RELATION OF AIR TO FOOD MANUFACTURE 77 



from a plant or from some other source where they may be 

 found in such a condition that it can use them directly. The 

 plant, on the other hand, is able to make these compounds 

 from very simple substances. 



Such compounds as starch and sugar are 

 composed of three simple substances, carbon, 

 hydrogen, and oxygen. We can easily prove 

 that this is the composition of starch by heat- 

 ing some in a test tube (fig. 39). The heat 

 decomposes the starch, and we have carbon 

 and water. Water, we already know, is com- 

 posed of hydrogen and oxygen, and therefore 

 we have here the three substances. 



83. The place where food is made. If a green 

 leaf is examined early in the morning, little or 

 no sugar or starch will be found in it. After 

 several hours' exposure to sunshine, however, 

 sugar and starch will be abundant in the leaf, 

 and the quantity will increase with longer 

 exposure to the sun's rays. Sugar is made in 

 the leaf from carbon dioxide and water and 

 converted into starch for storage. Under the 

 influence of sunshine this manufacture may 

 go on in any green part of the plant. Since 

 the leaves are the principal green parts of 

 plants, it is in the leaves that most of the 

 sugar-making is carried on. Sugar may be 

 used as food by the leaf or stored in the leaf 

 after conversion into starch, or may be trans- 

 ferred to other parts of the plant and then used or stored 

 as starch until it is needed by the plant. 



84. Some problems. We shall now want to know how the 

 carbon dioxide gets into the leaf, how the water gets there, 

 what chemical changes take place, and what the light does in 

 this process. We shall have to learn certain things about the 



Fit;.3U. Decom- 

 position of starch 



When starch is de- 

 composed hy heat, 

 a hlack deposit of 

 carbon remains in 

 the bottom of the 

 tube, and water 

 condenses on the 

 inside of the tube 



