IO4 Minnesota Plant Life. 



gen of the atmosphere. Further, they are able to assimilate am- 

 monia but sparingly, and such a nitrogenous compound as urea 

 is almost worthless as a food. Therefore, in three great fields 

 of interest to the human race, bacteria are active ; first, as causes 

 of disease through the poisons they secrete; second, as the 

 sources of ferments which are used in the various arts of brew- 

 ing, wine-making and the dairy, not to mention tanning, cigar- 

 manufacture, and a variety of technical processes which are de- 

 pendent upon the proper control of ferment-bacteria; third, 

 as fertilizers of the soil, where they live in countless billions and 

 under a variety of conditions play their part in fixing the nitro- 

 gen of the atmosphere in the form of salts to serve as food for 

 the plants of forest, field and plain. 



Energy produced by bacteria. It should be remembered 

 that among the activities of living things not only are there 

 processes which result in the production of particular substances, 

 but sometimes, also, in the development of particular forms of 

 energy. Heat, for example, is a form of energy which arises 

 in the human body, as a result of its vital activity, and light 

 arises in the body of the glow-worm, in many fungi, and in a 

 large variety of marine animals. It is not, therefore, astonish- 

 ing that heat and light should be among the results of bacterial 

 activity, and it may, therefore, be comprehended how two such 

 apparently unrelated phenomena as an epidemic of smallpox and 

 the phosphorescence of the ocean should each of them be but 

 the results of bacterial growth and nutrition. In the one in- 

 stance, the waste products formed are substances belonging 

 to the group of organic poisons. Secreted in the body they 

 injure its tone and may even shatter its mechanism, causing 

 death. In the other, light is one of the results of bacterial ac- 

 tivity, just as in the physiology of the fire-fly, and when innumer- 

 able germs are floating at the surface of the ocean they may give 

 to it that faint, uniform illumination which is recognized as bac- 

 terial phosphorescence a phenomenon which has been exactly 

 reproduced in the aquaria of laboratories by artificial cultures 

 of light-producing bacteria. 



Substances and forces harmful to bacteria. Bacteria differ 

 among themselves in the kinds of food which they demand, in 

 the nature of the waste products which they produce, and in the 



