206 THE LIVING ORGANISMS OF THE SOIL [chap. 



tion had always been wanting because of the great 

 care that had been taken to prevent the entry of any 

 accidental impurity. Eventually, both at Rothamsted 

 and by other investigators, the conclusions of Hellriegel 

 and Wilfarth were confirmed, that when leguminous 

 plants are grown under sterile conditions, without a 

 supply of combined nitrogen there is very limited 

 growth, no formation of nodules, and no gain of nitrogen. 

 But when the culture is seeded with soil extract there is 

 luxuriant growth, abundant nodule formation, and coin- 

 cidently, great gain of nitrogen, many times as much in 

 the products of growth as in the seed sown. Gilbert also 

 showed that there is a gradual accumulation and then 

 withdrawal of nitrogen from the nodules. Lastly, 

 Schloesing y^/i - and Laurent, by growing Leguminosai in 

 closed vessels, and analysing the air before and after 

 growth, found an actual disappearance of nitrogen gas, 

 agreeing with the amount gained by the plant during 

 growth. Thus, a conclusion was reached that the 

 leguminous plants can assimilate and fix the free 

 nitrogen of the atmosphere by the aid of bacteria living 

 symbiotically in the root nodules a conclusion which 

 served to explain, not only the discrepancies in the 

 previous experiments, but the long-accumulated experi- 

 ence of farmers that crops like clover and lucerne enrich 

 the soil, and form the best preparation for cereals like 

 wheat, which are particularly dependent on an external 

 supply of nitrogen. The mechanism of the fixation of 

 free nitrogen is still incompletely understood. It has 

 been found possible to grow these bacteria apart from 

 the leguminous plants, if they are cultivated on a medium 

 containing only a trace of nitrogen but supplied with 

 the ash constituents of the plant and also with 

 some carbohydrate like dextrose or maltose. The 

 quantities of nitrogen fixed in this way are always, 



