MATERIALS FROM PLANTS AND ANIMALS 425 



the individual has over himself and over his environment. It 

 should be the ideal of all thinking persons to make themselves 

 so far as possible in- 

 dependent of these 

 various devices that 

 interfere with or 

 modify the action 

 of the nervous sys- 

 tem. In the case of 

 extremely harmful 

 drugs, such as opium 

 and cocain, it is nec- 

 essary to make strict 

 regulations to pre- 

 vent people from be- 

 coming dependent 

 upon them. Most of 

 us should be able to 

 manage such things 

 without the police- 

 man or other state 

 officer, and to find 

 forms of enjoyment 

 that are fully sat- 

 isfying but without 

 any danger to our 

 health and our re- 

 sponsibility. 



315. Indirect uses 

 and services. Plants 

 and animals may 

 serve us without 

 our making direct 



Fig. 176. The carbon cycle 



Fires and all kinds of living things are constantly- 

 throwing carbon dioxid into the air. Green plants, 

 represented by the tree in the diagram, withdraw car- 

 bon dioxid from the atmosphere and return oxygen. 

 The material of the green plant is made up in part of 

 the carbon derived from the carbon dioxid. This mate- 

 rial serves as food for animals and as fuel for fires. 

 The animals oxidize this material, or they are eaten by 

 other animals. Finally, the carbon of larger plants and 

 animals is oxidized by simple organisms, such as bac- 

 teria and fungi, and is returned to the atmosphere. Ni- 

 trogen, oxygen, and other elements circulate about in a 



similar way 



use of the materials 



in their bodies. The by-product of photosynthesis, oxygen, 



comes to us through the air as part of the continuous exchange 



