151 



which was oiirichod on an average to 12 luilliuranis, partly in the 

 phmts, which had gained 20 to 30 milligrams, although, owing to the 

 narrow space in which they were confined, they were not able to 

 attain their full development. 



This last proof appears to have finally closed the discussion for- 

 merly inaugurated by Boussingault and which had not been com- 

 pletely closed by the analytic results e.\})lained above. 



Thus a few years have sufiiced to definitely decide this theory of a 

 direct assimilation of nitrogen by plants, first enunciated by Ville. 



What, now, is the mechanism or modus operandi of this assimila- 

 tion? We have just seen how Berthelot was led, by certain i)eculiar- 

 ities of his experiments, and, above all, by the complete cessation of 

 all fixation of nitrogen in soils that had been subjected to a tempera- 

 ture of 100°, to admit that nitrogen is assimilated directly by certain 

 inferior organisms Avhich force it into organic combination; but we 

 have also seen that the fixation of niti-ogen by naked soils is always 

 weak and generally insufficient for the necessities of a normal vege- 

 tation. 



It is true that when the aid of a leguminous plant is invoked the 

 fixation becomes more active and may become powerful enough to 

 compensate alone for all the known causes of loss; but how, then, 

 are we to account for the difterence in this respect found between the 

 Leguminosa:* and the Graminea?? Shall we be forced to admit that 

 the Leguminosa? are able, by themselves, to assimilate gaseous nitro- 

 gen, by a power possessed by them which is wanting in the other 

 species ? 



Berthelot has concluded, from his researches upon this subject, that 

 in the development of leguminous plants there comes into play some 

 micro-organism which facilitates the fixation of nitrogen upon the 

 root of the plant, or rather upon the mass formed by the root and 

 the soil, intimately connected one to the other; but this idea could 

 not be definitely adopted unless the existence of such a microbe were 

 proved by experiments. This result is fully demonstrated by a 

 series of very remarkable experiments made by Hellriegel, Wilfarth, 

 Frank, Prazmoff'ski, and others in (Jermany, and which have been 

 most successfully verified l)v Breal, Schloesing, jr., and Laurent in 

 France, and, finally, by LaAves and (Gilbert in England. 



Before proceeding to explain these researches I nuist call attention 

 to a well-established fact which had been well known for a great 

 many years, although no one before Hellriegel and Wilfarth ever 

 thought of seeing in it anything more than a phenomenon of nature. 



^^lien we examine the roots of a leguminous plant grown in good 

 soil we always see ii'regularly disposed on them tuberculous enlarge- 

 ments, a kind of nodosity | node, nodule, knot, or knob] formed of a 

 special tissue and appai'ently quite accidental. Examined with a 

 microscope the interior of the^e excrescences ai)pears to be filled with 

 corpuscles of varying forms, always animated with the '' Brownian ■' 

 movement, although they have sometimes a movement of their own. 

 These assume various shapes: sometimes they are like simple rods 

 similar in form to certain bacteria; sometimes they have the 

 appearance of vegetal)le cornlloids and take the branched T or Y 

 form more or less ramified. 



Botanists have for a long time discussed the nature of these excres- 

 cences, but at present it seems to be generally admitted that, mor])ho- 



