CHAPTER XI 



BACTERIA AND THE CIRCULATION OF NITROGEN 



(Continued) 



2. The Liberation of Organic Nitrogen by Putrefaction, and its 

 Mineralization by the Nitrifying Bacteria. 



ONE of the most conspicuous physiological differences between plants 

 and animals is the absence in plants of any nitrogen-containing excretions. 



Nitrogen, once assimilated by the plant, is built up into proteids, 

 alkaloids, colouring-matters (chlorophyll, indigo), and other compounds, but 

 as long as the plant lives is not excreted. It is set into circulation again 

 only by the death and decay of the tissues, for the living plant can be used 

 as a source of nitrogen only by parasites and by plant-eating animals. 



In the animal body nitrogen is present chiefly in the form of proteids 

 and their derivatives the albuminoids, e. g. mucin, glutin, keratin, elastin, as 

 well as other highly complex bodies such as lecithin, haemoglobin, chilin, 

 and nuclein. In the animal, as in the plant, the nitrogen of these substances 

 is set free at death, but, in addition, the animal, unlike the plant, gives off in 

 its secretions (milk) and excretions (dung, urine) a number of nitrogenous 

 compounds. It is to these substances, enriched still more by the nitrogen 

 contained in straw, that stable-manure owes its high value as a fertilizer for 

 plants. 



But the nitrogen in fresh stable-manure is in a form that is useless to 

 plants ; they cannot assimilate it. In the urine of herbivora nitrogen is 

 present mainly as hippuric acid, in human urine mainly as urea and uric 

 acid. In the excrements it is contained in the remnants of undigested 

 nitrogenous food, and the products of decomposition set up by the bacteria 

 of the intestine. These products include indol, skatol, leucin, tyrosin, and 

 many simpler nitrogen compounds even down to ammonia. But none of 

 these, not even ammonia, are directly available as nutritive materials for 

 green plants. Not until the nitrogen has been removed from the organic 

 molecule by the process of putrefaction, and has been united to a mineral 

 base by the process of nitrification, can it be taken up by the green plant. 



