52 THE SPIRIT OF THE SOIL 



out leguminous plants as distinct from all others was 

 apparent rather than real. The great generaliza- 

 tions, however, emerged: 



1. Plants derive their nitrogenous food material 

 from the foods present in the soil, and not from any 

 gases in the air. 



2. The nitrogenous plant foods present in the soil 

 are either directly or indirectly* derived almost 

 entirely from the Nitrogen of the soil air, which is 

 combined to form a suitable plant food by the 

 bacteria living in the soil. 



This discovery was obviously of fundamental 

 importance. Botanists had hitherto regarded the 

 soil as a complex mixture of chemicals from which 

 the plants derived their food. Now, however, they 

 began to appreciate the fact that the earth's surface 

 was no inert mass of soil, but was the seat of myriads 

 of living organisms necessitating the closest study. 



The discovery made by Hellriegel and Wilfarth 

 was rapidly followed up. A year after they had 

 announced the main facts, Marshall Ward, by careful 

 study of the tubercles of leguminous plants, traced 

 the whole process of nodule formation from the 

 infection of the root hairs of the plant by some soil 

 organism up to the formation of the mature nodule, 

 and showed that the tubercles could be produced at 

 will by the inoculation of the roots with soil infusions. 

 In 1888 Beyerinck isolated the organism by growing 



* When such a substance as stable manure is added to the 

 land, the nitrogenous bodies which it contains are plant 

 residues, and these have, either directly or indirectly, derived 

 their Nitrogen substance from the Nitrogen combined by 

 bacteria. 



