4. Plants. — These act in several ways: — 



(a) Mechanically. — The roots penetrate the rocks or soils, 

 rendering them porous, and thus admitting air and water. 

 Plants growing on rocks also tend to keep the surface moist, 

 and thus favour erosion. 



{b] Chemically : — 



(i) During life. — By the corrosive action of the liquid 

 seci eted by the roots and root-hairs.t 



(ii) After death. — By producing carbon dioxide and 

 various vegetable acids which have solvent properties 

 upon certain constituents of soils. 



The formation of a mass of pulverised rock, however, is not 

 all that is necessary for producing a fertile soil. Ordinary 

 plants require the presence of organic nitrogen compounds 

 of the nature of " humus," and the chief source of such matter 

 is the remains of previous plants. The question at once 

 suggests itself, how did the soil first obtain its organic matter 

 necessary for plant growth ? According to recent researches, 



t Recent work seems to point to carbon dioxide evolved by the root- 

 hairs as the corrosive agent, not any vegetable acid. 



