572 



Introduction to the Study of Science 



the finest grades of butter and cheese ; in tanning leather, curing 

 tobacco, making hemp and linen. Under the guidance of 

 scientific knowledge, cultures of pure bacteria are now prepared 

 for the market. These are adapted to several of the pur- 

 poses above indicated ; for example, in agriculture to inoculate 

 such legumes as alfalfa, clover, soy beans, vetches, and the 

 like. 



The nitrogen cycle. The importance of bacteria which are 

 mainly concerned in putrefaction and decay, in taking nitrogen 



from the air and fixing it 

 in compounds available for 

 green plants, and in other 

 ways converting substances 

 into suitable plant food ma- 

 terials, may be seen in the 

 accompanying diagram (Fig. 



FIG. 169. The nitrogen cycle. 



169). Nitrogen is one of the 

 chief materials of plant food, 

 but must be furnished in 

 compounds which plants can 

 absorb and assimilate. Such 

 compounds are nitrates. Ni- 

 Trace trogen exists in abundance 



the complete cycle of nitrogen from the 



air through plant and animal life to the ln Organic matter, SUch as 



air. Bacterial action is indicated by WOod, Straw, manure, and 



broken Hne e s S : PlantandanimalaCti nby anim al bodies; but green 



plants cannot make use of it 



in this form. Molds begin the decomposition of organic matter, 

 and bacteria carry on the process, utilizing the organic ma- 

 terials as food and throwing off new compounds, which are 

 directly available as food material of green plants. For 

 example, ammonia is a nitrogen compound, but in this form is 

 not useful for plants. It is taken as a food by a certain species 

 of bacteria, which in using it combine it with oxygen or oxidize 

 it, forming a nitrate. Again the free nitrogen of the air is 



