PHYSICAL AND CHEMICAL AGENTS 33 



whereby these bacteria restore the free nitrogen to 

 the soil is not clearly understood, but it is thought 

 that the process requires the combined efforts of 

 several species. It has also been learned that the 

 combined action of certain higher plants and some 

 species of bacteria arrests or reclaims the free nitro- 

 gen. Ordinary plants do not absorb the free nitro- 

 gen from the atmosphere, but a class known as 

 legumes — to which clover, peas, and beans belong 

 — together with certain soil bacteria is able to 

 extract the free nitrogen from the air which per- 

 meates the soil ; the evidences of which are small 

 tubercles or nodules growing upon the roots, which 

 contain nitrifying bacteria and nitrogen compounds. 

 From the knowledge of this nitrifying process has 

 grown the practice of planting sterile fields with 

 legumes, chiefly clover or peas, and plowing the crop 

 under while green, which restores the lost nitrogen 

 to the exhausted soU. 



The sa prophytes may the refore_be_ regarded_-as 

 benefactors, while the jparasites , to which belong the 

 pathogenic bacteria, exist at the expense of both 

 animal and vegetable life. 



The Effect of Chemicals upon Bacteria. — Many 

 species of bacteria in the course of their growth 

 produce acids and other injurious substances which 



