194 Minnesota Plant Diseases. 



tine have been made possible and an enormous saving of life 

 effected. Modern methods of medical and surgical practice 

 have been built upon such knowledge. Great as have been the 

 results in the past, still greater may yet be achieved in the future 

 by a more complete knowledge of these disease-causing bacteria. 

 This applies as well to those bacteria-causing diseases of 

 plants as to the diseases of man, though the former are not 

 so numerous nor so vital to man's interests. 



Dye-forming bacteria. Another great group of bacteria 

 have the peculiar property of producing coloring matters dur- 

 ing their nutritive processes. This coloring matter is in some 

 cases found in the cells of the bacteria and in others is a by- 

 product of nutrition. Red and yellow spots on bread are fre- 

 quently of this nature and milk is sometimes colored red from 

 a similar cause. The blue coloration of milk is also of bacterial 

 origin. Certain bacteria form a beautiful ''bacterial purple" 

 and are furthermore peculiar in that, by means of this color- 

 ing matter, they seem to be able to utilize the sun's rays in a 

 manner analogous to the leaf-green plants which convert sun- 

 light energy by the use of leaf-green. The production of in- 

 digo dyes from indigo plants is also dependent upon the activ- 

 ity of bacteria; other blue colorations, as in certain kinds of 

 cheese diseases, and again, green colorations may have bacterial 

 origins. 



Light- and heat-forming bacteria. In the conversion of 

 energy in which bacteria are engaged, many forms exhibit 

 still other peculiarities. Some utilize surplus energies in the 

 generation of light and such produce phosphorescence or 

 other illuminations. Sea phosphorescence is in part due to 

 these bacteria. Others again dissipate energy in the produc- 

 tion of heat and examples of these may be seen in heated 

 manure piles, in silos, in certain methods of curing hay, and 

 in tobacco curing. The generation of heat in all of these cases 

 is due to the activity of heat-producing bacteria. The tem- 

 perature may even be raised to such a degree that rapid com- 

 bustion of the materials may take place and such occurrences 

 are usually described as spontaneous combustion. 



Fermentation bacteria. Still another great group of bac- 

 teria are capable of causing fermentation in fluids a splitting 



