328 Biological Chemistry. 



substances without the addition of nitrogenous manures is 

 characteristic of the leguminosae ; other crops, such as 

 the cereals, require the addition of nitrogenous manures, 

 otherwise the soil becomes rapidly exhausted by the 

 removal of its nitrogen. These crops, in fact, remove from 

 the soil approximately the amount of nitrogen added to it. 

 To Hellriegel and Wilfarth is due the credit of explaining 

 the difference of the action of the leguminosse and other 

 plants. At the time of these investigations it was known 

 that certain bacteria possessed the capacity of utilizing the 

 nitrogen of the air. It was also known that leguminosse 

 usually formed certain small swellings on the roots, which 

 are known as nodules. It was found by Hellriegel and 

 Wilfarth that if leguuiinosae are grown on sterilized soil 

 the nodules do not develop. On the other hand, if the 

 sterilized soil is treated with an extract of ordinary soil on 

 which leguminosaa are grown, nodules will be formed. A 

 heated extract of the soil was found, however, to be in- 

 effective as regards the production of the nodules. These 

 results explain the discrepancy between the observations 

 of Lawes and Gilbert and of Boussingault, for the last- 

 named had carried out his experiments on the nitrogen 

 assimilation on plants growing in sterilized soil, whereas 

 the first-named investigators had used ordinary untreated 

 soil. The experiments also indicated the fact that the 

 production of root-nodules was due to the intervention of 

 bacteria that is, to a " symbiosis " between the higher plant 

 and a lower organism. Beyerinck and Prazmovski suc- 

 ceeded subsequently in isolating the nodule bacteria in the 

 form of pure cultures, and in indicating the morphological 

 relationship between the development of the nodules and 

 the bacterial growth. Subsequent researches revealed 

 later the existence of other bacteria capable of "fixing" 

 the nitrogen of the air, notably of Clostridium pas- 



