106 PRINCIPLES OF SOIL MICROBIOLOGY 



penetration of the microbe into the tissues of the plant. According to 

 Faber, the bacteria bringing about these formations can also fix nitrogen 

 when not working symbiotically with plants. The bacteria are carried 

 through the seed, into which they penetrate when the plant is still alive. 

 The amount of nitrogen thus fixed may be so considerable that in India 

 Pavetta plants are used as green manure. 



Direct nitrogen fixation by higher plants was first suggested by the 

 early chemists, Priestly and Ingenhouse. Positive results were reported 

 also by some recent investigators. 20 - 21 However the work of investiga- 

 tors, like Boussingault, 22 Lawes, Gilbert and Pugh, 23 and the more recent 

 work of Molliard 24 and numerous others definitely point to the fact that 

 non-leguminous plants are unable to fix any atmospheric nitrogen. 



Classification of nitrogen-fixing bacteria. The nitrogen-fixing bacteria 

 require organic compounds of carbon for structural and energy purposes. 

 These organisms can be classified on the basis of their source of carbon, 

 whether they derive it in a non-symbiotic manner or obtain it from the 

 growing plant, with which they live symbiotically. None of these 

 organisms are obligate, since they can also obtain their nitrogen from 

 organic or inorganic nitrogen compounds. 



I. Non-symbiotic nitrogen fixing bacteria 



1. Anaerobic bacteria 



Bac. amylobacter (Clostridium group, Amylobacter group, Granulo- 



bacter group) 

 Other butyric acid bacteria 



2. Aerobic bacteria 



(a) Azotobacter group 



(b) Radiobacter group 



20 Mameli, E., and Pollacci, G. Sur l'assimilatione diretta dell'azoto atmosfer- 

 ico libero nei vegetali. Atti. 1st. Bot. Pavia, Ser. 2, 15: 159-257. 1911; Centrbl. 

 Bakt. II, 32: 257. 1912. 



21 Lipman, C. B., and Taylor, J. K. Proof of the power of the wheat plant to 

 fix atmospheric nitrogen. Science, 56: 605-606. 1922; J. Frankl. Inst. 1924, 475- 

 506. 



22 Boussingault, J. B. Recherches sur la vegetation, entreprises dans le 

 but d'examiner si les plantes fixent dans leur organisme l'azote qui est a l'etat 

 gazeux dans l'atmosphere. Ann. Chim. Phys. (3) 43: 149-223. 1855. 



23 Lawes, J. B., Gilbert, J. H., and Pugh, E. On the sources of the nitrogen of 

 vegetation, with special reference to the question whether plants assimilate free 

 or uncombined nitrogen. Phil. Trans. Roy. Soc. London, 151: 431-577. 1861; 

 Rothamsted Mem. 1, No. 1; 3, No. 1. 



24 Molliard, M. L'azote libre et les plantes superieures. Rev. Gen. Bot. 28: 

 225-250. 1916. 



