ASSIMILATION OF CARBON 



55 



ia a cubic centimeter of air in the Park of Montsourie. In the same table are 

 shown the corresponding numbers, averages from ten years of observations, 

 in one of the squares in Paris (Place Saint- Gervais). The numbers are much 

 larger in cities than in the country. 



Microorganisms occur not only in the air but also in water and soil. The 

 water of rivers always contains bacteria, these being especially numerous in the 

 vicinity of cities. The following numbers of bacteria were found in a cubic 

 centimeter of water from the rivers and at the localities cited below. 



River Rhone 



River Spree 



above Lyons 75 



below Lyons 800 



above Berlin , 4,300 



below Berlin 97,400 



Microorganisms also occur in rain water, in snow and in hail. 



The soil always contains microorganisms, their number naturally depending 

 upon the amount of organic material present. Many more are found near the 

 surface than in the deeper layers. The following table gives an idea of their 

 distribution at various depths in a soil covered with forest growth (at Pfingst- 

 berg, in the vicinity of Potsdam). These are the numbers of microorganisms 

 found in a cubic centimeter of soil from various depths at different times of the 



