374 PEACTICAL BOTANY 



343. Dependent relations of soil bacteria. Most bacteria are 

 saprophytic. Upon and within the soil are many kinds which 

 get their nourishment chiefly from the organic products of 

 plants and animals. Saprophytic bacteria, as they grow upon 

 these substances, decompose them until they finally no longer 

 appear as organic materials (Sect. 154, Relation of bacteria 

 to decay). Many kinds of saprophytic bacteria, and some of 

 the parasitic ones, are able to live for a long time in soils 

 which contain only a very small amount of organic matter. 

 The dependent habit of these saprophytic forms is important, 

 since they reduce much organic matter to a condition in which 

 it is usable in the growth of other plants. Conspicuous among 

 these important products of decay are the nitrogen compounds 

 which are essential to growth of green plants. 



There are four groups of soil bacteria of particular interest 

 in this connection. First, there are saprophytic forms which 

 in their processes of nutrition make certain compounds of 

 nitrogen and hydrogen which are known as ammonia. This 

 bacterial action is known as ammonification, which means am- 

 monia-making. The bacteria which are responsible for the 

 action are called the "ammonification bacteria." Secondly, 

 there are the so-called nitrite bacteria, which in their processes 

 of nutrition change ammonia into compounds in which there 

 is one part of nitrogen to each two of oxygen. Such com- 

 pounds are known as nitrites. Thirdly, there are the nitrate 

 bacteria, which change nitrites into compounds in which there 

 is one part of nitrogen to each three of oxygen. Such com- 

 pounds are known as nitrates. These last two processes are 

 spoken of as nitrification. And fourthly, there are still differ- 

 ent bacteria (Sect. 37, Chapter III) which at times enter the 

 roots of certain kinds of plants, as clover, soy beans, peas, and 

 alfalfa. When some of these bacteria have entered the roots 

 they are surrounded by tissue so as to form nodules or tuber- 

 cles (Fig. 305). Within these tubercles the bacteria are able 

 to take uncombined nitrogen from the air and to combine 

 it with oxygen in such a way as to form nitrates. These 



