408 



BENEFICIAL ACTIVITIES OF BACTERIA 



chapter. When a pod-bearing plant, such as the clover, alfalfa, 

 bean, or pea, is young, its roots are attacked by the nitrogen-fixing 



bacteria found in the soil. They 

 penetrate the roots through the deli- 

 cate roothairs and establish them- 

 selves in the outer layer of the root . 

 cells. The root accommodates them 

 by building more cells in that region, 

 forming little nodules or tubercles. 

 In these nodules, the nitrogen-fixing 

 bacteria multiply. They take the 

 free nitrogen from the air which 

 permeates the soil, and build it into 

 nitrites which are later converted 

 to nitrates. ^Vhen the roots of these 

 plants are plowed under, they decay, 

 and the nitrates are liberated into 

 the soil. Soil may be inoculated with 

 cultures of nodule bacteria. A crop 

 of clover or alfalfa plowed under 

 supplies the soil with about 100 



They were both watered with a nutrient pOUuds of uitrOgCU tO the acrC. Such 



solution containing all the necessary ^ Jg ^^ Valuablc aS many loads 



nutrients but nitrogen. Explain the -t^ ^ 



relation of the tubercles to the growth ^f manurC. A gOod CrOD of COm Or 



of the plant. ^ ° ^ 



wheat will take from 50 to 75 

 pounds of nitrogen per acre from soil. There are certain free- 

 living soil bacteria which build nitrogen into nitrates when con- 

 ditions are favorable. The result of the activity of these organ- 

 isms is not unlike the nitrates formed by nitrogen-fixing bacteria. 

 But they differ from the nitrogen-fixing bacteria in that they do 

 not need roots of plants for their homes. 



Lightning and similar electric discharges in the air unite some free 

 nitrogen w^ith oxygen to form nitrates. These are washed from the 



The value of nitrogen-fixing bacteria 

 was demonstrated in the above experi- 

 ment. When the roots of the plants 

 were examined, those on the left had 

 numerous well-developed tubercles ; 

 those on the right had no tubercles. 



