116 THE ART OF CULTURE. 



We take it for granted, that the sea plants 

 require metallic iodides for their growth, and that 

 their existence is dependent on the presence of 

 those substances. With equal justice, then, we 

 conclude, that the alkalies and alkaline earths, 

 always found in the ashes of land-plants, are like- 

 wise necessary for their development. 



THE ART OF CULTURE. 



THE conditions necessary for the life of all vege- 

 tables have been considered in the preceding part 

 of the work. Carbonic acid, ammonia, and water 

 yield elements for all the organs of plants. Certain 

 inorganic substances salts and metallic oxides 

 serve peculiar functions in their organism, and 

 many of them must be viewed as essential consti- 

 tuents of particular parts. 



The atmosphere and the soil offer the same kind 

 of nourishment to the leaves and roots. The 

 former contains a comparatively inexhaustible sup- 

 ply of carbonic acid and ammonia ; the latter, by 

 means of its humus, generates constantly fresh 

 carbonic acid, whilst, during the winter, rain and 

 snow introduce into the soil a quantity of am- 

 monia, sufficient for the development of the leaves 

 and blossoms. 



The complete, or it may be said, the absolute 

 insolubility in cold water of vegetable matter in 

 progress of decay, .(humus,) appears on closer con- 

 sideration to be a most wise arrangement of na- 



