62 THE FOOD OF PLANTS CHAP. 



The charcoal is obtained from the air by decom- 

 posing carbonic acid gas. 



The water is obtained from the soil. 



The nitrogen is obtained entirely from the soil, 

 except in the case of leguminous plants, which can 

 also obtain it from the air, which surrounds their 

 roots in the soil. 



The ash is obtained from the soil. 



We have also learnt that the ash contains, among 

 other substances, potash and phosphoric acid, and 

 that these are apt to run short, as they exist in an 

 available form in the soil in very small quantities. 



We know that when a seed first begins to sprout 

 it only requires to be supplied with water, as it 

 contains food for the young plant in the form of oil, 

 starch, and other substances. The young plant, how- 

 ever, develops in two directions pushing a root into 

 the earth, and stem and leaves into the air. 



The root sucks up water from the soil, which, 

 passing through the plant, evaporates from the leaves. 



The water brings from the earth two kinds of 

 food in solution : mineral food derived from the 

 decayed rocks of which the soil is partly composed, 

 and vegetable food, the most valuable ingredient of 

 which is combined nitrogen in a soluble state, derived 

 from the decay of plants in the soil, or from the 

 organisms already described. 



The leaves obtain food from the air in the form 

 of carbonic acid gas, keeping the carbon or charcoal 

 they require, and setting free oxygen again. 



