Unity and Interdependence in Living World 651 



plant materials. Because of the irritating presence of these bacteria 

 in the roots of such plants, the latter are stimulated to form enlarged 

 nodules on their roots. The nitrogen-fixing organisms are in these 

 nodules. (2) Free nitrogen is also changed to nitrates by other bacteria 

 hving in the soil (genus Azotohacter and genus Clostridium). (3) 

 Other plants may secure their nitrates by bacterial decomposition of ani- 

 mal products (such as urea) and other nitrogenous materials. Certain 

 bacteria (ammonifying bacteria) act on nitrogenous compounds chang- 

 ing them into ammonia (NH3) through a process known as ammonifi- 

 cation. (4) Other bacteria {Nitrosomonas sp. and Nitrosococcus sp.) 

 oxidize the ammonia into nitrites (containing NO2). (5) Still other 

 types of bacteria (Nitrohacter sp.) oxidize the nitrites into nitrates. This 

 whole process of transforming ammonia into nitrites and the latter into 

 nitrates is called nitrification. The nitrates so formed are usable by 

 plants. ThuSj the two sources of nitrates for plants are: (a) the fixation 

 of free nitrogen from the atmosphere and (b) the production of nitrates 

 from ammonia by the process of nitrification. 



When plants die, their complex nitrogenous compounds are reduced 

 by bacterial action to ammonia, which can be used in the process of 

 nitrification. When animals die, their complex nitrogenous compounds 

 are also reduced by bacterial action to such simple compounds as ure.a, 

 which can be converted into ammonia, to be used as a starting point in 

 the nitrification process. Plants and animals both depend on these 

 chemical compounds for their food materials, the animals depending 

 upon the plants for at least part of their nitrogen supply from which 

 to build proteins. As has been shown above, the roots of certain plants 

 also contribute to the available supply of nitrates to be used by plants. 

 Animals in turn use plant nitrates for their various needs. 



There are still other types of bacteria which convert nitrates to nitrites, 

 oxides of nitrogen, and free nitrogen — a reverse process which removes 

 nitrogen from the soil. This process is known as denitrification. 



2. Carbon Cycle. — Carbon is also an essential constituent of both 

 plant and animal protoplasms. The stages in the carbon cycle may be 

 briefly summarized. The carbon dioxide of the atmosphere comes from 

 the respiration of animals, from the burning of wood, coal, oil, and gas, 

 from various manufacturing processes, and from volcanoes (Fig. 325). 

 Green plants which contain chlorophyll are able to combine the carbon 

 dioxide with water in the presence of sunlight to form plant carbo- 

 hydrates such as sugars, starches, and cellulose. This process is known 

 as photosynthesis. Plants utilize the materials produced by photosyn- 



