LUTHER BURBANK 



culturist mixes clover with the timothy in his 

 pastures and in his hay-field. 



At least a partial explanation df the high nitro- 

 gen content of the leguminous plants has been 

 furnished by the discovery that these plants have 

 the very unusual capacity to extract nitrogen from 

 the air. Most plants, as we have seen, are quite 

 powerless to take even the most infinitesimal 

 quantity of nitrogen from the air, and would starve 

 to death for lack of nitrogen even while their tis- 

 sues are perpetually bathed in it — as the tissues 

 of all aerial plants necessarily are — ^inasmuch as 

 the atmosphere contains nitrogen as its most 

 abundant element. 



But the leguminous plants are able to extract 

 nitrogen from the air directly; not, however, with 

 the aid of their leaves or stems, but only by way 

 of the roots, and there only with the aid of the 

 little tubercles that develop under the influence of 

 micro-organisms. 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 micro-organisms 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 micro-organisms 



[98] 



