THE BACTERIA 201 



188. How bacteria secure their food. Since they are so sim- 

 ple in structure, and since they live in direct contact with 

 their food supply, the bacteria absorb their nourishment directly 

 through their cell walls. Different kinds of bacteria may live 

 upon different kinds of organic matter, but almost every kind 

 of organic matter may serve to nourish some kind of bacteria. 

 Certain kinds of bacteria can thrive only in absence of free oxy- 

 gen, and a few others can construct food somewhat as green 

 plants do. It must also be noted that bacteria, like other liv- 

 ing things, produce and excrete substances which, if retained, 

 would be injurious to them. If excreted and accumulated 

 about the bacteria in great quantity, these substances would 

 soon kill them. If a jar of beef broth is carefully sealed after 

 any ordinary bacteria have been introduced into it, there will 

 at first be a rapid increase in their number, and the liquid will 

 become clouded with the organisms and their products. But 

 the excretions soon accumulate to such an extent that the 

 bacteria can no longer grow. They become dormant or may 

 die and settle to the bottom of the jar or collect in a jelly-like 

 mass at the surface. 



189. How bacteria reproduce themselves. When in favor- 

 able nutrient material, bacterial cells divide frequently. A 

 plant thus forms two new ones, each of which may soon (in 

 from twenty minutes to half an hour) divide again. This is the 

 simplest possible method of reproduction, consisting merely 

 of the dividing, or fission, of a single-celled plant. The possi- 

 bilities of this rate of reproduction are enormous. If all con- 

 ditions were to remain entirely favorable for reproduction, a 

 bacterium which divides but once an hour would in two days 

 produce offspring numbering 281,500,000,000, and " in three 

 days the progeny of a single cell would balance 148,356 

 hundred- weight." l Of course it is well known that ordina- 

 rily this rate of reproduction cannot be realized, because 

 growth conditions do not remain favorable. The food supply 



1 Jordan, E. O., General Bacteriology, p. 62. W. B. Saunders Company, 

 11)11. 



