34 STUDIES IN PHYSIOLOGY 



It usually takes about an hour for the division to take 

 place. Suppose, then, we start at 10 o'clock some morning 

 with a single healthy bacterium. If conditions are favor- 

 able, there would be two cells at 11 o'clock, and by 12 

 o'clock each of these two daughter cells would form two 

 granddaughter cells ; the colony would then number four 

 individuals. Should this process continue for 24 hours or 

 until 10 o'clock on the day after the single bacterium began 

 its race, the colony would number 16,776,216 bacteria. " It 

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

 den, 1 " that if the proper conditions could be maintained, a 

 rodlike bacterium, which would measure about a thousandth 

 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." 



Necessary Conditions for the Growth of Bacteria. Such start- 

 ling 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, oxy- 

 gen, 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 favor- 

 able for growth. The process we have been describing is 

 known as spore formation; the tiny protoplasmic sphere is 

 called a spore, and its dense covering a spore wall. In this 

 condition bacteria may be blown hither and yon as a part of 

 the dust. They may be heated even above the temperature 

 of boiling water without being killed. When at length they 



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

 Putnam's Sons, N. Y. 



