120 Minnesota Plant Life. 



erate free nitrogen to the atmosphere of the soil — and it is to 

 be understood that ordinary soil contains a considerable amoinit 

 of such s^-aseous material together with oxygen and carl)onic- 

 acid. 



Sulphur bacteria. The sul])hur and iron bacteria are of some 

 economic importance from their growth in mineral springs. 

 Sulphur bacteria, when present, occasion an odor of decaying 

 eggs, revealing the presence of a gas known as sulphuretted 

 hydrogen. During their nutrition granules of sulphur are de- 

 posited in their cells and they form a stage in that round of life 

 in which sulphur waste products are given off by animals, then 

 are broken up with the production of sulphur granules in the 

 bacteria, which upon the death of the germ may be re-combined 

 into sulphur salts and in this condition may again be used by 

 green plants for food. Animals, then, directly as do the herb- 

 devouring species and indirectly as do the flesh-devouring 

 forms, make use of the substances produced by green plants and 

 the waste is again begun and the process repeated. 



Iron bacteria and iron-ores. Iron bacteria form iron pre- 

 cipitates where they grow and assist in the rusting of iron in 

 some parts of the world. They frequently extract iron-rust 

 from the waters of springs and form flocculent red masses of the 

 oxide. There is reason to suppose that the masses of iron-ore 

 in Northern Minnesota upon the Mesaba range. were deposited 

 there l)y the activity of iron-bacteria, living in the w'arm waters 

 of an ancient ocean. We may imagine such a primeval sea. hot 

 like the geysers of the Yellowstone, its waters impregnated with 

 iron and furnishing a splendid field for the peculiar activities of 

 the iron-bacteria. Living in such an ocean for thousands of 

 years, as they may have done, there is nothing unreasonable in 

 attrilniting to them the deposits of iron ore which during these 

 latter da\s arc being mined for connnercial pin'poses. 



Manifold relations of bacteria to man. J"',nough lia> nt)v^• 

 been said about the bacteria of Minnesota to justify their api)el- 

 lation of most extraordinary plants. It seems inconceivable at 

 first thought, tliat iron-nn'ning. cheese-making. ])hosiihorescence 

 in the forest, the tanning of leather, the rotation of cro])s, the 

 ri])ening of guano, the i)liglu of ])ear-trees. ei)idemics ot ixphoid- 

 fever, the souring of milk and the >acrihce of innocent people 



