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 6t, (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. The ability of 

 a nation to obtain its needed food from the soil may thus come 

 to depend upon a continuous supply of cheap energy, such 

 as is furnished by waterfalls. It has been found that under 

 certain conditions some green plants are able to make use of 

 atmospheric nitrogen, but it is not likely that we shall be able to 

 get our farm crops to grow without suitable nitrogen fertilizers. 



