LEGUMINOUS PLANTS AND SYMBIOTIC FIXATION 103 
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 
alge, as well as a considerable list of other kinds of bacteria 
(see Fig. 22). 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 com- 
pounds and eventually convert them into available nitrates. 
LEGUMINOUS PLANTS AND SYMBIOTIC FIXATION 
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 eighteenth century and has been more or less fully realized 
by farmers since that time. To what this soil-enriching function 
