50 HOUSEHOLD BACTERIOLOGY 



green leaves through their chlorophyl cells are able 

 to take most of their carbon from the carbon dioxide 

 of the air. Some oxygen is also taken from the air, 

 but most of it is absorbed by the rootlets from the 

 ground air, the water in the soil, or from organic com- 

 pounds in solution in the water. Hydrogen is ob- 

 tained from water and other compounds containing 

 hydrogen and is taken in through the rootlets. 

 Nitrogen No plants can take their nitrogen directly from the 

 * ot Direct air. Although this gas with oxygen comprises the 

 major part of the atmosphere in which all vegetation 

 is bathed, it is not taken in through the leaves as the 

 carbon dioxide is. 



A government bulletin says : "Ever since anything 

 has been known in regard to plant nutrition and the 

 necessary part that various gases and minerals play 

 in the successful growing of crops, scientific men have 

 realized the great importance of conserving the world's 

 store of nitrogen and have made every effort either to 

 husband or to increase all available sources of supply. 

 In the early days, when it was first realized that 

 nitrogen was so essential to plant life in fact, was at 

 the very foundation of agriculture no particular 

 alarm was felt. Botanists had demonstrated that plants 

 obtained their carbon from the carbon dioxide of the 

 air, and since this gas is present in so much less quan- 

 tity than nitrogen it was believed that by no possible 

 means could the most essential of plant foods be ex- 

 hausted. However, when it was shown that plants 



