52 THE PLAOT-. 



be present at the time when it is required and in 

 sufficient quantity. In the plant, this food under- 

 goes such chemical changes as are necessary to growth. 



Tlie compound substances contained in plants are 

 of two classes, those not containing nitrogen, and 

 those which do contain it. 



The first class constitute nearly the whole plant. 



The second class, although small in quantity, are 

 of the greatest importance to the farmer, as from 

 them all animal muscle is made. 



Animals, like plants, are composed of both at- 

 mospheric and earthy matter, and their bodies are 

 obtained directly or indirectly from plants. 



The first class of compounds in animals comprise 

 the fat, and like tissues. 



The second class form the muscle, hair, gelatine 

 of the bones, etc. 



In order that they may be perfectly developed, 

 animals must eat nitrogenized and non-nitrogenized 

 food, and in the proportions required by their 

 natures. 



. They require phosphate of lime and other mineral 

 food which exists in plants. 



Aside from their use in the formation oi fat, sub- 

 stances of the first class are employed in the lungs 

 and blood-vessels as fuel to keep up animal heat, 

 which is produced (as in fire and decay) by their 

 decomposition. 



When the food is insufficient for the purposes of 

 heat, the animal's own fat is decomposed, and carried 

 to the lungs as fuel. 



