THE BIOLOGICAL ACTIVITIES OF BACTERIA 61 



nitrogen to the soil in suitable form for consumption by the plants 

 is performed largely by bacteria. 



It is well known that specimens of agricultural soil when allowed 

 to stand for any length of time without further interference will 

 increase in nitrogenous content, but that similar specimens, if 

 sterilized, will show no such increase. 18 The obvious conclusion to 

 be drawn from this phenomenon is that some living factor in the 

 unsterilized soil has aided in increasing the nitrogen supply. Light 

 was thrown upon this problem when Winogradsky, 19 in 1893, dis- 

 covered a microorganism in soil which possessed the power of 

 assimilating large quantities of nitrogen from the air. This bac- 

 terium, which he named ' ' Clostridium Pasteurianum," is an obliga- 

 tory anaerobe which in nature always occurs in symbiosis with two 

 other facultatively anaerobic microorganisms. In symbiosis with 

 these, it can be cultivated under aerobic conditions and thus grows 

 readily in the upper well-aerated layers of the soil. 



Although, until now, no other bacteria with equally well-devel- 

 oped nitrogen-fixing powers have been discovered, yet it is more 

 than likely that Clostridium Pasteurianum is not the only micro- 

 organism endowed with this function. In fact, Penicillium glaucum 

 and Aspergillus niger, two molds, and two other bacteria described 

 by Winogradsky, have been shown to possess this power slightly, 

 but in an incomparably less marked degree than Clostridium Pas- 

 teurianum. 20 According to the calculations of Sachse, 21 unsterilized 

 soil may, under experimental conditions, gain as much as 25 milli- 

 grams of nitrogen in a season, a statement which permits the cal- 

 culation of a gain of twelve kilograms of nitrogen per acre annually. 22 

 It is very unlikely, however, that such gains actually occur in nature, 

 where nitrogen-fixation and nitrogen-loss usually occur side by side. 



Agriculturally of even greater importance than the free nitrogen- 

 fixing bacteria of the soil are the bacteria found in the root tubercles 

 of a class of plants known as "leguminosse. " It has long been 

 known that this class of plants, including clover, peas, beans, vetch, 

 etc*, not only does not withdraw nitrogen from the soil, but rather 

 tends to enrich it. Upon this knowledge has depended the well- 



18 Berthelot, Compt. rend, de la soc. de biol., cxvi, 1893. 



19 Winogradsky, Compt, rend, de la soc. de biol., cxvi, 1893, ibid., t. cxviii, 1894. 



20 Tackc, Landwirtseh. Jahresber., xviii, 1889. 

 sl Sactec, "Agr. Chem.," 1883. 



- Pfcffer, Pfliigers Physiologic, p. 395. 



