234 



PLANT PHYSIOLOGY 



however, may be regarded as a sort of balanced parasitism. 

 First, the bacteria are the assaulting party, behaving like para- 

 sites in respect to the plant and causing a temporary delay in 

 development and even, in some instances when the plant is not 

 sufficiently strong, 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. 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. The 

 bacteria from beans readily 

 inoculate vetches but are 

 ineffective on alfalfa. In intro- 

 ducing new leguminous plants, 

 such as, for example, soybeans 

 or lupines, it is therefore 

 necessary to incorporate into 

 the soil artificial cultures of 

 such nodular bacteria as are 

 adapted to these plants (Fig. 

 A proper ''bacterial 

 fertilization," therefore, often 



Fig. 70. — Alfalfa grown in soil poor in 

 nitrogen: left, inoculated with specific 

 bacteria; right, not inoculated (after 7U) 

 Smith, et al.). 



increases the yield considerably. The rather widely used 

 preparation ''nitragin" is such a culture of nodule bacteria. 

 Inoculation with nodule bacteria may be found especially 

 necessary when leguminous plants are grown on drained swamps, 

 because these bacteria are usually absent in boggy soils. All 

 leguminous plants have 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 

 plants. Among the trees are found similar, but perennial, 

 tubercles, as on the roots of the alder ^Znwssp. and ^^eagrnus; while 

 on certain tropical trees and shrubs, belonging to the family of 

 Rubiaceae, the bacterial tubercles are on the leaves. In these 



