CHAPTER VII 

 RELATION OF AIR TO FOOD MANUFACTURE 



79. Questions for Discussion. 1. When bread is burned in bak- 

 ing there has usually been no flame, and the bread has not been 

 consumed as would be true if it were burned in a furnace. What 

 has really happened to the food? What is the black substance pro- 

 duced, and what is its origin? 2. What plants are the principal source 

 of commercial starch? 3. What plants are the principal source of 

 commercial sugar? In what countries is most of the world's sugar 

 produced? 4. How do you account for the yellowish color of a grow- 

 ing wheat field after several cloudy days? 5. Why do most plants 

 grow slowly in the shade? 6. Why do leaves of most land plants die 

 if submerged in water? Can you suggest any way in which dust, for 

 example from roadways or cement works, might injure leaves ? 7. What 

 change do green plants produce in the earth's atmosphere? If then' 

 were no green plants, would the air probably remain suitable for man's 

 use? 8. Trace back to its origin the energy which is used to propel a 

 locomotive. 9. Are carnivorous animals in any way dependent upon 

 plants ? 10. If we were to fill our sleeping rooms with growing plants, 

 would they contribute to the oxygen supply of the air at night? 

 11. What plants do you know that grow without contact with the 

 soil? 12. What makes the stalks of celery white? 13. If plants are 

 the source of all foods, how do they secure their own food? 



80. Sources of our food. A large part of our food consists 

 of plants or parts of plants (such as fruits and vegetables) 

 that are eaten in almost or quite the natural condition. Such 

 substances as flour and sugar are manufactured from plants. 

 Of course in all these cases there can be no doubt that our 

 food comes directly from plants. Another important class 

 of food materials includes meats and other animal products. 

 The animals feed upon plants, and thus there are only one or 

 two stages before we come back to plants as the source of our 



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