NUTRITION 67 



These figures show plainly that where there is the greatest 

 proportion of living cells there is also the greatest amount 

 of nitrogen. Where living cells are most active, as in green 

 leaves and at growing points, and where there is the great- 

 est and most compact accumulation of stored food, as in 

 some seeds, the larger part of the nitrogenous substance 

 will be found. In many seeds, and in such places of stor- 

 age as potato tubers, reserve food is accumulated mainly as 

 starch, to be worked up into nitrogenous compounds only 

 as needed. 



Nitrogen occurs in nature in the uncombined state, form- 

 ing nearly four-fifths of the earth's atmosphere, and also 

 united into compounds of very different degrees of complex- 

 ity, solubility, and availability. Some of the compounds 

 are the results of the constructive activities of living organ- 

 isms (see p. 71), others are the products of their waste 

 or decay (see p. 36), and still others originate indepen- 

 dently of living organisms. Some of the nitrogen com- 

 pounds absorbed by plants contain carbon, others do not. 



The forms in which nitrogen is obtained by the plant are, 

 therefore, much more varied than are the sources of carbon. 

 The supply of nitrogen compounds is also far less constant. 

 At times plants may obtain from the air, as well as from 

 the soil and from the water, simple compounds of nitrogen, 

 such as ammonia and other gaseous substances, formed in 

 the air only under electrical influences, or escaping into the 

 air from terrestrial sources. 



With the exception of certain bacteria, either free-living or 

 associated with higher plants, no living organisms can use 

 the free nitrogen so abundantly supplied to them in the air. 

 They are absolutely dependent upon compounds of nitrogen. 

 Animals obtain all their nitrogenous as well as non-nitro- 

 genous foods from plants; most plants must obtain their 

 nitrogenous food-materials in the form of nitrates. In the 

 soil and in fresh and salt water there occur, besides vari- 

 ous nitrates, other soluble nitrogen compounds, some en- 

 tirely free from carbon, others united with it in more or less 

 complex forms. Substances of the latter sort are the con- 

 stituents or the derivatives of animal excreta or of the dead 



