

Bacteria and Their Relations to Life 389 



containing medium. They are so small that the details of cell 

 structure are not well known. The protoplasm is surrounded 

 by a cell wall probably composed of cellulose and chitin. In 

 some forms protoplasmic threads, called flagella (singular, 

 flagellum) extend through the cell wall and provide organs of 

 locomotion. The flagellate forms are active individuals, that 

 become stationary later and lose the flagella. 



Many bacteria have each cell further surrounded by a gelat- 

 inous sheath. Sometimes the sheaths of many individuals 

 coalesce, forming slimy scums on stagnant water and on objects 

 in the water. 



When all the conditions are favorable, bacteria may multiply 

 very rapidly. This is accomplished by cell division, the cell 

 simply pinching in at the middle and separating, forming two 

 new individuals. As the daughter cells quickly grow to the size 

 of the original, this process may be repeated in 20 minutes to an 

 hour. A little calculating will show that if this process continued 

 for 24 hours there would be hundreds of million-millions of 

 individuals.^ Of course, long before any such number can 

 accumulate, the water and food supplies are consumed and the 

 products of their activities accumulate and cell division is 

 stopped. If this were not true, the whole organic world would be 

 turned to bacteria over night. 



Spores are formed by many bacteria, by the contraction of the 



^ Starting with one bacterium and counting a generation every half hour, 

 the number at the end of a day would be 281 million-millions, or about one 

 pint of bacteria. Starting the second day with one pint of individuals all 

 multiplying at the same rate, at the end of 48 hours there would be 281 million- 

 million pints of bacteria, or about 32 cubic miles. At the end of the third 

 day there would be enough to fill the ocean basins 3 milHon times, or sufficient 

 to make 33,000 bodies the size of the earth. 



Why do not bacteria capture the earth? First, because they produce 

 acids and other harmful substances in the medium, that stop their develop- 

 ment ; second, because they can obtain only the food that diffuses to them from 

 infinitely small distances beyond their own cell walls ; third, because they soon 

 meet unfavorable temperature, moisture, or light conditions ; and fourth, 

 because they are eaten by microscopic animals in large numbers. A short 

 life is the rule among bacteria. 



