THE BACTERIA AXD VIRUSES 359 



usually inhibited, while in well-aerated sandy soils it may be twice as active 

 as in close clays. 



Boussingault found that plants of the order Leguminosae were apparently 

 exceptions to the general rule, and this is now known to be due to their 



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Fig. 346. — Clostridium pastorianum. Gram stained. 



symbiosis with certain Bacteria, which enter roots from the soil and stimulate 

 the growth of root nodules in which the bacteria multiply (Fig. 347). This 

 organism, Rhizobium, of which the principal species is R. leguminosarum 

 (Fig. 348), has also the power of fixing atmospheric Nitrogen, at least when 

 living in a root, and it appears that the flowering plant benefits by absorption 

 of some of the Nitrogen compounds thus formed. The roots of Leguminosae 

 are consequently particularly rich in Nitrogen, and such crops as Clover, 

 which belongs to this family, have long been known to enrich the ground 

 on which they grow (Fig. 349). 



Chemosynthetic Bacteria 



Among the Bacteria there are some which display a very interesting type 

 of metabolism which marks them out as biologically ver>' primitive. This 

 is called chemosynthesis. It differs from photosynthesis in that the energy 

 for Carbon assimilation comes not from light but from the oxidation of 

 certain inorganic or simple organic substances. The reactions employed 

 by these Bacteria are all exothermic, that is to say, they yield free energy 

 which is utilized for building up carbohydrates in the cells. These plants 

 are therefore independent both of light and, mostly, of organic matter, so that 

 they would be capable of existence even under the conditions which must 



