NITROGEN FIXATION 237 



which it contains, but to the activity of the micro-organisms present, since 

 on heating it to a temperature of 70'' C., its effect is at once neutralized. That 

 it is not due, however, to the presence of some kind of nitrogen-combining 

 micro-organism in the soil is shown by the absence of any result in the case 

 of cereals. The organisms must be such as have special relationships with 

 the Leguminosae. Further, different Leguminosae require different micro- 

 organisms, since the extract of a soil in which beetroot has been grown in which 

 peas and various species of clover had been cultivated for a long time up to 

 the fruiting stage, but in which no Serradella or lupins had ever been grown, 

 furthered the growth of peas only, but not of Serradella and lupins. Hand in hand 

 with this stimulating influence of the soil extract goes the formation of special 

 nodules on the roots of Leguminosae, whose existence had been long known, but 

 as to whose origin and nature no explanation was forthcoming. 



Hellriegel and Wilfarth showed in the clearest way that the micro- 

 organisms present in the soil extract were also the cause of the formation of 

 the nodules on the roots of Leguminosae, and that these plants could assimi- 

 late atmospheric nitrogen only when the micro-organisms were present in 

 the root nodules. * To show that the Leguminosae make use of free nitrogen 

 for nutritive purposes ', write Hellriegel and Wilfarth, ' it is not sufficient 

 merely that any kind of lower organism should be present in the soil, but it 

 is essential that certain species of the latter should first of all enter into 

 symbiotic relation with the former.' 



Following De Bary (1879), ^^ ^^Y define symbiosis as a partner- 

 ship of two organisms of such a nature that both receive benefit by living 

 together, or where at least the benefits are not all on one side. In the latter 

 case, one would have to speak of it as parasitism. Hellriegel and Wilfarth 

 have not, however, explained in individual cases wherein lies the reciprocal bene- 

 fit to the leguminous plant and to the Bacteria which live with it, and this has 

 not even yet been made clear, in spite of the series of important memoirs by 

 Beijerinck (1888), Prazmowski (1890-91), Frank (1890) and others on the 

 formation and significance of the nodules. 



Without going into details, we may present here the essential results of 

 recent investigations on the leguminous nodules by a reference to Figs. 42 

 and 43. When certain motile rodlike Bacteria, known collectively as Bacterium 

 radicicola, have entered the roots of Leguminosae, they increase there to an 

 almost astonishing degree. Just as we have seen in the case of galls, the 

 Bacteria stimulate the cells of the root and produce local hypertrophy, so that 

 we may term these nodules ' bacterium galls '. In the majority of the cells 

 of the nodule one finds masses of Bacterium radicicola, which later on degene- 

 rate in peculiar ways, and exhibit large spherical or branched ' involution 

 forms ', rich in proteid. These involution forms (' bacteroids ') are then used 

 by the leguminous plant as proteid reserves, that is to say, they are broken 

 down and employed in the formation of fruit. Only part of the Bacteria 

 become altered into bacteroids, and are sacrificed for the good of the legu- 

 minous plant, the rest persist, and, after the destruction of the nodule, 

 remain in the soil, and serve to infect the new Leguminosae of the next year. 



Bacterium radicicola has been cultivated apart from the plant in suitable 

 nutritive media by several investigators, and the strongest possible proof is 

 forthcoming that the bacterium is the cause of the nodule formation. The 

 conditions of life of this organism have been established in the clearest possible 

 way, and we have to thank MAzi (1897) especially for proof that nitrogen is 

 combined by its means. Similar evidence was also furnished by other authori- 

 ties, but they provided Bacterium radicicola with ammonia or with no combined 

 nitrogen at all, in the expectation of proving even more clearly that a combina- 

 tion of nitrogen took place. Beijerinck employed asparagin, and thought 



