294 



Bacteria in Relation to Country Life 



produced slowly or rapidly, the sulfuretted hydrogen, 

 a gaseous substance, is not in itself available to higher 

 plants as a source of sulfur. It must be changed further 

 into sulfate, usually sulfate of lime (gypsum), before it 

 can be used to advantage. 



The sulfuretted hydrogen evolved in the decay of 

 animal and vegetable materials may be changed to 

 sulfate by purely chemical means, 

 or by a group of organisms desig- 

 nated as sulfur bacteria. The sul- 

 fur bacteria are found in ditches, 

 canals, swamps, seas, and mineral 

 springs, — in places where there is 

 a more or less constant supply of 

 sulfuretted hydrogen. These or- 

 ganisms possess, in a very marked 

 degree, the power of decomposing 

 the sulfuretted hydrogen and of 

 depositing granules of sulfur in 

 their cells. When grown in solu- 

 tions containing sulfuretted hy- 

 drogen, the cells are seen to be 

 filled with sulfur granules. The 

 latter make up, at times, 80 to 95 

 per cent of the fresh weight of the 

 bacteria. 



When the supply of sulfuretted hydrogen is dimin- 

 ished or exhausted, the sulfur granules gradually disap- 

 pear and the bacteria perish. Evidently, the organisms 

 are dependent on the sulfuretted hydrogen for their 

 energy. They secure the latter by causing the hydrogen 



Fig. 84. Sulfur bacteria. — 

 1. ThiospirUium Wino- 

 gradskii; X 2,000. 

 (Omelianski.) 2. Beg- 

 giaioa alba; X 2,440. 

 (Coisini.) 



