1 62 BACTERIA 



to the soil of sewage, that we must depend in the future for 

 storing nitrogen as nitrates. 



1. Some combined nitrogen is absorbed by the soil or 

 plant from the air, for example, fungi, lichens, and some 

 algae, and the absorption is in the form of ammonia and 

 nitric acid. This is admittedly a small quantity. 



2. Some free nitrogen is fixed within the soil by the 

 agency of porous and alkaline bodies. 



3. Some, again, may be assimilated by the higher chlo- 

 rophyllous plants themselves, independently of bacteria 

 (Frank). 



4. Electricity fixes, and may in the future be made to fix 

 more, nitrogen. If a strong inductive current be passed be- 

 tween terminals, the nitrogen from the air enters into com- 

 bination with the oxygen, producing nitrous and nitric acids. 



5. Abundant evidence has now been produced in support 

 of the fact that there is considerable fixation by means of 

 bacteria. 



Bacterial life in several ways is able to reclaim from the 

 atmosphere this free nitrogen, which would otherwise be 

 lost. The first method to which reference may be made is 

 that involving symbiosis. This term signifies " a living 

 together" of two different forms of life, generally for a 

 specific purpose. It may be to mutual advantage', a living 

 for one another, or it-may be, by means of an interchange 

 of metabolism or products, finally to produce or obtain some 

 remote chemical result. It is convenient to restrict the term 

 symbiosis to complementary partnerships such as exist be- 

 tween algoid and fungoid elements in lichens, or between 

 unicellular algae and Radiolarians, 1 or between bacteria and 

 higher plants. The partnerships between hermit crabs and 

 sea-anemones and the like are sometimes defined by the term 

 commensalism (joint diet). Symbiosis and commensaHsm 

 must be distinguished from parasitism, which indicates that 



1 Geddes, Nature, xxv., 1882. 



