282 APPLIED BIOLOGY 



In similar ways bacteria from, any source may be obtained in pure 

 cultures by means of solid culture media. In addition to gelatin, a 

 thin slice of potato, carefully sterilized, may be used ; and for certain 

 bacteria which must be grown in incubators agar-agar is added, 

 because it is not so easily melted as gelatin. 



256. Important Facts concerning Structure and Functions 

 of Bacteria. (a) Size. The average rod-shaped bacteria 

 are not more than -nrFoir f an i ncn m length and ^riinr in 

 diameter. Many of the spherical forms are not more than 

 STj-tfu-^ of an inch in diameter; some are still smaller and just 

 visible with the highest powers of the microscope, and the 

 germs (supposed to be bacteria) of the foot- and-mouth disease 

 of cattle are so small that they will pass through the pores of 

 the earthenware tubes in a Pasteur-Berkefeld filter, which 

 will remove all bacteria large enough to be seen with the 

 microscope. In fact, bacteria are so small that fractions of 

 inches are inconvenient measures; and biologists commonly 

 state dimensions in micromillimeters or microns, one of which 

 is nnnr f a millimeter, or about ^-grinr of an inch. A bac- 

 terium OTTO- mc h long would be 1 micron long. 



(b) Growth and Division. Under favorable conditions, a 

 bacterium rapidly grows to full size and then divides into 

 equal halves. Each of these absorbs food, grows, and soon 

 divides again. Certain bacteria have been observed to 

 divide every twenty minutes. Some one has computed that 

 if the descendants of a single individual bacterium were to 

 keep on dividing once an hour for two days there would be 

 more than 280 millions. You can easily verify these figures 

 by applying the mathematical formulas for geometrical pro- 

 gression. At the rate of one division every half hour, there 

 would be 4772 billions in 72 hours, and the weight would 

 probably be more than 7000 tons. These figures simply 

 illustrate the enormous reproductive powers of the bacteria 

 and help us to understand how a few bacteria introduced 

 may soon produce enough to cause disease or the decompo- 



