PLANTS AND ANIMALS 35 



of the higher plants and turn them into forms of matter 

 which can reenter the life cycle of the vegetable world. 

 Similarly, the colorless plants are the agents of decay 

 where animal remains are concerned. Especial atten- 

 tion should be given to the action of some of these 

 colorless plants the bacteria upon the waste-products 

 of animals. 



Carbon dioxid and water, the major wastes of animal 

 life, are set free in fit condition to be used at once in 

 photosynthesis. This is not true of the nitrogenous 

 excretions of animals. Take, for example, the compound 

 urea which is the principal form in which nitrogen is 

 eliminated by mammals. This is not directly valuable 

 to the higher plants when it is dissolved in the soil about 

 their roots. But there are bacteria which can transform 

 urea into other substances beneficial to them. The 

 result is not brought about by a single reaction but by 

 stages, probably by three successive processes and 

 three different kinds of organisms. 



Urea is first changed to compounds of ammonia. 

 These in their turn are oxidized to nitrites. The final 

 step is the further oxidation of the nitrites to nitrates. 

 The utility of the nitrates in protein synthesis as carried 

 on by green plants has already been noted. A striking 

 demonstration of it occurs in connection with well 

 waters. These have been long in the earth and are 

 rich in nitrates as compared with rain or surface waters. 

 If well waters are pumped into open reservoirs and 

 left standing in the sunlight they encourage the growth 

 of green plants (algae) to an extent which is most un- 

 desirable in a public supply. If reservoirs which are 

 roofed over are provided for the storage of such waters 

 no troublesome growth will develop. 



The term Cycle of Nitrogen has been used to denote 

 the endless process in course of which protein is built 

 up by plants from simple materials and with the aid of 

 the solar energy, to be destroyed by the respiratory 

 activity of the plant itself, or of an animal, and then 



