196 Bacteria in Relation to Country Life 



proof that there is in the soil and living outside of plants 

 still another class of bacteria that are capable of utiliz- 

 ing nitrogen gas for their growth. 



Thus it was learned that the supply of combined 

 nitrogen to the plant world would soon fail but for the 

 activities of bacteria which, Uving either within the 

 roots or in the soil by themselves, provide for the 

 continuance of life on this earth. They are different 

 from the other classes of bacteria already considered 

 in that they are concerned with the addition of nitrogen 

 compounds to the resources of the plant and animal 

 world, and are known, therefore, as nitrogen- fixing, or 

 nitrogen-gathering bacteria. The others can only change 

 the form of combination, and are concerned only with 

 the transformation of soil-nitrogen, a process that fre- 

 quently involves considerable losses of nitrogen. 



The nitrogen-transforming bacteria we have already 

 discussed. We will now turn our attention to the very 

 important nitrogen-fixing kinds. 



The nitrogen-fixing bacteria may be divided broadly 

 into two classes, non-symbiotic and symbiotic. The 

 former live in the soil itself and develop there even 

 when no crop is growing upon it. The latter may also 

 grow in bare soils, but attain a pronounced power of 

 gathering atmospheric nitrogen only after they had in- 

 vaded the roots of some leguminous crop and had at- 

 tained a certain development there. The term sym- 

 biotic is derived from the word symbiosis, which means 

 living together. It is applied to describe a condition 

 in which two organisms, rather different in character, 

 live together to the advantage of both. 



