BACTERIA 213 



are but few in comparison with the great number of species 

 found existing almost everywhere upon the earth's surface, 

 bacteria will be further considered here. In Experiment 

 113 we found that if substances are left exposed to the air 

 they soon undergo certain changes, which they are free 

 from when properly protected. These changes are due to 

 bacteria. 



The bacterium is a single-cell plant, probably the simplest 

 of all plants ; it can only be seen with a high-power micro- 

 scope. Bacteria are rod shaped, 

 thread-shaped, screw-shaped or have "~2^^ 



various other forms (Fig. 105). The 

 protoplasm in the cell of bacteria has 

 the power to assimilate food and 

 build more protoplasm. When the 

 cell has grown sufficiently, it divides 

 into two cells. 



A healthy bacterium grows fast * ^ 

 enough to be ready to divide about . 



once an hour. If it divided once *-^ 



^>, -* & 



an hour and each division continued 

 to divide once an hour, in the course 



rig. lUo. 



of twenty-four hours there would be 



nearly seventeen million bacteria produced. If this were 

 kept up for some weeks, the mass of bacteria would be as 

 large as the earth. Of course, this would mean that each 

 bacterium had plenty of room to live in and plenty of 

 food to live on and nothing to injure it. These con- 

 ditions are not found, and each bacterium has to struggle 

 for existence just as every other plant does. As it is, 

 however, bacteria are numberless. 



Since bacteria and fungi cause the "spoiling" of food, 

 it is necessary to find means of stopping their growth. It 

 has been found that thoroughly smoking fish and meat 



