590 PRINCIPLES OF SOIL MICROBIOLOGY 



these conditions, one cannot speak of plant immunity against further 

 invasion by bacteria. 125 



The "equilibrium" theory proposed by Siichting, 126 as an explanation 

 for the mutual relationship between the leguminous plant and nodule- 

 forming bacteria, is more plausible and has many facts to support it. 

 A state of equilibrium was considered to exist between the attacking 

 power of the bacteria and resisting power of the plant, due perhaps to 

 the fact that the bacteria produce a toxin and the plants an antitoxin. 

 The degree of equilibrium determines the extent of nodule formation, 

 the plant becoming immunized by an antibody and not by a substance 

 produced by the bacteria and the nitrogen supply being regulated by the 

 production of the antibody. When the leguminous plants are grown 

 in soil containing plenty of nitrates, their resisting power to the infec- 

 tion of the bacteria is greater than when grown on nitrogen free media. 

 The bacteria may vary in virulence, depending on the media in which 

 they are grown. Increasing virulence was also found to be directly 

 correlated with a shortening of the vegetation period of the plant. 



The nitrogen is fixed by the bacteria present within the nodules and is 

 made available for the growth of the host plants by the autolysis of these 

 nodules, or through the production of a bacteriophage by the plant, or 

 perhaps as a result of the action of enzymes produced by the plant. 

 The plant obtains its carbon from the C0 2 of the atmosphere by photo- 

 synthetic processes; a part of the carbohydrates thus synthesized is 

 transferred to the roots and used by the bacteria as a source of energy. 

 This enables the microorganism to fix atmospheric nitrogen much more 

 efficiently than it can ever do in artificial media, or even more efficiently 

 than any non-symbiotic nitrogen fixing organism. 



Wunschik 127 based his idea of the relation between the bacterium and 

 the host plant on the statement of Beijerinck that "when living plant 

 cells have to derive help from another organism, an equilibrium between 

 the growth of both must be reached." The equilibrium is in this case 

 between the vegetative energy of the plant and of the nodule forming 

 organism. Wunschik differentiated between the vegetative energy, 

 or ability to penetrate into the roots of the plant, and nitrogen-fixinsj; 

 capacity of the organism. The vegetative energy of the bacteria results 

 in the removal from the host plant of a part of its nutrients, thus causing 



i" Whiting, A. L.: 111. Agr. Exp. Sta., Bui. 179. 1915. 



128 Suchting, H. Kritische Studien iiber die Knollchenbakterien. Centrbl. 

 Bakt. II, 11: 377-388, 417-441, 496-520. 1904. 

 i" Wunschik, 1925 (p. 13S). 



