ORGANIC NITROGEN. ITS NATURE 55 
chrondin, and other compounds, none of which are likely to be more 
complicated than the original proteid of the food. Thus, in the 
bodies of plants and animals alike the nitrogen reaches a condition 
allied to proteid. But, while proteids may serve as food for 
animals and for the great class of colorless plants (fungi) they are 
quite out of the reach of the green plants, which are the great food 
producers of nature. Our next problem, then, must be to learn 
how these proteids are reduced to their original condition of nitrate. 
Part of the proteid thus built up into the body of the plant or 
the animal remains there until the animal or plant dies, and at 
death it is still a proteid and as complex as ever. In this form it 
may become incorporated into the soil when the animal or plant 
dies, or it may become eaten as food and pass through the body of 
another animal. But much of it will eventually reach the soil 
while still in the form of proteid. 
A second portion of the proteid is used up in the animal’s 
body to furnish energy and heat; it is metabolized, as we say. 
When it is thus used its complex chemical molecule is broken to 
pleces, and it is reduced to much simpler compounds. Butitis not 
decomposed sufficiently to bring the nitrogen back within the reach 
of plant life. The carbon in this proteid is in part removed from it 
and combined with oxygen, to be exhaled as COs. The molecule 
falls to pieces and various simpler by-products arise; but in the 
animal’s body, practically all of it eventually assumes the form of 
urea (CON2H,). Though this urea is a nitrogen molecule far 
simpler than proteid, still it is not simple enough for a plant food. 
Urea, or a closely allied compound, is the form in which nearly all 
of the nitrogenous material resulting from proteid metabolism in 
the animal body is excreted. Urea thus represents one stage in the 
destruction of proteid compounds, and to this stage the pro- 
teids are brought as the result of the metabolism in the life 
processes of animals. In some animals this urea is secreted as 
urine by the kidneys, but in others (birds) it is mixed with the 
feces; in all cases it contains the nitrogen which is no longer of any 
