NITROGEN AND LIFE 55 



part of the proteids, nitrogen compounds needed by all 

 living cells. Nitrogen is, therefore, a necessary part of 

 all animals and plants. The proteids make up a large 

 part of such food as meat, eggs, cheese, and milk. 



Animals cannot make their own proteid ; they must get 

 it by feeding upon plants. Plants make proteids out of the 

 carbon, hydrogen, oxygen, nitrogen, etc., which they ob- 

 tain from the soil and the air. Now, most plants can take 

 up nitrogen only when it is a part of certain compounds 

 dissolved in the water of the soil. Hence soils must con- 

 tain nitrogen compounds to be fertile. 



While most plants can make proteids only out of nitrogen com- 

 pounds present in soil, some plants seem to take up nitrogen directly 

 from the air. Such are beans, peas, clover, and alfalfa. It has been 

 found that the plants named have this power because " colonies" of 

 bacteria microscopic plants (cf. 324) 

 find a home upon their roots. It is the 

 bacteria that take the nitrogen out of the 

 air, and build it into the complex albumins 

 upon which the beans, etc. feed. But the 

 bacteria produce not only enough proteid 

 for the beans, etc., but they produce an 

 excess of it, and leave it in the soil. Be- 

 cause they themselves contain so much 

 proteid, beans, peas, and clover are valu- 

 able as food for man and animals; but 

 they are even more important because FIG. 48. 



they, or, rather, the bacteria growing 

 upon them, are the means of bringing into 



a soil the nitrogen it needs for other plants. As all farmers know, 

 clover is grown in a field, and "ploughed under," to enrich the soil 

 for crops of grain. Artificial fertilizers, containing nitrogen com- 

 pounds, are often added to the soil for the same purpose. 



