42 LUTHER BURBANK 



of microorganisms. It is, indeed, the micro- 

 organism that extracts and fixes nitrogen and 

 makes it assimilable for the plant. 



The tissues of the plant itself have no direct 

 share in the work, beyond giving hospitable 

 refuge to the microorganisms themselves. 



The little tubercles that form on the clovers and 

 the allied plants vary in size and shape with the 

 species of plant, although the microorganisms 

 that produce the tubercles and that assist the 

 plant in securing a supply of nitrogen are closely 

 related. There are, however, different groups of 

 microorganisms that are able to produce the 

 tubercles and help in nitrogen fixation. 



As microorganisms are not always present in 

 any given soil, it has been found sometimes de- 

 sirable to inoculate the soil in which various 

 clovers are to be grown. 



This may be done by scattering over the field 

 soil from a field in which tubercle-bearing plants 

 of the same species have been grown the previous 

 year. 



It has been clearly demonstrated that such in- 

 oculation of the soil may lead to much freer 

 growth of tubercles than would otherwise take 

 place, and to the increased vigor and growth of 

 the clover crop. The use of artificial cultures of 

 nitrifying bacilli has also been recommended. It 



