CHAPTER VI 



THE NITROGEN CYCLE 



SOURCES OF NITROGEN— THE PROTOPLASiMIC CYCLE 



If in our withered leaves you see 



Hint of your own mortality : — 



Think how, when they have turned to earth, 



New lovehness from their rich worth 



Shall spring to greet the light ; then see 



Death as the keeper of eternity. 



And dying Life's perpetual re-birth ! w. l. 



Sources of Nitrogen 



We have seen how a plant bnilds up its sugar and starch 

 from the carbon dioxide of the air and the water absorbed 

 by the roots from the earth; but to build up their proteins 

 plants also require nitrogen. The atmosphere contains some 

 78 per cent, of this colourless, inert gas, which combines 

 with difficulty with other elements. 



There are several sources of nitrogen compounds in Nature, 

 both inorganic and organic: 



(i) Occasionally during a thunderstorm electricity causes 

 the atmospheric nitrogen to combine with oxygen and to form 

 nitrous or nitric acids which are brought down to the earth 

 by the rain. 



(ii) Perhaps the Aurora Borealis is an important source of 

 nitrogen compounds. Astronomers have recently shown that 

 there are always some auroral effects in progress, so perhaps 

 we have here a continuous action similar to the synthesis of 

 nitrogen compounds in a laboratory by means of the electric 

 "silent discharge." 



(iii) But a far greater supply of nitrogen taken up by plants 

 and ultimately by animals is organic and is due to innumerable 

 microscopic organisms called Bacteria. Certain of these 

 organisms living in the soil and in the water, using sugars as 

 a source of their carbonaceous food, are yet able to fix the 

 nitrogen of the atmosphere. One of them, Clostridium, 

 flourishes best in the absence of oxygen ; that is to say, it is 



