142 PLANT BIOLOGY 



gether, a chain or a mass is formed. Oftentimes they separate 

 entirely from each other. In either case the whole mass of 

 bacteria is called a colony. 



It usually takes about an hour for the division to take place. 

 Suppose, then, we start at ten o'clock some morning with a 

 single healthy bacterium. If conditions are favorable, there 

 would be two cells at eleven o'clock, and by twelve o'clock 

 each of these two daughter cells would form two granddaugh- 

 ter cells; the colony would then number four individuals. 

 Should this process continue for twenty-four hours, or until 

 ten o'clock on the day after the single bacterium began its 

 race, the colony would number 16,777,216 bacteria. " It 

 has been calculated by an eminent biologist," says Dr. 

 Prudden., 1 " that if the proper conditions could be maintained, 

 a rodlike bacterium, which would measure about a thou- 

 sandth of an inch in length, multiplying in this way, would in 

 less than five days make a mass which would completely fill 

 as much space as is occupied by all the oceans on the earth's 

 surface, supposing them to have an average depth of one mile." 



149. Necessary conditions for the growth of bacteria. 

 Such startling possibilities as those suggested in the preceding 

 section fortunately can never become realities, for the 

 favorable conditions to which we have referred soon cease to 

 exist. Bacteria, like all other living organisms, require food, 

 oxygen, moisture, and a certain degree of warmth. Let any 

 one of these conditions be withheld, and the cells either die 

 or cease to be active. Sometimes, when food or moisture 

 begins to fail, the protoplasm within each cell rolls itself 

 into a ball and covers itself with a much thickened wall. 

 This protects it until it again meets with conditions favorable 

 for growth. The process we have been describing is known as 



1 " The Story of the Bacteria," by Dr. T. Mitchell Prudden. G. P. 

 Putnam's Sons, New York. 



