252 Science of Plant Life 



summer months and of our own bodies. They reproduce by 

 simple cell division, the cell simply pinching in two to form 

 new plants. Many forms produce spores, similar to the rest- 

 ing spores of the algas, by the contraction of the protoplasm 

 in the cell and the secretion of a new cell wall. Spores are 

 very 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 temper- 

 ature 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 pro- 

 duce spores. 



• 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 dioxid, water, and mineral salts. The sewage 

 that is turned into our rivers is chemically changed and dis- 

 posed 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 more rapidly and more efi&ciently. 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 



