156 ROOTS 
matter. They are of various kinds and perform various func- 
tions, the most important of which has to do with making 
nitrogen available for root absorption. The importance of the 
Bacteria concerned with providing available nitrogen in the soil 
is due to the fact that nitrogen is an indispensable constituent of 
protoplasm, and, although composing four-fifths of the air, it is 
only available when it occurs in the form of a soluble salt in the 
soil where it is taken in through the roots. To maintain in the 
soil an adequate supply of available nitrogen, which is constantly 
being lost by the removal of the crop and through drainage, is an 
important problem in maintaining soil fertility. The Bacteria 
provide available nitrogen in two ways. First, some kinds act 
on the ammonia which is usually abundant where there is humus, 
forming nitrates, which are soluble, and in which form the nitrogen 
is available to our higher plants. Second, certain kinds of Bac- 
teria use the free nitrogen of the air in forming the nitrogenous 
compounds of their bodies, which after death release these com- 
pounds to the soil, and in this way the soil has its nitrogen in- 
creased. Thus while some Bacteria change the nitrogenous 
substances already present in the soil into forms which can be 
used as a source of nitrogen by the higher plants, these forms, by 
adding nitrogenous compounds through the rapid multiplication 
and early death of their bodies, actually increase the nitrogen 
content of the soil, and for this reason are of most importance in 
maintaining the soil fertility. Although many of these Bacteria 
which fix the free nitrogen of the air live free in the soil, there are 
some, however, which live in the roots of some of the higher 
plants, especially in those of Clover, Alfalfa, Beans, and other 
Legumes. In this case they live in the nodules formed on the 
roots, and the relation between the Bacteria and the higher plant 
is said to be one of symbiosis, a name applied to such an intimate 
association of organisms. In this case both organisms are bene- 
fitted; for the Bacteria obtain some food from the higher plant 
and the latter obtains nitrogenous compounds from the dead 
bodies of the Bacteria. (Fig. 138.) 
However, not all kinds of soil Bacteria are so indispensable, 
for there are some which have harmful effects, in that they tend 
to lessen the nitrogen content of the soil by so thoroughly de- 
composing the nitrogenous compounds that the nitrogen escapes 
from the soil as free nitrogen or as ammonia gas. Among so 
