BACTERIOLOGY OF SOIL 211 



acid when grown in peat-infusion, which has a great 

 solvent action upon lead. It is to these organisms that 

 the plumbo-solvency of waters from peaty districts is 

 attributed. Experiments to determine the longevity of 

 the typhoid bacillus in various soils have given variable 

 results. Thus, Savage found it to live in highly-polluted, 

 unsterilised mud from a tidal river for two weeks in one 

 experiment, and for five weeks in another. Firth and 

 Horrocks found that in a sewage -polluted soil recovered 

 from beneath a broken drain the organism survived for 

 sixty-five days. 



When completely frozen, there is an unexpectedly 

 rapid multiplication of organisms. H. J. Conn thinks 

 there may be two groups of soil bacteria, one flourishing 

 in summer, the other in winter. 



Nitrogen Fixation. A number of organisms absorb or 

 * fix ' nitrogen from the atmosphere, notably Clostridium 

 Pastorianum, a sporulating anaerobe, some thermophilic 

 bacteria, and the aerobic ovoid organisms Azotobacter. 

 Botrytis cinerea, Aspergillus niger, Penicillium glaucum 

 in fact most fungi and a pseudo-yeast, Tulare No. 466 

 (Lipman), also fix nitrogen. The fixation of free atmo- 

 spheric nitrogen by Lolium temulentum, or darnel, is due 

 to the fungus which infests it. This grass has no sym- 

 biotic root organisms. Leguminous plants possess on their 

 roots little tubercles containing ' bacteroids.' Bacteroids 

 may occur as rods or branching structures. When 

 inoculated into suitable media, they give rise to the nodule 

 bacteria (B., Pseudomonas or Rhizobium radicicola) which 

 measure about 5 /n by l/u. The latter do not form spores, 

 are actively motile, and are strict aerobes. The organisms 

 enter the plant through the root hairs, where on multi- 

 plication they produce a filamentous zooglcea which 

 grows into the root and causes production of nodules. 

 These take up atmospheric nitrogen and convert it into 

 a form assimilable by the plant. Atmospheric nitrogen 

 is absorbed in greater quantity in the legume than in 

 artificial media. The plant, so far from welcoming the 

 organisms, offers resistance to their attack; this resistance 

 is most evident where free potassium nitrate is present, 

 when nodules do not form, because the ' nitrogen hunger ' 

 is lessened. The association of the plant and bacteria 

 cannot, therefore, be regarded as true symbiosis. 



