CHAPTER VII 

 RELATION OF AIR TO FOOD MANUFACTURE 



70. 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 sub- 

 stances 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 ani- 

 mals feed upon plants, and here again there are only one or 

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

 food. The more we think of this the more fully we shall 

 be convinced that almost all of the things that we eat come 

 directly or indirectly from plants. The next question is, How 

 do plants produce these food materials ? 



71. Source of plant food. It is commonly believed that 

 plants secure their food from the soil, but this is not wholly 

 correct. If it were true, a plant growing in a small pot would 

 consume the soil, but we all know that a plant may grow for 

 a long time and become larger and heavier than the soil in the 

 pot was at first, and yet the soil will not appear to have de- 

 creased. The plant takes water from the soil, and very care- 

 ful observation would show that an extremely small part of 

 the soil itself is used. The remainder of the material in the 

 plant is not supplied from this source. 



Another thing that leads to the same conclusion is that some 

 of the most important substances in plants are not found in the 

 soil or are very scarce there. An example of this is the case of 

 the black substance called carbon. Wood contains a great deal 

 of carbon, though we do not usually recognize it as such, because 



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