KEEPING THE BODY IN CONDITION 



CHAPTER IX 



THE MEANING OF FOOD 



Questions. 1. Why must we eat food ? 2. Must all living things have 

 food ? 3. How do plants take in food ? 4. Have all animals mouths ? 

 5. Is the food of one organism suitable for other organisms ? 



76. The material needs of protoplasm. We know that we 

 must have food to keep us alive, but the connection between 

 feeding and keeping alive is not always clear. Many people 

 think that feeding is the same as eating; yet the plants and 

 many animals have no mouths, and they also must feed. Since 

 it is the protoplasm that is alive, we should think of food in re- 

 lation to the peculiarities and activities of protoplasm. 



The fact of growth means the need for a material income 

 suitable for protoplasm building. It is impossible for proto- 

 plasm (or any other substance) to be made out of nothing. The 

 fact of movement calls for materials that can yield energy. The 

 nearly fluid state of active protoplasm, and the chemical changes 

 that are constantly going on within it, call for a water income. 

 We find also that various salts or mineral substances are a 

 necessary part of protoplasmic constitution, although we do 

 not know exactly how each one acts in this complex mixture. 

 Some of the salts seem to start chemical processes among other 

 materials that make up the protoplasm. Other salts (or ele- 

 ments, perhaps) appear to modify certain chemical processes, 

 just as the bromide used by the photographer slows up the 

 development of his negative ; these are called regulators. In 

 addition, nearly all protoplasm must have air, or rather oxygen, 

 which the protoplasm gets from the air. 



lOI 



