18 THE VITAL FUNCTIONS. 



been, in any case, absorbed.* But in the form 

 of carbonic acid, this element is received in 

 great abundance, through the medium of water, 

 which readily absorbs it : and a considerable 

 quantity of carbon is also introduced into the 

 fluids of the plant, derived from the decom- 

 posed animal and vegetable materials, which the 

 water generally contains. The peculiar fertility 

 of each kind of soil depends principally on the 

 quantity of these organic products it contains in 

 a state capable of being absorbed by the plant, 

 and of contributing to its nourishment. 



The soil is also the source whence plants derive 

 their saline, earthy, and metallic ingredients. 

 The silica they often contain is, in like manner, 

 conveyed to them by the water, which it is now 

 well ascertained, by the researches of Berzilius, 

 is capable of dissolving a very minute quantity 

 of this dense and hard substance. It is evident 

 that, however small this quantity may be, if it 

 continue to accumulate in the plant, it may in 

 time constitute the whole amount of that which 

 is found to be so copiously deposited on the sur- 

 face, or collected in the interior of many plants, 

 such as the bamboo, and various species of grasses. 

 The small degree of solubility of many substances 

 thus required for the construction of the solid 

 vegetable fabric, is, probably, one of the reasons 

 why plants require so large a supply of water 

 for their subsistence. 



* Elements of Agricultural Chemistry, Lect. VI. p. 234. 



