390 General Botany 



protoplasm into a rounded mass at one end, or near the middle 

 of the cell, and by the secretion of a secondary spore wall. In 

 this condition the protoplasm contains less water and is highly 

 resistant to drying, to high and low temperatures, and to poisons 

 which readily kill the ordinary bacterial cells. It is because the 

 spores of certain forms withstand the temperature of boiling 

 water that steam pressure is used in sterilizing cans of corn, 

 beans, peas, and other vegetables. Most of the common disease- 

 producing bacteria, however, do not produce spores. 



Forms of bacteria. Some of the largest bacteria form long 

 rows, or filaments of cells. These may be found commonly in 

 stagnant water or in streams that carry sewage. Among the 

 small forms it is customary to call the rod-shaped cells Bacillus 

 (plural, bacilli) the round ones. Coccus (plural, cocci), and the 

 spiral forms Spirillum. Some of these type-forms are shown in 

 Figure 242. 



Bacteria and sanitation. The bacteria of decay help to keep 

 the surface of the earth clean. They change the highly complex 

 organic substances that form the bodies of plants and animals 

 into simple substances that may be used again by other plants 

 in building foods. When plants and animals die, their bodies are 

 gradually transformed by the bacteria into carbon dioxide, water, 

 and mineral salts. The sewage that is turned into our rivers is 

 chemically changed and disposed of in the same way by these 

 minute plants. The great increase in the number and size of 

 our cities has made it necessary to build large sewage-disposal 

 plants where the bacteria can act rapidly and efficiently. This 

 prevents the pollution of streams and keeps the water suitable 

 for city water supplies. 



The modern processes of filtering and sterilizing the water 

 supplies of cities are carried on partly to remove sediment and 

 partly to remove disease-producing bacteria. Adding minute 

 quantities of alum and chloride of lime to the water and then 

 filtering it through sand not only renders the water clear but 



