VEGETATION. 347 



ing and rearrangement of the soil which are favorable to 

 fertility. 



481. Inorganic Food of Plants. The materials of which we 

 have spoken as ministering to the growth of plants are said 

 to be their organic food. They are composed of the four 

 grand elements carbon, oxygen, hydrogen, and nitrogen. 

 But there are other substances which are absorbed in va- 

 rious quantities in different plants, as silica, potash, lime, 

 phosphorus, etc. These are said to be their inorganic 

 food. Although the inorganic are not as essential to the 

 growth of plants as the organic substances, still the fact 

 that the most important of them are present to some ex- 

 tent in all plants shows that every plant requires some 

 amount of them for its full development. If a plant fails 

 to find them its growth is feeble, and it withers before at- 

 taining maturity. 



482. Ashes of Plants. If a plant be burned, we obtain in 

 the ashes the inorganic portion of it. The organic part 

 has flown off in the form of gas, the carbon having formed 

 carbonic anhydride with oxygen, the hydrogen water with 

 oxygen, and ammonia with nitrogen. The ashes show how 

 small a proportion of the substance of plants is inorganic. 

 Ordinarily every hundred grammes of wood affords but 

 two of ashes, the other ninety-eight grammes having been 

 dissipated in the air. 



483. Mineral Classification of Plants. The ashes of differ- 

 ent plants differ very much in their inorganic constituents. 

 A knowledge, therefore, of their composition in this respect, 

 derived from a chemical examination of their ashes, is very 

 important for an intelligent application of manures in rais- 

 ing different crops. The inorganic substances which are 

 found to predominate in the ashes of a plant must be con- 

 sidered as indispensable to its nourishment ; and if the soil 

 be deficient in them they must be supplied by the culti- 



