WATER 317 



stoves, furnaces, charcoal-heaters and from tobacco as 

 ordinarily smoked. 



Water (H 2 O). Of not less importance than air to 

 plants and animals is water. It is formed during the 

 combustion of coal, wood and oil when the hydrogen in 

 them unites with the oxygen of the air. It is also 

 formed in the bodies of animals when the hydrogen in 

 the food which they eat unites with the oxygen which 

 they breathe. Water is of great importance for both 

 plants and animals because it dissolves food and waste 

 materials, so that they can be readily carried to and 

 from all parts of their structure. Moreover, it is only 

 when dissolved that food is capable of nourishing them 

 and of undergoing the various changes necessary for 

 growth arid the production of energy by oxidation in 

 animal tissues. 



The demand of living organisms for a definite amount 

 of water is so urgent that animals will seek for it at 

 any cost of exertion or danger. Plants in their help- 

 lessness can only wilt and die. 



The amount of water in the body of a man who weighs 

 150 pounds is about 87 pounds. The daily loss which 

 requires to be made good is about 6 pounds or 6 pints. 

 Of this, 3 pints is usually taken as drink, and the 

 remainder as liquid food. 



Soil. In addition to air and water as sources of food 

 supply for plants, there is the soil. Besides furnishing 

 a firm support for them, the soil is so fine of texture, so 

 filled with decaying organic matter and so well shaded 

 by vegetation that it holds the moisture given it by 

 the rain and gives it back to even the smallest plants. 

 The soil further supplies substances needed by plants of 

 both organic and mineral nature. In it are found 

 the minerals potassium, calcium, magnesium and phos- 



