58 



NATURAL HISTORY 



Plants and animals are made out of the same chemical elements. 

 Burn some beans or a piece of beefsteak, a piece of wood or a bit of 

 living bone, an entire green plant or a dead mouse, and the chemist 

 would tell us that the same chemical elements are present in animals 

 and plants ; that certain of these elements passed off in the smoke, 

 others into the air as colorless gases, leaving still others as a 

 whitish ash. All living things are composed mainly of carbon, 

 oxygen, hydrogen, nitrogen, with about twelve other chemical ele- 

 ments found in very minute quantities. These elements are all 

 present in the immediate environment of plants and animals, air, 

 water, and soil. 



How they get from the basic environment into living things can 

 be briefly stated. Carbon, which is contained in all organic foods 

 and in this condition is taken into the animal body, can only be 

 absorbed in the form of carbon dioxide by food-making green 

 plants. This gas, which is present in the atmosphere to the average 

 amount of about 0.03 per cent, gets there as a result of oxidative 

 processes taking place in plants and animals, as well as by the com- 

 bustion of organic substances. Factories and volcanoes alike form 

 their quota of carbon dioxide to diffuse out into the atmosphere. 

 The cycle of the passage of carbon from plants to animals and from 

 animals back to plants is shown in the accompanying figure. 



Hydrogen, another component part 

 of living things, cannot be used in 

 its pure state by either plants or 

 animals. In water (H2O) , it becomes 

 an important part of the food of 

 animals, and as water vapor it is 

 used in starch-making by green 

 plants. 



Oxygen is freely available to both 



plants and animals. As a gas, making 



up over 20 per cent of the air, capable 



of being dissolved in water for aquatic 



plants and animals, it is used by all 



living things in respiration. Green 



plants add this gas to the air during the process of starch-making. 



Nitrogen is one of the most important elements found in living 



things. Making up 79 per cent of the air, it is not usable in the 



form of a gas except by the nitrogen-fixing bacteria. 



The carbon and oxygen cycles in a 

 balanced aquarium. Trace the pas- 

 sage of an atom of carbon from a 

 green plant back to the plant. 



