FEEDS FOE ANIMALS 71 



The animal body cannot make energy; neither can a steam 

 engine. These machines, both the animal and mechanical 

 machine, must have as feed or fuel, the substances that will 

 burn. That is, when oxygen is brought into contact with 

 them under certain conditions, they will break down into 

 simpler compounds and give off the energy stored in them. 

 The conditions under which this happens in ordinary combus- 

 tion as in a stove or firebox under a boiler, is a high tempera- 

 ture. In the animal body it is the metabolism or the work of 

 the living cell that brings about the transformation to liberate 

 the energy stored in these compounds. 



The fact that these substances are chemical compounds of 

 considerable complexity means that they are a source of energy. 

 The animal uses this energy for living, walking, digesting its 

 food, and doing all the internal work called metabolism. 



Storage of Energy. — How does this energy happen to be 

 stored in the form of chemical compounds in plants and what 

 is energy? Energy is the ability to do work. So when the 

 steam engine uses fuel it can do work. So, also, when the 

 animal eats feed it can do work. This energy comes origi- 

 nally from the sun. As the grass and all other plants grow 

 they use up this energy as it comes from the sun in the form of 

 heat, light and other rays. It is appropriated by means of 

 the green called chloraphyll. Thus a plant cannot grow unless 

 it is green and it cannot grow unless the sun shines. Of course, 

 plants can grow for a short time on the food stored in the 

 seed or in the tuber. But in order to store food for man or 

 animals, plants must be green and they must have sunlight. 



Thus it is seen that plants live to make food for animals 

 and the animal can live because the plant made food for it. 

 The two together make a complete cycle. The plant takes 

 the elements and simple compounds and by means of the 

 energy coming from the sun builds these up into higher or 

 more complex compounds, so to speak, bottling up the energy 

 from the sun. The animal then takes this bottled-up energy 

 in the form of protein, carbohydrates and fat and lets it out. 

 It uses the energy and gives the elements and simple com- 

 pounds back to nature so that they can be used over again. 



Why is a desert so very hot in the summertime? And why 



