124 APPLIED BIOLOGY 



very simple substances absorbed from water in soil (e.g., 

 sodium nitrate used as a soil fertilizer), and to unite ele- 

 ments from these with those of sugar to make the substance 

 known as protein or albumen, which contains nitrogen in 

 addition to carbon, hydrogen, and oxygen. Most plants 

 can do this. 



Some very simple plants (bacteria) can use free nitrogen 

 from the air. Animals, on the other hand, must get their 

 nitrogenous (nitrogen-containing) food in the form of 

 proteins, for no animal has the power to make these from 

 carbohydrates and such simple substances as plants get from 

 the soil. Hence all animals depend upon plants for their 

 protein food, either getting it directly by eating plants as 

 food or indirectly by eating flesh from animals which derive 

 their protein from plants. 



Animals, then, depend upon plants for all their food-supply. 

 They require as foods carbohydrates, fats, and protein; and 

 only plants are known to make these. 



Especially should we note that both animals and plants 

 depend upon sunlight, which is necessary for combining 

 elements from carbon dioxide and water into carbohydrates, 

 which in turn are necessary for making proteins. Hence the 

 energy manifested in the activities of animals comes from 

 food formed by plants and indirectly from sunlight. This 

 is what is meant by the statement in popular books that the 

 " energy in foods is stored sunlight." 



Stored Energy. It should be noted further that plants by 

 photosynthesis of carbohydrates store energy which may later 

 be converted (by oxidation) into energy of action by either 

 plants or animals. Besides foods, we may name as examples 

 of stored energy such cases as gunpowder, a waterfall, coal, 

 a wound-up watch-spring, etc. Such stored energy is said 

 to be potential; that is, it has latent power. Energy being 

 liberated from burning coal, exploding powder, etc., is called 

 kinetic or energy of action. 



