CHANGES BROUGHT ABOUT BY NON-PATHOGENIC ORGANISMS 71 



its development do not often obtain. Bacteria of the nitrogen- 

 fixing aerobic type belong to the group called Azotobacter. These 

 organisms are abundant in many soils and fix considerable quan- 

 tities of nitrogen, gaining energy therefor by oxidizing the car- 

 bonaceous materials from dead plant tissues. The addition of 

 straw, for example, to a soil will furnish sufficient food so that 

 these bacteria will bring about an appreciable increase in the 

 nitrogen content. The importance of the molds in this connection 

 is not fully understood, but several species have been described 

 which are capable of fixing nitrogen. 



The microorganisms which fix nitrogen in symbiosis with 

 higher plants may be divided into two groups, those bacteria 

 which grow upon the roots of legumes and the molds which 

 grow on the roots of certain other plants. All plants belonging 



Fig. 38. Bacillus radicicola: a, Normal bacillar form; 6, bacteroids or involu- 

 tion forms. 



to the legume or pulse family, such as clover, alfalfa, peas, beans, 

 etc., usually bear upon their roots tubercles or nodules which, 

 when opened, are found to be made up of cells tightly packed with 

 bacteria. It has been shown experimentally that these organisms 

 growing within the roots in some way take up free nitrogen from 

 the air and eventually turn it over to the host plant, so that the 

 legumes are not dependent for their development upon nitrogen 

 which may be present in the soil, but can make use of the 

 free nitrogen of the air as well. These plants are, therefore, 

 very important in agriculture in the maintenance and increase of 

 soil fertility. This organism, known as Bacillus radicicola (Fig. 

 38), enters the young growing root through a root hair and causes 

 a kind of tumor formation in the tissues of the root, resulting in 

 the development of the nodule. The organism is at first a straight 

 rod, but later, when growing inside the cells of the host, it becomes 



