Minnesota Plant Diseases. 191 



multiplication is thus well illustrated. When unfavorable con- 

 ditions confront a rapidly dividing bacterium, spore formation 

 may take place. The spore, just as most fungus spores, pro- 

 vides itself with thick walls and is thus protected during the 

 continuance of the unfavorable surroundings. These spores 

 may be formed inside of the cells, or by the mere transforma- 

 tion of the ordinary cells by wall thickening and condensation 

 of protoplasmic contents. When placed again under favorable 

 conditions the spore may grow out in various ways into the 

 ordinary bacterial cells. No breeding act is known among the 

 bacteria. 



Physiology of bacteria. The physiological activities of the 

 bacteria are most varied and interesting. They are of immense 

 economic importance for upon them are built a host of indus- 

 trial processes as well as many diseases in man and in plants. 

 Many geologic deposits, as iron ores, may possibly owe their 

 existence to bacterial activities. 



Air-loving and air-shunning bacteria. Most bacteria resem- 

 ble other plants in their requirements for air gases during their 

 life processes. Oxygen, one of these gases, is utilized in burn- 

 ing up certain compounds and in this combustion energy is 

 liberated to run the protoplasmic machinery. This use of oxy- 

 gen is common to animals as well as plants and the ultimate 

 products of the burning are carbonic acid gas and water. Such 

 bacteria may be known as air-loving bacteria or perhaps, more 

 strictly speaking, oxygen-loving. There is another class of 

 bacteria which is capable of obtaining such energy as the air- 

 loving bacteria derive from combustion of compounds in a dif- 

 ferent way, viz. : by the breaking down of complex organic 

 compounds into simpler, during which process the necessary 

 energy is liberated. This process may take place, moreover, 

 when no air is present, and in certain cases the exclusion of air, 

 and particularly of oxygen gas, is necessary. Such bacteria are 

 known as air-shunning bacteria. They have a method of break- 

 ing down complex substances different from that of the com- 

 bustion method and may carry on such a process even when the 

 air is entirely excluded. 



Among the many air-loving forms the vinegar bacteria may 

 be mentioned while the rancid-butter bacteria are examples of 

 air-shunning kinds. 



