198 FIXATION OR ASSIMILATION OF NITROGEN 



serradella, and some other species of leguminous plants 

 are missing, and until they are supplied either in soil 

 from a field which has grown a good crop of these 

 plants or in pure cultures, such plants grow very 

 unsatisfactorily. 



4. As already indicated, there is considerable variety 

 in the morphology of the nodule organisms, but all the 

 forms are doubtless modifications of a single species 

 produced by variation in environment. Many of the 

 forms exhibit striking differences also in regard to their 

 power of producing nodules. The most luxuriant growth 

 of these root excrescences upon any particular plant is 

 brought about in the speediest and most effective manner 

 by inoculation with cultures derived from nodules of the 

 same species of plant. Thus upon the roots of peas they 

 are most easily produced by organisms derived from pea 

 nodules, although their formation can be stimulated by 

 those obtained from beans and vetches. Those taken 

 from the roots of Robinia are active upon beans and 

 certain clovers. Several varieties, such as those obtained 

 from lupins, clovers, and serradella, are somewhat ex- 

 clusive in their behaviour, and do not affect plants 

 belonging to other genera. The bacteria from peas 

 produce nodules upon beans, but not upon lupins, nor 

 upon plants belonging to the genera Trifolium, Medicago, 

 Anthyllis, or Ornithopus. Although what may be 

 termed the virulence of the organism, or its prompt 

 and unfailing power of nodule production, is largely 

 dependent upon the origin of the bacterium, it is also 

 greatly influenced by nutrition ; pure cultures which have 

 been grown for some time upon gelatine or media 

 containing considerable amounts of combined nitrogen 

 lose much of their power in this respect. The com- 



