80 BACTERIOLOGY. 
the leguminous plants are enabled to assimilate nitrogen 
from the atmosphere, thus yielding harvests of grain, 
ete., 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 zodgloea-like 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 asually derived in small quantities from the 
meat.’ According to Theobald Smith, all anaérobic or 
facultative anaérobic bacteria form acids from sugar; 
the strict aérobic 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.). 
