IT EVOLVES CARBONIC ACID. 47 



tutes the principal part of all the strata of brown 

 coal and peat. 



Humus acts in the same manner in a soil per- 

 meable to air as in the air itself; it is a continued 

 source of carbonic acid, which it emits very slowly. 

 An atmosphere of carbonic acid, formed at the 

 expense of the oxygen of the air, surrounds every 

 particle of decaying humus. The cultivation of 

 land, by tilling and loosening the soil, causes 

 a free and unobstructed access of air. An atmo- 

 sphere of carbonic acid is, therefore, contained 

 in every fertile soil, and is the first and most 

 important food for the young plants which grow 

 in it. 



In spring, when those organs of plants are absent, 

 which nature has appointed for the assumption of 

 nourishment from the atmosphere, the component 

 substance of the seeds is exclusively employed in 

 the formation of the roots. Each new radicle fibril 

 which a plant acquires may be regarded as consti- 

 tuting at the same time a mouth, a lung, and a 

 stomach. The roots perform the functions of the 

 leaves from the first moment of their formation ; 

 they extract from the soil their proper nutri- 

 ment, namely, the carbonic acid generated by the 

 humus. 



By loosening the soil which surrounds young plants, 

 we favour the access of air, and the formation of 

 carbonic acid ; and on the other hand the quantity 

 of their food is diminished by every difficulty which 



