SAPROPHYTISM AND SYMBIOSIS 789 



beaches. The tubercles live but a single season, hence in perennial legumes there 

 is a new bacterial infection each year. 



Nitrogen fixation and nitrification. Although it is an abundant and 

 important constituent of plants, only a comparatively small number of 

 plants, namely, certain bacteria and fungi, are known to be able to utilize 

 directly the free nitrogen that exists so abundantly in the air. This 

 incorporation of free nitrogen into nitrogenous compounds within the 

 body is known as nitrogen fixation, and it is a process of vast importance 

 to the entire organic world. The first organism definitely ascertained 

 to have the power of nitrogen fixation was Clostridium Pasleurianum, one 

 of the anaerobic soil bacteria. A number of nitrogen-fixing organisms 

 are now known, embracing various widely distributed species of Azoto- 

 bacter (a genus of aerobic bacteria) and of Bacillus, such as B. radicicola, 

 the organism inhabiting legume tubercles, and B. amylobacter, which is 

 thought by some investigators to include forms that have been referred 

 to Granulobacter and Clostridium. 



Probably the most important of all nitrogen-fixing organisms is Azotobacter 

 chroococcum, which in temperate climates is abundant in nearly all soils and also in 

 fresh and salt water, being absent chiefly in bogs and in some virgin soils. Nitrogen 

 fixation by this organism is best accomplished in aerated soils and is facilitated by 

 lime and phosphorus and retarded by acids. Contrary to earlier views, humus 

 facilitates nitrogen fixation by Azotobacter, probably because of its microorganisms; 

 it has been suggested, for example, that cellulose-destroying bacteria furnish carbo- 

 hydrates in available form for nitrifying organisms, and it has been shown that the 

 addition of sugar to cultures facilitates nitrogen fixation. In addition to the bacteria, 

 some yeasts and a number of fungi are now thought to be able to fix free nitrogen 

 (p. 797). Recently some investigators have claimed that the hairs of many plants 

 are able to fix nitrogen; the fact that the amount of nitrogenous materials in such 

 hairs is inconsiderable and that this small amount is no less when the hairs develop 

 in an atmosphere devoid of nitrogen make the claim very doubtful At the same 

 time it must be admitted that the known methods of nitrogen fixation seem inade- 

 quate to account quantitatively for the large and relatively constant supply of avail- 

 able-nitrogen in the soil, particularly in view of the considerable amount of denitri- 

 fication that is effected through the activity of various bacteria. 



In the processes of organic decay the complex proteins are broken up 

 into simpler substances, such as organic acids, amins, and ammonia. 

 Some of these, as ammonia, are oxidized through the agency of bacteria, 

 constituting a process that is known as nitrification. The first step in 

 this process is the formation of nitrites, and from these by further oxida- 

 tion are formed nitrates. Different erganisms are concerned in the two 



