SCHIZOPHYTA 45 1 



parasitic, (ii) Metatrophic, those which cannot live unless they have 

 organic substances at their disposal, both nitrogenous and carbonaceous. 

 They occur in the open, and are saprogenic and sometimes parasitic 

 (facultative parasites), (iii) Par atrophic, those which develop normally 

 only within the living tissues of other organisms, and are true and 

 obligatory parasites, such as the germs of Tubercle or Diphtheria. 



This classification may be extended, however, to all other organisms. 

 All green autophytes are prototrophic in the same sense as the first 

 group of Bacteria. All fungi and animals are metatrophic, except 

 the parasitic forms, which are paratrophic. Thus in point of fact the 

 Bacteria exemplify types of nutrition which run parallel with those 

 seen in larger organisms. 



Bacterial germs are widely diffused in air, water, and soil, as well as on or 

 within living organisms, whether animals or plants. Their activities are so 

 various that it will be best to illustrate them by a few examples rather than 

 to mention many. 



Among the Bacteria that live in water Crenothrix polyspora is notorious for 

 choking pipes of water-supply, and making the water undrinkable, though 

 apparently not poisonous. Lille, Rotterdam, Berlin, and Cheltenham have 

 suffered from it. It is probably world-wide in distribution : but being an 

 Iron-Bacterium, it finds a special opportunity for development in the water- 

 conduits of towns. It shares with other Iron-Bacteria, such as Leptothrix 

 ochracea, the power of oxidising oxide of iron to the hydrated oxide, which is 

 deposited in the walls of its cells, and when these are massed together, it 

 appears as those ochre-coloured deposits of bog iron ore not uncommon in 

 the beds of ferrugineous streams. These are filamentous Bacteria, and they 

 grow attached at their base to solid objects (Fig. 347). Somewhat similar are 



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Fig. 347- 

 Crenothrix polyspora. (After Ellis.) (Mature specimen, x 250.) 



the Sulphur-Bacteria, such as Beggiatoa, which grow in sulphurous springs. 

 They separate sulphur from sulphuretted hydrogen, which is then deposited 

 in their cells. The green and purple sulphur bacteria are remarkable for the 

 fact that they depend for their carbon supply on carbon dioxide and sunlight. 

 They are, in fact, photosynthetic, though the process differs in certain respects 



from that in green plants. 



Both the Iron- and the Sulphur-Bacteria are prototrophic, that is independent 

 of organic compounds for their nutrition. It is otherwise with Bacillus radicicola 

 already described in relation to the nodules of the Leguminosae (pp. 127, 

 235). These bacteria exist freely in the soil, and when the opportunity offers 

 they penetrate the root-hairs. The Bacillus within the nodule is paratrophic 



