44 



of course indisputable, but it is solely because it 

 'affords the plant a surplus of organic food by the 

 products of its decomposition, which are principally 

 carbonic acid, ammonia, and water, and which are 

 either returned to their original element, the atmos- 

 pheric air, to be absorbed by the leaves, or dissolved 

 by the moisture in the soil and assimilated through 

 the roots. We can therefore easily understand the 

 importance of decaying vegetable matter in a loose, 

 porous soil, where all particles are accessible to the 

 influence of the atmospheric air and its oxygen, 

 out of contact with which vegetable matter is 

 inert, and instead of increasing the fertility of the 

 soil, has, on the contrary, a sterilizing effect a 

 striking example of which we have in the extensive 

 bogs of northern climates. A great amount of 

 carbonic acid is undoubtedly derived from decaying 

 vegetable matter in the soil ; but by far the greater 

 portion is absorbed directly from the atmospheric 

 air, which must therefore be regarded as the prin- 

 cipal source of this important element. 



The absorption of carbonic acid from the atmos- 

 phere is that phenomenon which is tenned vegetable 

 respiration, and is conducted through the small 

 pores or openings in the cuticle of the leaves, called 

 stomata. Atmospheric air, the bulk of which is 

 oxygen and nitrogen, contains, besides ammonia, 

 invariably a quantity of carbonic acid, which, 

 although relatively very . -small, being only two 



