MAN'S NEED OF FOOD 



79 



a manufacturing process. The two processes may be 

 compared as follows: 



factory green leaf 



work rooms cells in the leaf 



machinery chlorophyll 



energy light 



raw materials soil water and carbon dioxide 



product carbohydrates (starch and sugar) 



by-product oxygen 



From the carbohydrates plants can produce other 

 substances such as fats, oils, and proteins. 



What are the factors that control our food supply? 

 You have learned that most of our food comes either 

 directly or indirectly from plants. Plants are living 

 things and as such must have favorable conditions un- 

 der which to grow. Among these factors are proper 

 temperature, sufficient soil moisture, fertile soil, and 

 sunlight. Let us consider the importance of each of 

 these factors in crop production. 



It has been found that seeds germinate better and 

 crops grow faster when soil temperature is between 

 6'5 and 75 degrees Fahrenheit. It is possible for man 

 to influence and control soil temperature to a certain 

 degree. If manure is added to the soil a certain chem- 

 ical change is brought about by microscopic organ- 

 isms called bacteria. This chemical change produces 

 a certain amount of heat, some of which is useful in 

 warming the soil. In the unit on weather you learned 

 that the process of evaporation is a cooling process. 

 This process is important in the regulation of soil 

 temperature. If moisture is permitted to evaporate 

 from the ground surface, it absorbs heat from the 

 soil and as a result cools it. Evaporation of soil mois- 

 ture is prevented by cultivation and mulching. 



Another important factor in crop production is the 

 proper amount of soil moisture. Everyone has ob- 

 served that plants and crops vary in the amount of 

 soil moisture that they require. The willow tree us- 

 ually grows where there is an abundance of water in 

 the soil while the cactus may grow where there is 

 relatively little. Most of our farm and garden crops 

 fall between these two extremes. Water commonly 

 exists in soil either as a film around the soil particles 

 or by filling the spaces between them. 



Providing the proper amount of soil moisture pre- 

 sents a different problem in various sections of the 

 country. For example, in certain sections there is an 

 over supply of water, and some of it must be drained 

 away. In other places the rainfall is so light that mois- 

 ture must be supplied by irrigation. 



If crop production is to be kept at the highest pos- 

 sible point, the fertility of the soil must be maintained. 

 As you have learned, plants take their nourishment 

 from the soil. This withdrawal tends to remove cer- 

 tain important mineral substances which must be re- 



placed if the fertility of the soil is to be maintained. 

 Nature has provided certain ways in which some of 

 the used substances may be replaced, but certain oth- 

 ers must be replaced through fertilizers by man. You 

 will learn more about these processes in a later unit 

 of this book. 



It is important that crops have sunlight not alone 

 for the heat which it gives to the soil but also, as you 

 have learned, because plants need the energy of sun- 

 light to build food. 



REFERENCES FOR FURTHER STUDY 

 Texts 



Caldwell and Curtis, Science for Today, Chap. 35 



Clement, Collister, and Thurston, Our Surroundings, Chaps. 



26-33 

 Hunter and Whitman, My Own Science Problems, Unit 1 1 ; 



Science in Our World of Progress, Unit 15 

 Lake, Harley, and Welton, Exploring the World of Science, 



Chaps. 26, 27 

 Pieper and Beauchamp, Everyday Problems in Science, Unit 



4 

 Powers, Neuner, and Bruner, The World around Us, Chap. 



22 ; This Changing World, Unit 6 ; Man's Control of His 



Environment, Chap. 28 



Skilling, Tours through the World of Science, Tour 19 

 Van Buskirk and Smith, The Science of Everyday Life, Unit 



3 



Watkins and Bedell, General Science for Today, Unit 11 

 Webb and Beauchamp, Science by Observation and Experi- 

 ment, Units, 4, 8 (parts) 

 Wood and Carpenter, Our Environment: Its Relation to Us, 



Unit 8; Our Environment: How We Use and Control It, 



Chap. 18 



Special references 



Atwood and Heiss, Educational Biology 



Kinne and Cooley, Food and Health 



Sanford, The Story of Agriculture in the United States 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. The scientific reasons why we eat. 



2. The sources of our food supply. 



3. A knowledge of how plants make food. 



4. The concept that animals are dependent upon 

 plants, the only living things that can build up food. 



TEST OF MASTERY OF THE TOPIC 



In your notebook complete the statements, answer the 

 questions, and comply with the instructions. 



1. What are the three main services performed by food? 



2. For each of the following foods, name the part of 

 the plant in which the food is stored; for example, sweet 

 potato root. 



Tomato, apple, banana, potato, lettuce, cauliflower, radish, 

 turnip, corn, beans, cabbage, onion, carrot, walnut. 



3. Food is manufactured by . 



4. The green substance present in the leaves of green 

 plants is called . 



