32 BOTANY: PRINCIPLES AND PROBLEMS 



for experiment has shown that without their presence the soil 

 would soon become unfit to support a vegetation of higher plants. 

 Most notable among these micro-organisms are the Bacteria 

 (Fig. 18), tiny, single-celled plants which lack the green pigment 

 chlorophyll. Many of these — the bacteria of decay — decompose 

 the complex organic substances found in humus into such simple 

 end-products as carbon dioxide, water, and ammonia, thus 

 releasing great quantities of nutrient materials which would 

 otherwise be locked up and useless in dead bodies of animals 



^6 e® 



B 





^' 



Fig. 18. — Some important soil bacteria. A, nitrite bacteria, Nitromonas; 

 X 2000. B, nitrate bacteria, Nitrobacter; X 2000. C, a common decay-produc- 

 ing organism, Bacterium mycoides; X 1500. D, nitrogen-fixing bacteria, Rhizo- 

 bium leguminosarum; X 750. 



and plants. Still other bacteria in the soil cause chemical changes 

 of various sorts there, the results of which are of great moment 

 to the higher plants. Notable among these are bacteria con- 

 cerned with the transformations of nitrogen and its compounds, 

 for* through their activity alone is the available supply of this 

 necessary element maintained in the soil. The continual circula- 

 tion of nitrogen through its various successive stations in organ- 

 isms, air, and soil is known as the Nitrogen Cycle (Fig. 19). 

 Complex nitrogenous substances returned to the soil in the bodies 

 of dead animals and plants are broken down by the bacteria of 

 decay into simpler compounds, which are finally reduced to 

 ammonia. Since most plants can use nitrogen only when it 

 occurs in the form of nitrate salts, however, this ammonia is not 

 directly available to them but must first be converted into nitrate 

 salts through the process of nitrification. This is carried on by 



