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 



