 

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

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

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

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

 istinguishable from climatic ones by their dependence on collections 

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

 ibterrancan. 



Stagnant zvater determines the origin of those formations that are 

 rmed sivainps, which are again subdivided into several groups, of 

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

 :st characterized. Swamp-formations are but slightly affected by 

 mospheric precipitations, and therefore exhibit essentially the same 

 :getation both in a woodland climate and in a grassland climate ; on 

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

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

 e former because the chemical processes on which the formation of 

 :at depends come into play at a low temperature only. 



UMPER jVj 



