268 THREAD-BACTERIA 



fail to develop any nodules, and are just as dependent on a 

 supply of soil-nitrates as other green plants. It appears that 

 different strains of these Bacteria infect different Leguminous 

 plants, and that normally those of a particular strain only attack 

 other individuals of the same species. Similar nodules of a 

 larger size occur on the roots of the Alder and the Bog Myrtle 

 (Mvrica gale). 



Brief reference may be made to the so-called Thread-Bacteria, 

 whose exact relation to the other forms is, however, dubious. 

 In many respects they show a closer approximation to the Blue- 

 green Algae than to the Bacteria proper. A common form, 

 Beggiatoa, which is like a colourless Oscillatoria, is frequent in 

 waters rich in sulphuretted hydrogen ; the organism in question 

 obtains the energy necessary for the building up of organic 

 substances by the oxidation of the sulphuretted hydrogen to 

 sulphur. The Iron-Bacteria (Leptothrix, Crenothrix, etc.), whose 

 branched threads are abundant in ferruginous waters, and are 

 often the cause of the brown deposit of ferric hydroxide, are 

 further examples of these forms. 



