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TEXTBOOK OF PLANT PHYSIOLOGY 



already shown, a temporary delay in development and, in some 

 instances, when the plant is not sufficiently strong, even its death. 

 Later, however, the upper hand is gained by the plant, which then 

 absorbs from the tubercles the nitrogen compounds produced by 

 the bacteria and finally digests the greater part of them. 



The bacteria found in nodules have been isolated in pure 

 cultures and have been named Bacterium radicicola. Being 

 adapted to a parasitic life in the roots of plants they do not grow 

 readily in an artificial medium. Moreover, under artificial con- 

 ditions, they show hardly any 



signs of assimilation of molec- 

 ular nitrogen. Several races 

 of such bacteria have been 

 isolated, each of them corre- 

 sponding to a definite group 

 of leguminous plants, whose 

 roots they can inoculate. Thus, 

 for instance, the bacteria from 

 beans readily inoculate 

 vetches, but are ineffective on 

 alfalfa. In introducing new 

 leguminous plants, such as, 

 for example, soybeans or 

 lupines, therefore, it is neces- 

 sary to incorporate into the 

 soil cultures such nodular bac- 

 teria as are adapted to these 

 plants (Fig. 27). Thus a 

 proper " bacterial fertilization ' ' 

 often increases the yield considerably. Inoculation with nodular 

 bacteria may be found especially useful when leguminous plants 

 are grown on drained swamps, because these bacteria are usually 

 absent in boggy soils. All leguminous plants possess the capacity 

 of entering into symbiosis with tubercle bacteria, this being an 

 important physiological peculiarity of this group of plants. It is 

 rarely found in other groups of plants. Among the trees are 

 found similar, but perennial tubercles on the roots of the alder 

 and Eleagnus; while on certain tropical trees and shrubs, belong- 

 ing to the family of Rubiaceae, the bacterial tubercles are found on 

 the leaves. In these cases, Bacterium radicicola are not present, 



Fig. 27. — Alfalfa grown in soil poor in 



nitrogen: left, inoculated with specific 



bacteria; right, not inoculated {after 



Smith, et al.). 



