with Geographic Botany. — Soil. 473 



soil is with them of very trifling importance. Some plants 

 undoubtedly have predilections for particular soils, but these 

 are exceptions to the mass of vegetation, and in some of these 

 cases it is the property of retaining moisture or collecting 

 heat which directs the bias of the plant, more than any inhe- 

 rent peculiarity of the soil. 4. Some water plants float on the 

 surface without any communication with the bottom, as the 

 Lemna and many Cellulares ; the marine algae, though at- 

 tached, are without the least semblance of a soil to draw 

 from, and some are of such enormous length, that it would in 

 all probability take a long time for any nutrient matter to find 

 its way from the root to the opposite extremity ; whilst the 

 Sargassum vulgare, if not without a root, is at least well able 

 to survive a separation. 



A natural soil is a very heterogeneous substance ; its base 

 might be expected to proceed from the disintegration of the 

 predominant rock in its vicinity, and such it usually is ; many 

 foreign materials are soon mixed with it, substances drifted 

 by the wind, the decaying parts of vegetation, the droppings 

 of animals, in some cases not an unimportant integrant, even 

 occasionally the dead remains of animals themselves ; but in a 

 state of nature these latter are soon invaded by a host of 

 beings which ere long remove every vestige of their prey. All 

 these substances are included in the twofold division of inor- 

 ganic and organic ; the former being derived from the mineral 

 world, the latter from plants and animals. 



The organic portion, or mould, is that constituent of the 

 soil which is the actual food of plants, and whilst the inor- 

 ganic part acts as a mechanical agent in retaining or diffusing 

 moisture, this is destined, after being rendered soluble by the 

 action of the atmosphere, to furnish nourishment to vegeta- 

 tion. As mould results chiefly from vegetable decomposition, 

 many varieties may be distinguished according to the kind of 

 plants furnishing it. One, well known, is found on our heaths, 

 or in situations where the Ericae have been growing, and is par- 

 ticularly suited for the rearing of other members of their fa- 

 mily. The mould formed by the decay of the Eucalyptus of 

 New Holland would seem to have such a prejudicial etfect on 

 the soil as to exclude other vegetation, and assemblages of 

 tree-ferns have apparently the same effect. The black soil at 

 the bottom of pools and ditches of stagnant water owes its 

 richness to the large quantity of vegetable remains in a state 

 of rapid decomposition and rendered soluble for future nutri- 

 tion. 



The different mineral productions which form the mass of 

 our globe, regarded as to their efficiency in creating soils, have 



Ann. ^ Mag. N. Hist. Vol. ix. 2 I 



