1014 FOOD OF PLANTS, OF ANIMALS. 



the plant by its roots. Another source of ammonia is animal 

 manure, particularly urine, which in a putrid state is rich in 

 salts of ammonia. There can be little doubt that nitrogen, in 

 the form of nitric acid, can also be assimilated by plants, as 

 appears by the favourable action of nitrate of soda, nitrate of 

 ammonia, and other nitrates upon vegetation. 



According to the observations of M. Boussingault, the 

 Jerusalem artichoke and leguminous plants generally can assi- 

 milate the free nitrogen of the atmosphere, to a small extent, 

 but the cereals and other plants are entirely destitute of 

 that power.* The quantity of nitric acid or nitrate of am- 

 monia produced in the atmosphere by lightning, must be utterly 

 insignificant, although some importance has been assigned to 

 this as a source of the azotised food of plants. 



Of the fixed earthy and saline constituent of plants which are 

 derived from the soil, and are found in their ashes when burnt 

 none is more generally necessary than the silicate ofpotas7i, 

 which is produced in most soils by the gradual decomposition, 

 under atmospheric influences of the felspathic minerals they 

 contain, or is added in the form of the ashes of burnt wood and 

 plants. Earthy phosphates are quite essential to the cereals, 

 and are added to the soil in animal manure ; hence the constant 

 remark that the cereals, like the domestic animals naturally 

 follow man in his migrations. 



The vegetable kingdom is undoubtedly the great laboratory 

 in which organic substances are produced, for from the sub- 

 stances enumerated, water, carbonic acid and ammonia, and not 

 from any store of original matter in the soil are the principles 

 in plants derived. The steps of the conversion of these into 

 the organic principles in the organism of the plant escape detec- 

 tion, but the general character of vegetable action is of a 

 reducing nature, such as the Sun's light favours, carbonic acid 

 certainly and probably water being decomposed, their carbon 

 and hydrogen retained, and their oxygen returned to the 

 atmosphere. 



Food of Animals. The organic matters produced by plants 

 form the food of animals ; for animals produce little or no 

 organic matter, but on the contrary destroy it. Indeed the 



* Ann. de China, et cle Phys. t. 76, p. 5 and t. 69, 353. 



