206 INTRODUCTION TO BOTANY 



by tissue, so that they form nodules, or tubercles (figs. 167 

 and 168). Within these tubercles the bacteria are able to 

 take the uncombined nitrogen from the air of the soil and 

 combine it with other substances in such a way as to form 

 nitrates. The tubercle bacteria are known as the nitrogen- 

 fixing bacteria, since they take free nitrogen from the air and 

 combine it with other things so as to make plant foods. 



Since the ordinary grain-producing plants must have nitro- 

 gen in order to grow, and since they can use it only in the 

 nitrate form, the significance of these tubercle bacteria to 

 agricultural plants is evident. Agriculturists have known for 



a long time that, after growing 

 a crop of clover or peas, the soil 

 is in better condition for growing 

 other crops, but the way in which 

 this is brought about, and the 

 relation of bacteria to the process, 

 are matters of recent knowledge. 



The clover, alfalfa, peas, and beans 

 FIG. 168. Bacteria, or bacteroids , , ., . , , 



(meaning "like bacteria") which g row better when the tubercle 



grew in the root tubercles * bacteria are present (fig. 169), 



and the added nitrates left by 



decay of the tubercles and the plants upon which they grew 

 enrich the soil for subsequent crops. 



There are many other kinds of soil bacteria, one kind liv- 

 ing in such a way as to denitrify nitrate compounds, thus 

 working in a manner opposite to some of the above groups. 



192. Bacteria and the destruction of foods. From what has 

 been said in preceding sections it is evident that all our fruits, 

 vegetables, meats, etc. are at times in danger of destruction 

 by bacteria or other organisms. How to prevent this destruc- 

 tion has been one of the important problems since civilization 

 began. Surplus production of food is useless unless some of 

 it may be kept for future needs. 



1 From Lipman, Bacteria in Relation to Country Life. The Macmillan 

 Company. 



