64 ELEMENTARY BIOLOGY 



The interdependence of living things in the matter of food 

 manufacture has been shown from the point of view of the 

 oxygen cycle, of the carbon cycle, and of the nitrogen cycle. 

 This idea may be more clearly perceived from the diagram 

 on page 63 (Fig. 22). 



With the outbreak of the Great War the nitrogen problem 

 took on a new aspect, for nitrogen compounds are as necessary 

 for the manufacture of explosives as they are for the manu- 

 facture of protoplasm, that is, the raising of crops. The 

 leading nations have established chemical factories for the 

 production of nitrogen compounds out of atmospheric nitrogen 

 by means of electricity obtained from waterfalls. 



