264 FIXATION OF FREE NITROGEN BY BACTERIA 



are able to thrive in soils destitute of nitrogen. The faculty of nodule formation 

 has also been ascribed to other plants. A. B. FRANK (IV.) goes the farthest in 

 this respect, and, indeed, assumes that all plants are in a position to take up 

 free nitrogen, an opinion also recently expressed by J. STOKLASA (II.). LIEB- 

 SCHER (I.) confines his opinion within narrower limits, but ascribes the power of 

 fixing free nitrogen to oats and mustard as well. The statements of Frank and 

 Liebscher have, however, been disproved by the searching criticisms of, e.g. 

 WILFARTH (I.), U. Kreusler, P. Wagner, F. Nobbe, and L. HILTNER (I.); and 

 J. H. AEBY (I.) has shown that mustard does not possess the faculty with which 

 it is credited by Liebscher. The same results were obtained by CH. E. COAXES 

 and W. R. DODSON (I.) in 1896, in their experiments on the cultivation of the 

 cotton plant (Gossypium). 



194. The Nodule Bacteria. 



The discoveries reported in the foregoing paragraph, and for which we have 

 chiefly to thank Hellriegel and Wilfarth, lead up to the question whether this 

 proved absorption of free nitrogen is effected in the root-nodule itself, or whether, 

 by the influence of this formation, the entire plant particularly the foliage 

 becomes capable of taking up this gas from the atmosphere and fixing it in 

 combination ? 



This latter opinion, which was chiefly supported by A. B. Frank, has been 

 investigated by P. KOSSOWITSCH (I.), who was, however, unable to discern any 

 absorption of atmospheric nitrogen by the parts of the plants above ground. 

 Attention must, therefore, be concentrated on the root-nodules themselves ; but 

 before going into the question whether and in what manner the nitrogen is fixed 

 by them, it will be necessary to become more closely acquainted with the living 

 organisms they contain, viz., the nodule bacteria. 



The discovery of these bacteria by Woronin in 1866 was not followed im- 

 mediately by their general recognition in scientific circles. For example, H. 

 DE VRIES (II.) in 1877 looked upon them as non-essential. Moreover, when 

 J. BRUNCHORST (I.), in 1885, examined them more closely, he came to quite a 

 different conclusion, and defined the supposed bacteria as organised albuminoids 

 collected in the interior of the nodule cells, and therefore termed them bacteroids 

 (on account of their external resemblance to bacteria). Hence the term bac- 

 teroidal tissue, applied to the internal portion of the nodules in which these 

 organisms appear in large numbers. Remarkably enough, Brunchorst's opinion 

 found favour in the eyes of A. B. FRANK (V.), although conflicting \vith his own 

 discoveries made in 1879. A. TSCHIRCH (I.) also ranged himself on the side of 

 Brunchorst. 



A complete revolution of opinion took place in 1888, when BEYERINCK (XIV.) 

 indubitably established the fungoid nature of these supposed pseudo-bacteria, 

 by isolating them from the nodules, and cultivating them further in artificial 

 media. The pure culture obtained from the individual species of Papilionacecu 

 exhibited certain slight but undeniable differences, which, however, were not so 

 extensive as to make their discoverer feel justified in classifying the organisms as 

 separate species, so he defined them as varieties of a single species, for which he 

 proposed the name of Bacillus radicicola. The artificial formation of nodules 

 induced by inoculating the roots of Leguminosae with such pure cultures was 

 successfully attempted a year later by Prazrnowski, and will be noticed in 195. 

 The bacillus in question develops either feebly or not at all on ordinary nutrient 

 gelatin, this substratum being too rich in nutrient materials. Beyerinck recom- 

 mends a decoction of the leaves of PapilionacecK, with the addition of 7 per cent, 

 of gelatin, \ per cent, of asparagin, and per cent, of cane-sugar. The requisite 



