266 BACTERIA IX THE SEA [PART III 



bacteria are, with regard to their mode of nutrition, and probably 

 also with regard to their morphology, absolutely the simplest 

 organisms known to us. They have no chlorophyll and can live 

 and multiply absolutely in the dark. They require as food 

 only ammonia or nitrous acid as nitrogen sources, and carbon 

 dioxide as a source of carbon. With these extremely simple 

 substances and with traces of the essential mineral salts they can 

 form proteid substance and convert this into living protoplasm. 

 They are aerobic germs deriving their oxygen from the atmosphere. 

 Their carbon dioxide is also obtained from the air or from the 

 ground water, and from the latter source they also obtain their 

 mineral salts. Their energy is derived from the oxidation of the 

 ammonia or nitrous acid. 



Nitrification of the ammonia resulting from the putrefactive 

 decomposition of dead proteid is carried out in two stages : (1) the 

 oxidation of the ammonia to nitrous acid, and (2) the oxidation of 

 the latter compound to nitric acid. These acids combine, of course, 

 with whatever bases or alkalies are present in the soil in which the 

 nitrification takes place, usually lime or soda. This mineralisation 

 of the ammonia is the last stage in the conversion of the unstable 

 proteid substance into the stable and inoffensive mineral salts. 



Nitrogen-fixing bacteria. The ultimate source of the nitrogen 

 of the tissues of plants and animals is the atmospheric gas. But 

 elementary nitrogen cannot be utilised by the vast majority of the 

 organisms of either kingdom until it is combined with oxygen, and 

 with some base to form nitrates or nitrites, or is in the form of some 

 compound of ammonia : then the plants can utilise it ; or until it 

 is combined with oxygen, hydrogen, carbon, sulphur and phos- 

 phorus to form proteid : then the animals can utilise it. If then a 

 considerable proportion of the combined nitrogen of the tissues of 

 plants and animals has been combined from the elementary state 

 there must be some means of bringing about this synthesis. Now 

 when we attempt to combine atmospheric nitrogen and oxygen in 

 the laboratory the most powerful means are required — either the 

 electric spark or furnace. Yet we find that this synthesis is 

 carried out by the vital activity of micro-organisms. 



It has been known for many years that the leguminous plants 



