20 BOTANY 



several other elements, which collectively we may call mineral 

 matters. If we were to separate a plant or animal chemically 

 into various organic compounds, we should find it composed of 

 various groups of tissues, the chemical compositions of which are 

 more or less alike. For example, the living part of a plant 

 corresponds chemically with the living part of an animal. The 

 starch found in grains or roots of plants has nearly the same 

 chemical formula as the animal starch found in the liver of man; 

 the oils of a nut or fruit are of composition closely allied to the 

 fat in the body, or in a sheep or cow. These building materials 

 of a plant or animal may be placed in one of the three following 

 groups of organic substances: carbohydrates, materials containing 

 a certain proportion of carbon, hydrogen, and oxygen; organic 

 fats and oils, which contain chiefly hydrogen and carbon; and 

 nitrogenous, or 'proteid substances, which contain nitrogen in addi- 

 tion to the above-mentioned elements. The above three kinds of 

 organic materials also form the organic foods of all animals and 

 plants. 



Foods. — What is a food ? We know that if we eat a certain 

 amount of proper foods at regular times, we shall go on doing a 

 certain amount of work, both manual and mental. We know, 

 too, that day by day, if our general health is good, we are 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. — Food substances may be classed into a number of 

 groups, each of Avhich may be detected by means of its chemical 

 composition. Such food substances are known as nutrients. Let 

 us now examine a few of the nutrients that we are likely to meet 

 in our daily life, and see how we could test chemically for their 

 presence. 



