iv SUBSTANCES OF A LEAF 31 



Water, about four-fifths. 



A portion that burns away with flame. 



Charcoal. 



Ash. 



Part of the Food of the Plant is derived from 

 broken-down Rocks 



These experiments have shown us that the plant 

 consists of a portion that can be burned away, and 

 another or mineral portion, the ash, which cannot 

 be burned away. 



The material contained in the ash (which is a 

 mixture of several things, as we shall see presently) 

 must come from the mineral portion of the soil, the 

 broken-down rocks, of which that portion of the soil 

 is composed. 



Let us consider, just now, only this part of the 

 food of the plant, the part derived from the crumbled 

 rock of which the soil is partly composed, and con- 

 veyed in solution in water into the plant. 



If we search in the soil for the mineral substances 

 contained in the ash of the plant, we find that they 

 are present in an available form in very small quanti- 

 ties, so that they will be temporarily used up if plants 

 are grown repeatedly on the same soil, and then removed. 



The Effect of Farming on the Soil 

 Now let us try and understand what will happen 

 on a piece of wild uncultivated land. The plants 



