20 



THE SMALLEST LIVING THINGS 



oxide, marsh gas, sulphuretted hydrogen, etc. Nitrogen is an 

 element absolutely necessary for living things to synthesize 

 (build up) more protoplasm. It is only available to animals 

 when taken into the digestive cavity in the form of proteins 

 derived from other animals or from plants. Through the putre- 

 factive agency of bacteria some of the nitrogen-holding sub- 

 stances of dead animal and plant protoplasm become available 

 as food for other types of pond-dwelling or earth-dwelling organ- 

 isms. In this respect the putrefactive bacteria play much the 

 same role as do the digestive ferments in the intestines of higher 

 types of animals. Here, through ferment action, protein food 

 substances are broken down by hydrolysis,* to nitrogen-holding 

 polypeptides and amino acids, simple substances which are ca- 

 pable of being utilized as foods by the cells of the body. 



Although most of the all-important nitrogen in dead bodies 

 of animals and plants is thus made useful to living organisms by 



the putrefactve bacteria, 

 some nitrogen escapes in 

 gaseous form, usually am- 

 monia (NH 3 ), into the at- 

 mosphere. Animals can ob- 

 tain their nitrogen only from 

 animal or plant proteins, 

 which they take in as food. 

 The great majority of plants, 

 including all the higher 

 plants, can get their nitrogen 

 in the form of salts (nitrates 

 chiefly), which, when dis- 

 solved in water, are absorbed 

 by the roots. Very few plants have the power to utilize the free 

 nitrogen which constitutes a large percentage of the air, but 

 certain types of bacteria — known as the "nitrogen-fixing" bac- 

 teria — have the ability to do this and they play the part of a 

 middleman between the nitrogen of the air and the living proto- 

 plasm. Thus certain forms of bacteria (Azotobacter) found in 

 the soil actually use the nitrogen of the air as food to build up 



DENITRIFYING 

 BACTERIA 



Fig. 9— THE NITROGEN CYCLE 



* Hydrolysis is a chemical process in molecules of a substance whereby, by the 

 addition of water, the substance is split into simpler substances. 



