SECT. 11 PHYSIOLOGY 231 



organisms which are strikingly autotrophic as regards nitrogen, while 

 they are heterotrophic as regards their carbon-assimilation. These 

 organisms are able to utilise the nitrogen of the atmosphere. Their 

 existence was first established at the end of last century by the work 

 especially of AVinogradski, Hellriegp:l and Wilfarth ('^^). 



In the first place there are certain Bacteria, such as Clostridium 

 Pasieurianum and related forms and Azotohader chroococcum, which live 

 independently in cultivated soil and in water under very various 

 external conditions. They fix free nitrogen and thus possess a 

 very important power both for their own success and for that 

 of many other organisms ; tliis property is of the greatest importance 

 in agriculture. An increasing number of the lower Fungi have been 

 shown by recent researches to have the same power though in less 

 degree. In addition to these free-living forms there are micro-organisms 

 Avhicli occur parasitically in higher plants and have the same property. 

 The best investigated among these are the various forms of Bacillus 

 rudicicola, which infest the roots of Leguminosae and frequently give 

 rise to enoi'mous numbers of gall-like tubercles upon them (Figs. 201, 

 202). The Leguminosae thus appear to differ from all other green 

 plants in their mode of accumulating nitrogen ; this was first established 

 by Gilbert and Lawes in England and Lupitz in Germ.any. 



The Bacteria penetrate tluougli the root-hairs into the cortex of the roots, 

 and there give rise to the tubercles. These tubercles become filled with 

 a bacterial mass, consisting principally of swollen and abnormally developed 

 (h3Tpertrophied) bacterioids, but in part also of Bacteria, which have remained 

 in their normal condition. The former seem to be eventually partially consumed 

 by the host plant, while the latter remain with the dead roots in the soil, to 

 provide for future reproduction. So far as is known, we have here a case of mutual 

 parasitism like those termed symbiosis by De Bary. While the Bacterium lives 

 on carboliydrates and at first also on albuminous substances supplied by the host 

 plant, the latter profits by the power of fixing free nitrogen possessed by the 

 Bacteria. While the Bacteria remain alive they furnish a steady supply of nitro- 

 genous substance to the leguminous plant, and ultimately the remaining substance 

 of the degenerated Bacterioids is absorbed. It has been calculated that Lupins are 

 able in this way to obtain 200 kg. of nitrogen per hektar. When the large amount 

 expended on nitrogenous manures (Chili-saltpetre and ammonium sulphate) is 

 borne in mind the agricultural importance of this natural fixation of nitrogen will 

 be evident. It has been attempted to further it by infecting fields with soil rich 

 in the bacteria, or with pure cultures of specially active forms ("nitragin "). If 

 the soil in which a Leguminous plant is grown contains a sufficiency of nitrates, 

 the plants may live at their expense ; since the presence of nitrates exerts an 

 injurious influence on Bacillus radicicola, practically no nodules are formed under 

 such circumstances. 



Besides the Leguminosae, in which order only Gleditschia triacanthos has 

 hitherto been found free from nodules, Elaeagmis and Alnus are able, according to 

 NoBBE and Hiltner, to utilise free atmospheric nitrogen when their roots bear 

 nodules ; these are due to infection by another of the lower organisms. The same 

 investigators gave the experimental proof that a species of Podocarpus wliich has 



