BACTERIA— YEAST PLANTS. 9 



that to the bacteria are due many diseases 

 of man and the lower animals, and perhaps 

 of common plants as well. Among such dis- 

 eases are anthrax and splenic fever in cat- 

 tle, and small-pox, scarlet fever, and proba- 

 bly consumption, cholera, and other scourges 

 of the human race. Pure water contains no 

 life, but the water of ditches, of stagnant 

 pools, of impure cisterns, contains myriads 

 of these minute plants. The souring of 

 milk and many changes of fermentation are 

 due to them. They are also the cause of de- 

 cay. While they themselves depend for life 

 upon the organic products of other plants 

 and of animals, they are the direct means of 

 reducing all organic life to decay and disin- 

 tegration. They hold the keys of life ; they 

 complete the grand cycle of nature by which 

 all living things return to the earth from 

 whence they came. 



A little higher in the scale of existence 

 are the Yeast Plants, These 

 minute bodies are propagated 

 rapidly in yeast and other 

 ferments, and by their physi- 

 ological action produce im- ^'S- 8. 

 portant chemical changes. The invisible 

 plants which spring up in bread yeast give 



