Petrie. — On Vegetable Physiology. 437 



it has enslaved and forced to intense metabolic activity it 

 does not destroy them. The next stage is one in which the 

 struggle for mastery between the pseudo-parasite and its host 

 is most intense. For a time the bacteroids take everything 

 the cell can give them ; they multiply and fill the cells, and 

 then gradually pass into a passive condition. The legu- 

 minous plant now gains the mastery, and the cells of the 

 nodules absorb and pass on into the plant almost the entire 

 store of nourishing substances which the bacteroids have 

 accumulated. 



Now, there can be no reasonable doubt that the fixation 

 of the free nitrogen occurs in the underground part of the 

 plant. The known anatomical structure of the nodules, 

 especially the complex and very regular system of vascular 

 bundles that connect them with the mother-roots, and 

 the abundant supply of carbo-hydrates passed into the 

 nodules, make it extremely probable that the seat of the 

 action lies in the nodules themselves. We may therefore 

 suppose either that the inert nitrogen molecules are forced 

 into tlie organic synthesis in the cells of the bacteroids, or 

 that the metabolic activity of the cells composing the nodules, 

 and it may be adjacent parts of the roots also, is so exalted 

 by the stimulating action of the bacteroids as to be able to 

 build up compound proteids by the synthesis of gaseous 

 nitrogen with the already elaborated carbo-hydrates. It is 

 difficult to arrange experiments that will settle decisively 

 which of these views is the correct one. Attempts to demon- 

 strate that the bacteroid cultivated outside the plant can 

 assimilate free nitrogen have hitherto failed. But we must 

 not lay too much stress on such negative results, because the 

 cultivation of an organism so highly adapted to its symbiotic 

 life as this evidently is may well fail outside the cells of 

 the host. On the other hand, it is improbable that the cells 

 of the bacteroids would assimilate, and by their disintegration 

 and absorption pass on to the infected plant, such quantities 

 of nitrogen as are known to be fixed from the air by the 

 Leguminosae. On the whole, we may conclude that the view 

 adopted by Hellriegel and many other inquirers, that the 

 cells of the nodules are the actual seat of the fixation of 

 the free nitrogen of the air, is the true one. 



How the fixation is brought about we cannot in the present 

 state of knowledge explain, though theoretical considerations 

 carry us some way towards an explanation of the phenomenon. 

 In the ordinary course of plant metabolism nitrogenous bodies 

 — e.g., amides — preceded, it may be, by simpler compounds, are 

 built up by the living cell protoplasm from carbo-hydrates and 

 nitrates, the energy required to effect this synthesis being 

 chiefly if not wholly derived from the indirect oxidation of 



