SCIENCE-GOSSIP 



7 1 



+ H a O. But once given a nitrite it is a matter 

 of comparative ease to raise this to a nitrate, 

 HN Oo + = HNO :1 . Yet we still have to see how 

 this second step takes place in nature. It was 

 Winogradsky who again solved the difficulty ; a 

 short time after his former discovery he effected 

 the isolation of a second bacterial form which was 

 capable of bringing about the final process of 

 oxidation. Thus the formation of nitrates in the 

 soil must 

 be regard - 

 e d as a 

 double act 

 of fermen- 

 tation, in 

 which one 

 bac t e r i a 1 

 form chan- 

 g e s the 

 ammonia 

 of the 

 soil into 

 a nitrite 

 and then 

 a second 

 micro- 

 organism steps in and raises this to a still higher 

 state of oxidation. During all the years in 

 which these advances were being made a certain 

 discontent was simmering against the theory 

 which gave over the vast ocean of free, atmospheric 

 nitrogen, as a perfectly useless source, to the plant. 

 The first reasonable doubts, based upon observed 

 facts, which were cast upon this hypothesis 

 emanated from the famous experimental farm at 

 Rothamsted. In growing certain 

 leguminous plants under carefully 

 watched conditions there was found 

 to be a larger quantity of nitrogen 

 in them after the lapse of some time 

 than could be accounted for by the 

 available sources in the soil. There 

 was only one conclusion to be drawn 

 from these facts and this was that, 

 by some means not yet understood, 

 these plants had assimilated the 

 nitrogen of the air. In and about 

 the year 1886, Hellriegel and Wilfarth 

 in Germany, Marshall Ward and 

 others in this country, carried on a 

 series of experiments with leguminous plants which 

 all gave similar results. If a plant, the quantity 

 of nitrogen in which was known, was grown in a 

 pot of carefully analysed soil it was found that 

 there was a considerable increase in nitrogen 

 within this system after several weeks, which 

 could only be attributed to a " fixation " or 

 abstraction of this element from the air. It is 

 upon the researches of Laurent and Schlocsing 



Fig. 3. — Transverse section, from root-nodule of scarlet-runner. 



that we depend, however, more than upon any 

 others, for placing this matter upon a sure basis. 

 These investigators were able to show, by enclosing 

 the plant and soil in a confined and analysed 

 portion of air, that exactly in proportion as the 

 plant gained in nitrogen did the surrounding air 

 become poorer in this gas. It was found, moreover, 

 that only particular plants, viz. : those belonging 

 to the Leguminosae and certain Alga;, could thus 



assimilate 

 free nitro- 

 gen. The 

 point in 

 which le- 

 g u minous 

 plants, 

 capable of 

 nitrogen 

 fixation, 

 differ from 

 others is 

 in the 

 possession 

 of the 



Fig. 4. — Cells from pith 

 of root-nodule of scarlet- 

 runner. 



curious 

 nodules 

 mentioned in the commencement of this article. 



The German observers, Hellriegel and Wilfarth, 

 were the first to notice this feature and point out 

 the further facts that these tuberosities were 

 crowded with minute organisms and were charac- 

 terised besides, by containing the richest stores of 

 nitrates in the whole plant. It was concluded, in 

 consequence, that the Leguminosa; possessed the 

 power of free nitrogen fixation through the presence 

 of the nodules, and that the ultimate 

 cause was to be found in the micro- 

 organisms which swarmed within 

 these. How exactly these " bac- 

 teriads," as these small bacteria-like 

 organisms of the tuberosities are 

 named, aid in this process of nitri- 

 fication and precisely what relation 

 they bear to the plant is still a matter 

 of discussion. 



According to a paper lately 



published by Professor Marshall 



Ward (see "Nature" March 29th, 



1894) there are four possible theories 



at present existing. 



(r) The living cells of all plants may have the 



power of "fixing" nitrogen, and this only 



becomes more evident where root-nodules 



are present. 



(2) The soil-bacteria, already considered, and 



the organisms of the tuberosities may have 



powers of directly fixing the nitrogen of 



the air as part of their life processes, and 



that the nitrates thus formed are subse- 



