142 



PLANTS WITHOUT CHLOROPHYLL 



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A pure culture of bacteria. Notice 

 that the bacteria are all the same 

 size and shape. 



Then the needle, which cools very quickly, is dipped in a colony 

 containing the bacteria we wish to study. This mass of bacteria 



is quickly transferred to another 

 sterilized plate, and this plate is 

 immediately covered to prevent 

 any other forms of bacteria from 

 entering. When we have suc- 

 ceeded in isolating a certain kind of 

 bacterium in a given dish, we are 

 said to have a pure culture. Hav- 

 ing obtained a pure culture of 

 bacteria, they may easily be studied 

 under the compound microscope. 



Size and Form. - - In size, bac- 

 teria are the most minute plants 

 known. A bacterium of average 

 size is about y^wo of an mcri m 

 length, and perhaps SOTFO- f an 

 inch in diameter. Some species 



are much larger, others smaller. A common spherical form is 

 TOTOT of an mcri m diameter. They are so small that several million 

 are often found in a single drop of impure water or sour milk. 

 Three well-defined forms of bacteria are recognized : a spherical 

 form called a coccus, a rod-shaped bacterium, the bacillus, and a 

 spiral form, the spirillum. Some bacteria are capable of move- 

 ment when living in a fluid. Such movement is caused by tiny 

 lashlike threads of protoplasm called flagella. The flagella pro- 

 ject from the body, and by a rapid movement cause locomotion 

 to take place. Bacteria reproduce with almost incredible rapidity. 

 It is estimated that a single bacterium, by a process of division 

 called fission, will give rise to over 16,700,000 others in twenty-four 

 hours. Under unfavorable conditions they stop dividing and form 

 rounded bodies called spores. This spore is usually protected by 

 a wall and may withstand very unfavorable conditions of dryness or 

 heat ; even boiling for several minutes will not kill some forms. 



Where Bacteria are most Numerous. - - As the result of our 

 experiments, we can make some generalizations concerning the 



