FIXATION OF NITROGEN 61 



the plant in a form in which it can absorb it . Present 

 knowledge does not make it possible to state in 

 definite chemical symbols what are the exact Nitro- 

 gen compounds formed. What can be seen when 

 the bacteria are cultivated in pure cultures is that a 

 slime forms round the bacteria. The mass of slime 

 and bacteria can be shown to consist of Carbon, 

 Oxygen, Hydrogen, and Nitrogen, and as the only 

 source of the nitrogen is the air, it can be stated defi- 

 nitely that the bacteria have seized the Nitrogen 

 from the air and combined it with Carbon, Oxygen, 

 and Hydrogen. It can also be stated definitely that 

 the nitrogen compounds so formed are such that 

 the plant can absorb them and build them into the 

 nitrogenous bodies or proteins that their tissues 

 require. 



In the early stages of the process, as we have seen, 

 it is the bacteria that are the attacking party. The 

 leguminous plant, however, waits and takes its 

 revenge. The V or Y shaped bodies become more 

 numerous, but as the plant grows older many of them 

 are dissolved and absorbed. Some, however, remain 

 behind, and when the plant eventually dies they 

 migrate back into the soil and await their oppor- 

 tunity for invading the roots of the next crop of 

 leguminous plants. 



If the ground in which leguminous plants are 

 grown is rich in nitrogen, as, for instance, as a 

 result of liberal treatment with well-rotted stable 

 manure or with nitrates, no nodules are formed on 

 the plant roots. A natural explanation is that the 

 growth of such plants is vigorous, and that their 

 oot hairs are strong enough to resist the attacks of 



