v] THE SOURCES OF FOOD 141 



evolved. Even among the plants the scarcity of 

 nitrogen food has led to the adoption of the parasitic 

 habit, to their symbiotic association with animals, 

 and to the insectivorous method of feeding. One 

 must think of modes of nutrition as being just as 

 adaptable to changed conditions as are the structures, 

 habits, and life-histories of organisms. 



On this view then the 79% of nitrogen which 

 exists in the atmosphere, and the relatively small 

 quantities of nitrogen compounds which are present in 

 the soil and sea are the results of denitrification 

 carried on in the past at a greater rate than at the 

 present time ; and the tendencies of vital processes 

 as we know them are towards a still greater amount 

 of denitrification. Yet we need not be alarmed, for 

 living processes are essentially regulatory, and both 

 nature and art are combining to solve the nitrogen 

 question ; art, in that it is now possible by the 

 advance of modern chemistry to combine atmospheric 

 nitrogen with oxygen on a large scale ; and nature by 

 reason of the adaptability of organisms. We know 

 that a number of species of plants, the Legumin- 

 osae (peas, beans, clover, etc.), are able to take up 

 elementary nitrogen from the atmosphere by the 

 assistance of associated bacteria contained in their 

 root tissues. Such nitrogen-fixing bacteria have also 

 been discovered to be widely distributed in the sea 

 and on the land. They can assimilate the free gas, 



