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 
Pe) 8 a crop of clover or peas, the soil 
ad Cee 18 in better condition for growing 
Q other crops, but the way in which 
this is brought about, and the 
dt =i relation of bacteria to the process, 
A are matters of recent knowledge. 
; _ The clover, alfalfa, peas, and beans 
Fic. 168. Bacteria, or bacteroids 
(meaning like bacteria”) which STOW 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. Frovii Lipman, Bacteria in Relation to Country Life. The Macmillan 
Company. : 
