80 BACTERIOLOGY. 



the leguminous plants are enabled to assimilate nitrogen 

 from the atmosphere, thus yielding harvests of grain, 

 etc., which are highly nitrogenous, upon soils which are 

 naturally poor in nitrogen. This explains the reason 

 why poor, sandy soils become gradually fruitful when 

 pease, lupine and other varieties of legumes are grown 

 upon them and then turned under with the plough. It 

 is not known exactly how this assimilation of nitrogen 

 occurs, but it is assumed that the zoogloea-li ke bacteria, 

 called bacteroids, constantly observed in the nodules, 

 either alone or in a special degree, possess the property 

 of assimilating and. combining nitrogen. It seems, 

 moreover, to have been recently established that, in- 

 dependently of the assistance of the legumes, certain 

 nodule-bacteria exist free in the soil, which accumulate 

 nitrogen by absorbing it from the air (Stutzer). 



Formation of Acids from Carbohydrates. Free acids 

 are formed by many bacteria in culture media contain- 

 ing sugar; the production of acid in ordinary bouillon 

 takes place on account of the presence of grape-sugar, 

 which is usually derived in small quantities from the 

 meat. 1 According to Theobald Smith, all anaerobic or 

 facultative anaerobic bacteria form acids from sugar; 

 the strict aerobic species do not, or so very slowly 

 that the acid is concealed by the almost simultaneous 

 production of alkali. The formation of acid occurs 

 sometimes with and sometimes without the production 

 of gas. Excessive acid production may cause the death 

 of the bacteria from the increase in acidity of the 

 culture media. 



1 According to Theobald Smith, 75 per cent, of the beef ordinarily bought 

 in the markets contains appreciable quantities of sugar (up to 0.3 per cent.). 



