24 ENVIRONMENT OF LIVING THINGS 



be adding weight to our body, and that added weight comes as the 

 result of taking food into the body. What is true of a boy or girl 

 is equally true of plants. If food is supplied in proper quantity 

 and proportion, they will live and grow ; if the food supply is cut 

 off, or even greatly reduced, they will suffer and may die. From 

 this, the definition which follows is evident. A food is a substance 

 that forms the material for the growth or repair of the body of a plant 

 or animal or that furnishes energy for it. 



Nutrients. Organic food substances may be classed into a 

 number of groups, each of which may be detected by means of its 

 chemical composition. Such groups of food substances are known 

 as nutrients. Let us now examine the nutrients. 1 



Carbohydrates. Starch and sugar are common examples of 

 this group of substances. The former we find in our cereals, bread, 

 cake, and most of our vegetables. Several forms of sugar are 

 commonly used as food ; for example, cane sugar, beet sugar, and 

 glucose or grape sugar. Glucose, found as the natural sugar of 

 grapes, honey, and fruits, is manufactured commercially by pour- 

 ing sulphuric acid over starch. It is used as an adulterant for 

 many kinds of foods, especially in sirups, honey, and candy. 



Fats and Oils. Fats and oils form an important part of the 

 composition of plants and animals. Examples of food in the 

 form of fat are butter and cream, the oils in nuts, olives, and 

 other fruits, and fat in animals. 



Proteids. Nitrogenous foods, or proteids, contain the element 

 nitrogen in addition to carbon, hydrogen, and oxygen of the car- 

 bohydrates and fats and oils. They include some of the most 

 complex substances known to the chemist, and, as we shall see, have 

 a chemical composition very near to that of living matter. Pro- 

 teids occur in several different forms. White of egg, lean meat, 

 beans, and peas are examples of substances composed in a large 

 part of proteids. 



Inorganic Foods. Water and various salts, some of which, 

 as lime, may be found in drinking water, form important parts in 

 the diet of plants and animals. Later we shall see that green 

 plants, although they use precisely the same foods as we do, take 

 into their bodies the chemical elements which form foods. From 

 1 For a fuller explanation of nutrients, see Chapters VI and XXIV. 



