SYMBIOTIC BACTERIA AND LEGUMINOUS PLANTS. 95 



perform their functions best when growing with certain other soil 

 organisms. They grow well with other fungi and with some 

 algae, organisms generally found associated with them in the soil. 



Other Nitrogen Fixers. It has been claimed that other organ- 

 isms are capable of fixing atmospheric nitrogen. Some species 

 of molds have been placed in this class, and certain species of alga, 

 as well as a considerable list of other kinds of bacteria. Indeed, the 

 power of fixing nitrogen has been said by some to be a fairly common 

 property of bacteria. Concerning this subject little is known at 

 present, but it is quite likely that the list of nitrogen-fixing organisms 

 will be considerably extended in the next few years. 



Nitrogen Fixation in the Soil. As yet little is known concern- 

 ing the actual efficiency of these bacteria in the soil, although they 

 are certainly very active. Soils do gain nitrogen and continue to 

 do so for periods of years. A long series of tests has shown that 

 crops can be removed from some soils year after year with 

 no diminution in the amount of nitrogen that may be found 

 there. In these soils no legumes have been grown, and hence it 

 would seem that the supply of nitrogen must have come from the 

 supply which the bacteria gather from the air. The general 

 belief to-day is that this method of fixation of nitrogen is of very 

 great significance in all soils, and plays a much larger part in the 

 maintenance of the soil nitrates than we formerly supposed. It 

 is probable that the bacteria do not fix the nitrogen in a form 

 immediately available to plants. They probably build it into some 

 more highly complex compound, very likely of a proteid nature, 

 which is incorporated in their own bodies. Later the processes 

 of decomposition and nitrification act upon these compounds and 

 eventually convert them into available nitrates. Of this series 

 of changes, however, little is yet known. 



SYMBIOTIC BACTERIA AND LEGUMINOUS. 

 PLANTS. 



The Value of Legumes. It has been known for a long time 

 that leguminous plants in some way enrich the soil, even the Romans 

 having commented upon the fact. The idea was revived in the 



