8 SUBSTANCES OF WHICH PLANTS CONSIST. 



into the fire, the latter remains behind in the form of ash. 

 Both these portions are dependent upon, and in whole or in 

 part supported by, the food which the plant derives from the 

 soil. The mineral part is drawn from the soil alone, by the 

 roots ; while, of the organic part, one portion comes from the 

 soil through the roots, and another portion from the air through 

 the leaves. 



The organic part of plants consists of four elementary or 

 simple bodies : carbon, hydrogen, oxygen, and nitrogen. In 

 all the parts of plants these four are associated also with minute 

 quantities of sulphur and phosphorus, which are more or less 

 completely dissipated into the air when the plant is burned. 



Of the four substances above mentioned, the nitrogen ap- 

 pears to be drawn by plants almost exclusively from the soil 

 a fact of much importance to the practical man, in whose soil, 

 of course, it must be present, if plants are to grow well upon 

 it. The hydrogen and oxygen are drawn partly from the soil 

 and partly from the air chiefly in the form of water, which 

 consists of these two elementary substances. The carbon is 

 derived only in small proportion from the soil, being for the 

 most part sucked in from the air by the leaves, in the form of 

 carbonic acid gas. Sulphur and phosphorus come from the 

 soil only. 



The mineral part of the plant, which forms from half a per 

 cent to fifteen or even twenty per cent of the whole weight of the 

 dried plant, consists of from eight to twelve different substances. 

 These are potash, soda, lime, magnesia, oxide of iron, oxide of 

 manganese, alumina, chlorine, sulphuric acid, phosphoric acid, 

 silica, and probably fluorine. Of these substances, alumina is very 

 rarely present in appreciable quantity, at least in our usually 

 cultivated plants, and may therefore be neglected. Silica 

 exists chiefly in the stems of the grasses, in the stalks of corn, 

 in those of the cane and bamboo, in the husk and chaff of grain, 

 &c., and in small proportion only in the softer parts and juices 

 of plants. Potash, soda, chlorine, and sulphuric acid, are for 

 the most part found in the sap ; lime, magnesia, and oxide of 

 iron, in the solid parts of plants. Phosphoric acid is necessary 

 to, and is found in every part of, a plant ; but it collects in 



