COMPOSITION OF PLANTS. 51 



amount of materials which the plants cannot take, or do not 

 need. A fertile soil is capable of retaining plant food, while 

 sandy soils, owing to their excellent natural drainage, are not 

 fertile unless frequently supplied with manure. 



Two different kinds of plants growing in the same field will 

 usually be found to contain certain substances in different pro- 

 portion. Some are essential, others not ; some in large quantity, 

 others in small quantity, yet, strange as it may seem, by the 

 chemical composition of a plant, we cannot always tell what 

 manures will benefit it most. 



Composition of Plants. — The combustible part of plants is 

 made up of five chemical elements — carbon, oxygen, hydrogen, 

 nitrogen and sulphur ; without these no plant is ever produced. 

 The carbon, hydrogen and oxygen form the cellulose, lignose, 

 pectin, starch, sugar, fat and vegetable acids. The same elements 

 united with nitrogen form the amides and alkaloids ; and further 

 united with sulphur the still more important albuminoids. 



The incombustible ash always contains five elements — potas- 

 sium, magnesium, calcium, iron and phosphorus, besides sulphur. 

 Iron is present in only very small quantity. Besides these, an 

 ash will generally contain sodium, silicon and chlorine, some- 

 times manganese, and perhaps minute quantities of other elements. 



The earlier chemists spoke of the combustible portions of 

 plants as "organic," and the incombustible portions as '^ inor- 

 ganic." This distinction is no longer considered accurate. 



Excepting oxygen, these elements are taken from compounds, 

 such as water, carbon dioxide and the substances combined as 

 shown in the following : 



Nitrates. "j f Ammonia. 



Sulphates. Potash. 



Carbonates. ^ __ Qp j Lime. 



Phosphates, f ' ' Iron, 



Silicates. Soda. 



Chlorides. J . l IVI agnq^ia. 



