SAPROPHYTES 405 



aid in keeping the nitrogen in circulation, they do not add to 

 the total stock. 



462. Nitrogen Fixing Bacteria. A second group of bacteria 

 which have to do with the nitrogen content of the soil consists 

 of several species which recently have been isolated and 

 proved to be able to take the free atmospheric nitrogen and fix 

 it in compounds which ultimately become available for the higher 

 plants. These bacteria live in the soil and derive their carbo- 

 hydrate food from the humus just as others do, and, if fixed 

 nitrogen is abundant in the soil, they also use this. In the 

 case, however, of the scarcity or absence of fixed nitrogen, they 

 are able to draw on the great ocean of free atmospheric nitrogen. 

 Unlike the first group discussed above, these bacteria are thus 

 able to add to the total nitrogen content of the soil. It is doubtless 

 true that these organisms have been, at least, important factors 

 in bringing about the gradual increase in the nitrogen content 

 of the soil that has occurred during the ages. 



In ancient times, the custom of FALLOWING land was largely 

 practised. This consists of cultivating the land for a season 

 to keep down the weeds without attempting to raise a crop on 

 it. It was believed by the ancients that in some way through 

 this practice, the land was enabled to produce a very much 

 better crop the following season, and this belief was abundantly 

 supported by practical experience. We now have, at least, a 

 partial explanation of this fact in these bacteria which add 

 atmospheric nitrogen to the soil. 



463. Nodule Bacteria. The third group of bacteria that 

 affect the nitrogen of the soil, like the second group, have the 

 power of fixing atmospheric nitrogen but they seem to be able 

 to do this only when they live within the root tissue of certain 

 higher plants, mainly those that belong to the botanical family 

 which includes clover, alfalfa, beans, peas, and the like. "When 

 these organisms grow in the roots of the host pla-nt, they cause 

 little outgrowths, or knots, to form on the roots, which are 

 called NODULES, and the bacteria themselves are often 

 spoken of as NODULE BACTERIA (Fig. 272). The nitrogen 



