276 THE FERTILITY OF THE SOIL 



or other soluble carbon compounds which the bacteria 

 absorb from them. The exact facts appear not yet 

 to be determined in respect to the relation existing be- 

 tween these classes of organisms. It has been recently 

 reported that some of these algae can fix their own 

 nitrogen. 



Tubercle Bacteria. — It is a widely known fact that 

 clover and some other leguminous plants enrich the soil. 

 This bit of knowledge had passed into common slang in 

 the phrase, " too poor to grow beans," long before its 

 scientific explanation had been worked out. On the roots 

 of such plants there occur small swellings variously called 

 nodules or tubercles. Inside them are found numerous 

 bacteria of a special sort. When the plant is young its 

 roots are attacked by the bacteria which penetrate into 

 them through the delicate root hairs and establish them- 

 selves in the outer layer of root-cells. The root reacts by 

 multiplying these cells in number, thereby forming a sort 

 of tumor-like outgrowth in which the bacteria are con- 

 fined. Here they fix nitrogen, deriving the energy to do 

 so from the carbon compounds obtained from the root- 

 cells. In return the root uses their nitrogen compounds. 



Mycorrhiza are masses of fine hyphae or filaments 

 of certain sorts of fungi which attack the roots of some 

 kinds of trees and shrubs. They penetrate the root and 

 take from it organic carbon compounds. Outside, they 

 ramify through the soil. They have the power to fix nitro- 

 gen, which they share with the roots. This arrange- 

 ment appears to be so satisfactory to both partners that 

 some of these trees no longer even form root hairs but 

 absorb all their water and salts through the fungus. 

 This arrangement illustrates another interesting adapta- 

 tion of organisms to their environment; for most of these 

 trees grow in soils which are poor in nitrates and also 

 unfit for the growth of the nitrifying bacteria. 



Denitrifying Bacteria are those which reduce 

 nitrates, i.e., break up the compound and liberate the 



