The Azotobacters 203 



There are, also, symbiotic relations between azoto- 

 bacter and algae, one-celled, green plants. The algae 

 are widely distributed on land and sea and are found in 

 cultivated and uncultivated soils, in fresh and salt 

 water. As green plants, they are able to build their 

 sugars and starches out of carbon dioxid and water. 

 The azotobacter living with them use the sugars and 

 starches thus produced as the fuel for the production 

 of nitrogen compounds from the free nitrogen of the 

 air. The algae are, therefore, provided by the bacteria 

 with nitrogen compounds, while the bacteria are pro- 

 vided by the algae with sugar. It is probable that the two 

 organisms furnish very considerable quantities of com- 

 bined nitrogen to other inhabitants of the sea. At any 

 rate both Azotobacter chroococcum and Clostridium 

 Pastorianum have been found in large numbers in sea- 

 water. 



Agricultural importance of the two classes of bacteria. 

 Very little is known as yet of the actual additions 

 to the store of combined nitrogen in the soil by the 

 aerobic and anaerobic nitrogen-fixing bacteria. There 

 are a few instances on record when they were appar- 

 ently responsible for marked gains of nitrogen. It 

 is reported by Khun, for instance, that after growing 

 non-leguminous crops on the same soil (sandy loam) 

 for twenty years, the yields became no smaller, not- 

 withstanding the fact that only non-nitrogenous fer- 

 tilizers were added. Similarly, in a portion of the 

 Geesecroft field, at Rothamsted, abandoned to itself 

 between the years 1882 and 1904, and bearing no le- 

 guminous vegetation, there was an average annual 



