76 PLANT PHYSIOLOGY 



under conditions wholly unnatural to all the plants experi- 

 mented upon, and must therefore be taken with reserve. 

 Leguminous plants grown in glass vessels undeniably do 

 better when their roots are infected by bacteria than when 

 their roots are sterile, but it has not been proved that 

 leguminous plants do better with their roots infected than 

 with sterile roots when they are grown where their roots 

 can be properly aerated. Infected Leguminosse benefit the 

 soil more than do sterile ones, but what the gain to the 

 plants themselves may be, remains to be shown, for the 

 bacteria are plainly parasites.* 



The bacteria found in and causing the root-tubercles of 

 the Leguminosse, Eleagnus, etc., have not yet been isolated 

 from the soil and are known only in the tubercles and in 

 cultures inoculated from tubercles. The isolation of another 

 species of bacteria which fix free atmospheric nitrogen 

 (Clostridium Pasteurianum) has, however, been accom- 

 plished by Winogradsky.f Other species will doubtless be 

 found in the cultivated and undisturbed soils of field and 

 forest. The green and blue-green algae growing on the soil 

 were suspected of being able to fix uncomhined nitrogen, 

 but it has been demonstrated that they cannot do this 

 alone. J It may be that nitrogen-fixing soil-bacteria and 

 low algae live together in an association similar to that of 

 bacteria and leguminous plants. § 



The number of organisms which can use free nitrogen will 

 undoubtedly be found to be small, for in the present balance 

 of nature little more nitrogen need be added to the soil 

 than is yearly returned to it in the excrementitious matters 

 and in the dead bodies which fall upon it. In these waste 

 matters are organic nitrogen compounds upon which de- 



* Peirce, G. J. Loc. cit. 



t Winogradsky, S. Sur I'assimilatlon de I'azote gazeux de I'atmosph&re 

 par les microbes. Comptes Eendus, t. 116, 1893. t. 118. 1894; also 

 Archives des Sciences Biologiques St. Petersburg, Bd. 3, 1895. 



J Kossowitsch, P. Dntersuchungen iiber die Frage ob die Algen freien 

 Stickstoff assimiliren. Botanische Zeltung, 1894. 



§ Pfeffer, W. Pflanzenphysiologie, Bd. I., p. 386. Engl, transl. I., p. 

 396. See also Kriiger nnd Schneiderwlnd in Landw. Jahrb., Bd. 29, Noe. 

 4 and 5, pp. 771, 804 1900. 



