IO2 Minnesota Plant Life. 



ers of the group are comparatively much larger, the largest be- 

 ing barely fifteen-thousandths of a millimeter in diameter. 

 Sometimes they cling together in chains or threads thus becom- 

 ing filaments of considerable length. In form, bacteria are 

 either spherical, ovoid, cylindrical, twisted like corkscrews, or 

 irregularly shaped. They never possess leaf-green, though some 

 of them are capable of producing coloring substances of other 

 kinds. It is now impracticable to group the bacteria as was 

 done years ago into different species based upon form. The 

 older students called a spherical bacterium a coccus, an ovoid 

 bacterium kept its name, while a cylindrical-elongated form was 

 called a bacillus ; but it has been shown that one and the same 

 organism may successively assume the coccus, the bacterium 

 and the bacillus shape. To-day it is more common to classify 

 them physiologically, and there are recognized the following 

 principal groups, not including, perhaps, all of the forms, but 

 excluding only the unimportant ones. The groups are as fol- 

 lows: 



A. Disease-producing bacteria: Of these there are two 

 classes; those producing diseases in animals and those produc- 

 ing diseases in plants. 



B. Ferment-producing bacteria: Of these there are four 

 principal classes; those producing butyric acid, in which class 

 are included most of the bacteria of putrifaction ; those pro- 

 ducing lactic acid, including the forms which make milk sour, of 

 which there is a great variety; those producing alcoholic fer- 

 mentation, like the yeasts; and those producing vinegar fer- 

 mentation. 



C. Heat-producing bacteria : Sometimes the cause of spon- 

 taneous combustion of cotton waste and the heating of ensilage 

 and hay. 



D. Light-producing bacteria: To which is attributed in 

 part the phosphorescence of the ocean. 



E. Color-producing bacteria : Including a variety of forms 

 which produce different sorts of coloring substances. 



F. Nitrifying bacteria: Which fix atmospheric nitrogen. 



G. De-nitrifying bacteria : Which decompose urea. 



H. Sulphur bacteria: Which produce granules of sulphur 

 in mineral springs. 



I. Iron bacteria : Which secrete iron rust or iron ores. 



