38 



THE NITROGEN CYCLE 



anaerobic. There is another one also occurring in sea water 

 known as Azotohacter, which in the presence of oxygen is 

 capable of fixing free nitrogen. 



(iv) There is yet a further method by which the same gas 

 can be built up into the food of the green plant. 

 If one examines the roots of clovers or vetches 

 or peas or beans or any other plant belonging 

 to the family Leguminosae one will find cer- 

 tain swellings or nodules looking like 

 tumours on the finer roots. Within 

 these nodules are a number of bacteria 

 of the genus Pseudomonas. Normally 

 these bacteria live in the soil, but when 

 they come across the thin wall of a 

 root-hair of a suitable plant they press 

 their way through it and then, quickly 

 multiplying, they infect the tissues of 

 the root and cause the formation of 

 nodules. As they propagate, the cells 

 of the nodules become swollen. Many 

 millions of such bacteria may be found in the 

 roots of one plant. The bacteria use the ready- 

 made sugar which the plant has built up as * 

 nourishment, and in return they take up from 

 the air, which is circulating in the interstices 

 between the cells of the root, free nitrogen. 

 After a time the green plant re-asserts itself 

 and digests the bacteria, using up the ni- 

 trogen which the latter have fixed. On the 

 death of the plant a few survivors remain 

 and serve to re-infect the soil and other roots, in t? f f 



and the nitrates which have been built up groad Bean with 

 in the nodules help to fertilize the soil. The nodules. After 

 use of such plants as clovers, peas, and Strasburger. 

 beans as rotation crops has been known for centuries. They 

 greatly enrich the soil. 



(v) Still another source of the fixation of nitrogen, and 

 a most important one, is the decaying and putrefying of 

 plants and animals and their refuse. The nitrogen in this 



